Present day concepts of the whole broad problem of bird migration are
made up of a few facts and many guesses. The evolutionary origin of
migration, the modern necessities that preserve its biologic utility,
the physiological processes associated with it, the sensory mechanisms
that make it possible, the speed at which it is achieved, and the
routes followed, all have been the subject of some investigation and
much conjecture. All, to a greater or less extent, remain matters of
current controversy. All must be considered unknowns in every logical
equation into which they enter. Since all aspects of the subject are
intimately interrelated, since all have a bearing on the probabilities
relating to any one, and since new conjectures must be judged largely
in the light of old conjectures rather than against a background of
ample facts, the whole field is one in which many alternative
explanations of the established phenomena remain equally tenable.
Projected into this uncertain atmosphere, any statistical approach
such as determinations of flight density will require the accumulation
of great masses of data before it is capable of yielding truly
definitive answers to those questions that it is suited to solve. Yet,
even in their initial applications, density analyses can do much to
bring old hypotheses regarding nocturnal migration into sharper
definition and to suggest new ones.
The number of birds recorded through the telescope at a particular
station at a particular time is the product of many potential
variables. Some of these--like the changing size of the field of
observation and the elevation of flight--pertain solely to the
capacity of the observer to see what is taking place. It is the
function of the density and direction formulae to eliminate the
influences of these two variables insofar as is possible, so that the
realities of the situation take shape in a nearly statistically true
form. There remain to be considered those influences potentially
responsible for variations in the real volume of migration at
different times and places--things like the advance of season,
geographic location, disposition of terrain features, hourly activity
rhythm, wind currents, and other climatological causes. The situation
represented by any set of observations probably is the end result of
the interaction of several such factors. It is the task of the
discussions that follow to analyze flight densities in the light of
the circumstances surrounding them and by statistical insight to
isolate the effects of single factors. When this has been done, we
shall be brought closer to an understanding of these influences
themselves as they apply to the seasonal movements of birds. Out of
data that is essentially quantitative, conclusions of a qualitative
nature will begin to take form. It should be constantly borne in mind,
however, that such conclusions relate to the movement of birds _en
masse+ and that caution must be used in applying these conclusions to
any one species.
Since the dispersal of migrants in the night sky has a fundamental
bearing on the sampling procedure itself, and therefore on the
reliability of figures on flight density, consideration can well be
given first to the horizontal distribution of birds on narrow fronts.
A. HORIZONTAL DISTRIBUTION OF BIRDS ON NARROW FRONTS
Bird migration, as we know it in daytime, is characterized by spurts
and uneven spatial patterns. Widely separated V's of geese go honking
by. Blackbirds pass in dense recurrent clouds, now on one side of the
observer, now on the other. Hawks ride along in narrow file down the
thermal currents of the ridges. Herons, in companies of five to fifty,
beat their way slowly along the line of the surf. And an unending
stream of swallows courses low along the levees. Everywhere the
impression is one of birds in bunches, with vast spaces of empty sky
between.
Such a situation is ill-suited to the sort of sampling procedure on
which flight density computations are based. If birds always traveled
in widely separated flocks, many such flocks might pass near the cone
of observation and still, by simple chance, fail to enter the sliver
of space where they could be seen. Chance would be the dominating
factor in the number of birds recorded, obscuring the effects of other
influences. Birds would seldom be seen, but, when they did appear, a
great many would be observed simultaneously or in rapid succession.
When these telescopic studies were first undertaken at Baton Rouge in
1945, some assurance already existed, however, that night migrants might
be so generally dispersed horizontally in the darkness above that the
number passing through the small segment of sky where they could be
counted would furnish a nearly proportionate sample of the total number
passing in the neighborhood of the observation station. This assurance
was provided by the very interesting account of Stone (1906: 249-252),
who enjoyed the unique experience of viewing a nocturnal flight as a
whole. On the night of March 27, 1906, a great conflagration occurred in
Philadelphia, illuminating the sky for a great distance and causing the
birds overhead to stand out clearly as their bodies reflected the light.
Early in the night few birds were seen in the sky, but thereafter they
began to come in numbers, passing steadily from the southwest to the
northeast. At ten o'clock the flight was at its height. The observer
stated that two hundred birds were in sight at any given moment as he
faced the direction from which they came. This unparalleled observation
is of such great importance that I quote it in part, as follows: "They
[the birds] flew in a great scattered, wide-spread host, never in
clusters, each bird advancing in a somewhat zigzag manner.... Far off in
front of me I could see them coming as mere specks...gradually growing
larger as they approached.... Over the illuminated area, and doubtless
for great distances beyond, they seemed about evenly distributed.... I
am inclined to think that the migrants were not influenced by the fire,
so far as their flight was concerned, as those far to the right were not
coming toward the blaze but keeping steadily on their way.... Up to
eleven o'clock, when my observations ceased, it [the flight] continued
apparently without abatement, and I am informed that it was still in
progress at midnight."
Similarly, in rather rare instances in the course of the present
study, the combination of special cloud formations and certain
atmospheric conditions has made it possible to see birds across the
entire field of the telescope, whether they actually passed before the
moon or not. In such cases the area of the sky under observation is
greatly increased, and a large segment of the migratory movement can
be studied. In my own experience of this sort, I have been forcibly
impressed by the apparent uniformity and evenness of the procession of
migrants passing in review and the infrequence with which birds
appeared in close proximity.
As striking as these broader optical views of nocturnal migration are,
they have been too few to provide an incontestable basis for
generalizations. A better test of the prevailing horizontal
distribution of night migrants lies in the analysis of the telescopic
data themselves.
[Illustration: FIG. 25. Positions of the cone of observation
at Tampico, Tamps., on April 21-22, 1948. Essential features
of this diagrammatic map are drawn to scale, the triangular
white lines representing the projections of the cone of
observation on the actual terrain at the mid-point of each
hour of observation. If the distal ends of the position lines
were connected, the portion of the map encompassed would
represent the area over which all the birds seen between
8:30 P. M. and 3:30 A. M. must have flown.]
The distribution in time of birds seen by a single observer may be
studied profitably in this connection. Since the cone of observation
is in constant motion, swinging across the front of birds migrating
from south to north, each interval of time actually represents a
different position in space. This is evident from the map of the
progress of the field of observation across the terrain at Tampico,
Tamaulipas, on April 21-22, 1948 (Figure 25). At this station on this
night, a total of 259 birds were counted between 7:45 P. M. and
3:45 A. M. The number seen in a single hour ranged from three to
seventy-three, as the density overhead mounted to a peak and then
declined. The number of birds seen per minute was not kept with stop
watch accuracy; consequently, analysis of the number of birds that
passed before the moon in short intervals of time is not justified. It
appears significant, however, that in the ninety minutes of heaviest
flight, birds were counted at a remarkably uniform rate per fifteen
minute interval, notwithstanding the fact that early in the period the
flight rate overhead had reached a peak and had begun to decline. The
number of birds seen in successive fifteen-minute periods was
twenty-six, twenty-five, nineteen, eighteen, fifteen, and fifteen.
Also, despite the heavy volume of migration at this station on this
particular night, the flight was sufficiently dispersed horizontally
so that only twice in the course of eight hours of continuous
observation did more than one bird simultaneously appear before the
moon. These were "a flock of six birds in formation" seen at 12:09 A. M.
and "a flock of seven, medium-sized and distant," seen at 2:07 A. M.
In the latter instance, as generally is the case when more than one
bird is seen at a time, the moon had reached a rather low altitude,
and consequently the cone of observation was approaching its maximum
dimensions.
The comparative frequency with which two or more birds simultaneously
cross before the moon would appear to indicate whether or not there is
a tendency for migrants to fly in flocks. It is significant, therefore,
that in the spring of 1948, when no less than 7,432 observations were
made of birds passing before the moon, in only seventy-nine instances,
or 1.1 percent of the cases, was more than one seen at a time. In
sixty percent of these instances, only two birds were involved. In one
instance, however, again when the moon was low and the cone of
observation near its maximum size, a flock estimated at twenty-five
was recorded.
The soundest approach of all to the study of horizontal distribution at
night, and one which may be employed any month, anywhere, permitting the
accumulation of statistically significant quantities of data, is to set
up two telescopes in close proximity. Provided the flight overhead is
evenly dispersed, each observer should count approximately the same
number of birds in a given interval of time. Some data of this type are
already available. On May 19-20, at Urbana, Illinois, while stationed
twenty feet apart making parallax studies with two telescopes to
determine the height above the earth of the migratory birds, Carpenter
and Stebbins (_loci cit._) saw seventy-eight birds in two and one-half
hours. Eleven were seen by both observers, thirty-three by Stebbins
only, and thirty-four by Carpenter only. On October 10, 1905, at the
same place, in two hours, fifty-seven birds were counted, eleven being
visible through both telescopes. Of the remainder, Stebbins saw
seventeen and Carpenter, twenty-nine. On September 12, 1945, at Baton
Rouge, Louisiana, in an interval of one hour and forty minutes, two
independent observers each counted six birds. Again, on October 17,
1945, two observers each saw eleven birds in twenty-two minutes. On
April 10, 1946, in one hour and five minutes, twenty-four birds were
seen through one scope and twenty-six through the other. Likewise on May
12, 1946, in a single hour, seventy-three birds were counted by each of
two observers. The Baton Rouge observations were made with telescopes
six to twelve feet apart. These results show a remarkable conformity,
though the exceptional October observation of Carpenter and Stebbins
indicates the desirability of continuing these studies, particularly in
the fall.
On the whole, the available evidence points to the conclusion that night
migration differs materially from the kind of daytime migration with
which we are generally familiar. Birds are apparently evenly spread
throughout the sky, with little tendency to fly in flocks. It must be
remembered, however, that only in the case of night migration have
objective and truly quantitative studies been made of horizontal
distribution. There is a possibility that our impressions of diurnal
migration are unduly influenced by the fact that the species accustomed
to flying in flocks are the ones that attract the most attention.
These conclusions relate to the uniformity of migration in terms of
short distances only, in the immediate vicinity of an observation
station. The extent to which they may be applied to broader fronts is a
question that may be more appropriately considered later, in connection
with continental aspects of the problem.
B. DENSITY AS FUNCTION OF THE HOUR OF THE NIGHT
There are few aspects of nocturnal migration about which there is less
understanding than the matter of when the night flight begins, at what
rate it progresses, and for what duration it continues. One would think,
however, that this aspect of the problem, above most others, would have
been thoroughly explored by some means of objective study. Yet, this is
not the case. Indeed, I find not a single paper in the American
literature wherein the subject is discussed, although some attention has
been given the matter by European ornithologists. Siivonen (1936)
recorded in Finland the frequency of call notes of night migrating
species of _Turdus_ and from these data plotted a time curve showing a
peak near midnight. Bergman (1941) and Putkonen (1942), also in Finland,
studied the night flights of certain ducks (_Clangula hyemalis_ and
_Oidemia fusca_ and _O. nigra_) and a goose (_Branta bernicla_) and
likewise demonstrated a peak near midnight. However, these studies were
made at northern latitudes and in seasons characterized by evenings of
long twilight, with complete darkness limited to a period of short
duration around midnight. Van Oordt (1943: 34) states that in many cases
migration lasts all night; yet, according to him, most European
investigators are of the opinion that, in general, only a part of the
night is used, that is, the evening and early morning hours. The
consensus of American ornithologists seems to be that migratory birds
begin their flights in twilight or soon thereafter and that they remain
on the wing until dawn. Where this idea has been challenged at all, the
implication seems to have been that the flights are sustained even
longer, often being a continuation far into the night of movements begun
in the daytime. The telescopic method fails to support either of these
latter concepts.
[Illustration: FIG. 26. Average hourly station densities in
spring of 1948. This curve represents the arithmetic mean
obtained by adding all the station densities for each hour,
regardless of date, and dividing the sum by the number of
sets of observations at that hour (CST).]
_The Time Pattern_
When the nightly curves of density at the various stations are plotted
as a function of time, a salient fact emerges--that the flow of birds is
in no instance sustained throughout the night. The majority of the
curves rise smoothly from near zero at the time of twilight to a single
peak and then decline more or less symmetrically to near the base line
before dawn. The high point is reached in or around the eleven to twelve
o'clock interval more often than at any other time.
[Illustration: FIG. 27. Hourly station densities plotted as
a percentage of peak. The curve is based only on those sets
of data where observations were continued long enough to
include the nightly peak. In each set of data the station
density for each hour has been expressed as a percentage of
the peak for the night at the station in question. All
percentages for the same hour on all dates have been averaged
to obtain the percentile value of the combined station
density at each hour (CST).]
Figure 26, representing the average hourly densities for all stations on
all nights of observation, demonstrates the over-all effect of these
tendencies. Here the highest density is reached in the hour before
midnight with indications of flights of great magnitude also in the hour
preceding and the hour following the peak interval. The curve ascends
somewhat more rapidly than it declines, which fact may or may not be
significant. Since there is a great disproportion in the total volume of
migration at different localities, the thought might be entertained that
a few high magnitude stations, such as Tampico and Progreso, have
imposed their own characteristics on the final graph. Fortunately, this
idea may be tested by subjecting the data to a second treatment. If
hourly densities are expressed as a percentage of the nightly peak, each
set of observations, regardless of the number of birds involved, carries
an equal weight in determining the character of the over-all curve.
Figure 27 shows that percentage analysis produces a curve almost
identical with the preceding one. To be sure, all of the individual
curves do not conform with the composite, either in shape or incidence
of peak. The extent of this departure in the latter respect is evident
from Figure 28, showing the number of individual nightly station curves
reaching a maximum peak in each hour interval. Even this graph
demonstrates that maximum densities near midnight represent the typical
condition.
[Illustration: FIG. 28. Incidence of maximum peak at the
various hours of the night in 1948. "Number of stations"
represents the total for all nights of the numbers of station
peaks falling within a given hour.]
The remarkable smoothness and consistency of the curves shown in Figures
26 and 27 seem to lead directly to the conclusion that the volume of
night migration varies as a function of time. Admittedly other factors
are potentially capable of influencing the number of birds passing a
given station in a given hour. Among these are weather conditions,
ecological patterns, and specific topographical features that might
conceivably serve as preferred avenues of flight. However, if any of
these considerations were alone responsible for changes in the numbers
of birds seen in successive intervals, the distribution of the peak in
time could be expected to be haphazard. For example, there is no reason
to suppose that the cone of observation would come to lie over favored
terrain at precisely the hour between eleven and twelve o'clock at so
many widely separated stations. Neither could the topographical
hypothesis explain the consistently ascending and descending pattern of
the ordinates in Figure 28. This is not to say that other factors are
without effect; they no doubt explain the divergencies in the time
pattern exhibited by Figure 28. Nevertheless, the underlying
circumstances are such that when many sets of data are merged these
other influences are subordinated to the rise and fall of an evident
time pattern.
Stated in concrete terms, the time frequencies shown in the graphs
suggest the following conclusions: first, nocturnal migrations are not a
continuation of daytime flights; second, nearly all night migrants come
to earth well before dawn; and, third, in each hour of the night up
until eleven or twelve o'clock there is typically a progressive increase
in the number of birds that have taken wing and in each of the hours
thereafter there is a gradual decrease. Taken at its face value, the
evidence seems to indicate that birds do not begin their night
migrations _en masse_ and remain on the wing until dawn and that in all
probability most of them utilize less than half of the night.
Interestingly enough, the fact that the plot points in Figure 26 lie
nearly in line tempts one to a further conclusion. The curve behaves as
an arithmetic progression, indicating that approximately the same number
of birds are leaving the ground in each hour interval up to a point and
that afterwards approximately the same number are descending within
each hour. However, some of the components making up this curve, as
later shown, are so aberrant in this regard that serious doubt is cast
on the validity of this generalization.
Because the results of these time studies are unexpected and
startling, I have sought to explore other alternative explanations and
none appears to be tenable. For example, the notion that the varying
flight speeds of birds might operate in some way to produce a
cumulative effect as the night progresses must be rejected on close
analysis. If birds of varying flight speeds are continuously and
evenly distributed in space, a continuous and even flow would result
all along their line of flight. If they are haphazardly distributed in
space, a correspondingly haphazard density pattern would be expected.
Another explanation might be sought in the purely mathematical effects
of the method itself. The computational procedure assumes that the
effective area of the sample is extremely large when the moon is low,
a condition that usually obtains in the early hours of the evening in
the days surrounding the full moon. Actually no tests have yet been
conducted to ascertain how far away a silhouette of a small bird can
be seen as it passes before the moon. Consequently, it is possible
that some birds are missed under these conditions and that the
effective field of visibility is considerably smaller than the
computed field of visibility. The tendency, therefore, may be to
minimize the densities in such situations more than is justified.
However, in many, if not most, cases, the plotting of the actual
number of birds seen, devoid of any mathematical procedures, results
in an ascending and descending curve.
[Illustration: FIG. 29. Various types of density-time curves.
(A) Near typical, Ottumwa, April 22-23; (B) random fluctuation,
Stillwater, April 23-24; (C) bimodal, Knoxville, April 22-23;
(D) sustained peak, Ottumwa, April 21-22; (E) early peak, Oak
Grove, May 21-22; (F) late peak, Memphis, April 23-24.]
A third hypothesis proposes that all birds take wing at nearly the
same time, gradually increase altitude until they reach the mid-point
of their night's journey, and then begin a similarly slow descent.
Since the field of observation of the telescope is conical, it is
assumed that the higher the birds arise into the sky the more they
increase their chances of being seen. According to this view, the
changes in the density curve represent changes in the opportunity to
see birds rather than an increase or decrease in the actual number of
migrants in the air. Although measurements of flight altitude at
various hours of the night have not been made in sufficient number to
subject this idea to direct test, it is hardly worthy of serious
consideration. The fallacy in the hypothesis is that the cone of
observation itself would be rising with the rising birds so that
actually the greatest proportion of birds flying would still be seen
when the field of observation is in the supine position of early
evening.
It cannot be too strongly emphasized that the over-all time curves
just discussed have been derived from a series of individual curves,
some of which differ radically from the composite pattern. In Figure
29, six dissimilar types are shown. This variation is not surprising
in view of the fact that many other causative factors aside from time
operate on the flow of birds from hour to hour. Figure 29A illustrates
how closely some individual patterns conform with the average. Figure
29B is an example of a random type of fluctuation with no pronounced
time character. It is an effect rarely observed, occurring only in the
cases where the number of birds observed is so small that pure chance
has a pronounced effect on the computed densities; its vacillations
are explicable on that account alone. Errors of sampling may similarly
account for some, though not all, of the curves of the bimodal type
shown in Figure 29C. Some variation in the curves might be ascribed to
the variations in kinds of species comprising the individual flights
at different times at different places, provided that it could be
demonstrated that different species of birds show dissimilar temporal
patterns. The other atypical patterns are not so easily dismissed and
will be the subject of inquiry in the discussions that follow. It is
significant that in spite of the variety of the curves depicted, which
represent every condition encountered, in not a single instance is the
density sustained at a high level throughout the night. Moreover,
these dissident patterns merge into a remarkably harmonious, almost
normal, average curve.
When, at some future date, suitable data are available, it would be
highly desirable to study the average monthly time patterns to
ascertain to what extent they may deviate from the over-all average.
At present this is not justifiable because there are not yet enough
sets of data in any two months representing the same selection of
stations.
_Correlations with Other Data_
It is especially interesting to note that the data pertaining to this
problem derived from other methods of inquiry fit the conclusions
adduced by the telescopic method. Overing (1938), who for several
years kept records of birds striking the Washington Monument, stated
that the record number of 576 individuals killed on the night of
September 12, 1937, all came down between 10:30 P. M. and midnight.
His report of the mortality on other nights fails to mention the time
factor, but I am recently informed by Frederick C. Lincoln (_in
litt._) that it is typical for birds to strike the monument in
greatest numbers between ten and twelve o'clock at night. At the
latter time the lights illuminating the shaft are extinguished, thus
resulting in few or no casualties after midnight. The recent report by
Spofford (1949) of over 300 birds killed or incapacitated at the
Nashville airport on the night of September 9-10, 1948, after flying
into the light beam from a ceilometer, is of interest in this
connection even though the cause of the fatality is shrouded in
mystery. It may be noted, however, that "most of the birds fell in the
first hour," which, according to the account, was between 12:30 A. M.
and 1:30 A. M. Furthermore, birds killed at the Empire State
Building in New York on the night of September 10-11, 1948, began to
strike the tower "shortly after midnight" (Pough, 1948). Also it will
be recalled that the observations of Stone (_loc. cit._), already
referred to in this paper (page 410), show a situation where the
flight in the early part of the night was negligible but mounted to a
peak between ten and eleven o'clock, with continuing activity at least
until midnight.
All of these observations are of significance in connection with the
conclusions herein advanced, but by far the most striking correlation
between these present results and other evidences is found in the
highly important work of various European investigators studying the
activity of caged migratory birds. This work was recently reviewed and
extended by Palmgren (1944) in the most comprehensive treatise on the
subject yet published. Palmgren recorded, by an electrically operated
apparatus, the seasonal, daily, and hourly activity patterns in caged
examples of two typical European migrants, _Turdus ericetorum
philomelas_ Brehm and _Erithacus rubecula_ (Linnaeus). Four rather
distinct seasonal phases in activity of the birds were discerned:
_winter non-migratory_, _spring migratory_, _summer non-migratory_,
and _autumn migratory_. The first of these is distinguished by morning
and evening maxima of activity, the latter being better developed but
the former being more prolonged. Toward the beginning of migration,
these two periods of activity decline somewhat. The second, or spring
migratory phase, which is of special interest in connection with the
present problem, is characterized by what Palmgren describes as
nightly migratory restlessness (_Zugunruhe_). The morning maximum,
when present, is weaker and the evening maximum often disappears
altogether. Although variations are described, the migratory
restlessness begins ordinarily after a period of sleep ("sleeping
pause") in the evening and reaches a maximum and declines before
midnight.
This pattern agrees closely with the rhythm of activity indicated by
the time curves emerging from the present research. Combining the two
studies, we may postulate that most migrants go to sleep for a period
following twilight, thereby accounting for the low densities in the
early part of the night. On awakening later, they begin to exhibit
migratory restlessness. The first hour finds a certain number of birds
sufficiently stimulated so that they rise forthwith into the air. In
the next hour still others respond to this urge and they too mount
into the air. This continues until the "restlessness" begins to abate,
after which fewer and fewer birds take wing. By this time, the birds
that began to fly early are commencing to descend, and since their
place is not being filled by others leaving the ground, the density
curve starts its decline. Farner (1947) has called attention to the
basic importance of the work by Palmgren and the many experimental
problems it suggests. Of particular interest would be studies
comparing the activity of caged American migrant species and the
nightly variations in the flight rates.
_The Baton Rouge Drop-off_
As already stated, the present study was initiated at Baton Rouge,
Louisiana, in 1945, and from the outset a very peculiar density time
pattern was manifest. I soon found that birds virtually disappeared
from the sky after midnight. Within an hour after the termination of
twilight, the density would start to ascend toward a peak which was
usually reached before ten o'clock, and then would begin, surprisingly
enough, a rapid decline, reaching a point where the migratory flow was
negligible. In Figure 30 the density curves are shown for five nights
that demonstrate this characteristically early decline in the volume
of migration at this station. Since, in the early stages of the work,
cooerdinates of apparent pathways of all the birds seen were not
recorded, I am unable now to ascertain the direction of flight and
thereby arrive at a density figure based on the dimension of the cone
and the length of the front presented to birds flying in certain
directions. It is feasible, nevertheless, to compute what I have
termed a "plus or minus" flight density figure stating the rate of
passage of birds in terms of the maximum and minimum corrections which
all possible directions of flight would impose. In other words,
density is here computed, first, as if all the birds were flying
perpendicular to the long axis of the ellipse, and, secondly, as if
all the birds were flying across the short axis of the ellipse. Since
the actual directions of flight were somewhere between these two
extremes, the "plus or minus" density figure is highly useful.
[Illustration: FIG. 30. Density-time curves on various nights
at Baton Rouge. (A) April 25, 1945; (B) April 15, 1946;
(C) May 10, 1946; (D) May 15, 1946; (E) April 22-23, 1948.
These curves are plotted on a "plus or minus" basis as
described in the text, with the bottom of the curve
representing the minimum density and the top of the curve
the maximum.]
The well-marked decline before midnight in the migration rates at
Baton Rouge may be regarded as one of the outstanding results emerging
from this study. Many years of ornithological investigation in this
general region failed to suggest even remotely that a situation of
this sort obtained. Now, in the light of this new fact, it is possible
for the first time to rationalize certain previously incongruous data.
Ornithologists in this area long have noted that local storms and
cold-front phenomena at night in spring sometimes precipitate great
numbers of birds, whereupon the woods are filled the following day
with migrants. On other occasions, sudden storms at night have
produced no visible results in terms of bird densities the following
day. For every situation such as described by Gates (1933) in which
hordes of birds were forced down at night by inclement weather, there
are just as many instances, even at the height of spring migration,
when similar weather conditions yielded no birds on the ground.
However, the explanation of these facts is simple; for we discover
that storms that produced birds occurred before midnight and those
that failed to produce birds occurred after that time (the storm
described by Gates occurred between 8:30 and 9:00 P. M.).
The early hour decline in density at Baton Rouge at first did not seem
surprising in view of the small amount of land area between this
station and the Gulf of Mexico. Since the majority of the birds
destined to pass Baton Rouge on a certain night come in general from
the area to the south of that place, and since the distances to
various points on the coast are slight, we inferred that a three-hour
flight from even the more remote points would probably take the bulk
of the birds northward past Baton Rouge. In short, the coastal plain
would be emptied well before midnight of its migrant bird life, or at
least that part of the population destined to migrate on any
particular night in question. Although data in quantity are not
available from stations on the coastal plain other than Baton Rouge,
it may be pointed out that such observations as we do have, from
Lafayette and New Orleans, Louisiana, and from Thomasville, Georgia,
are in agreement with this hypothesis.
A hundred and seventy miles northward in the Mississippi Valley, at
Oak Grove, Louisiana, a somewhat more normal density pattern is
manifested. There, in four nights of careful observation, a pronounced
early peak resulted on the night of May 21-22 (Figure 29E), but on the
other three nights significant densities held up until near twelve
o'clock, thereby demonstrating the probable effect of the increased
amount of land to the south of the station.
Subsequent studies, revealing the evident existence of an underlying
density time pattern, cast serious doubt on the explanations just
advanced of the early decline in the volume of migration at Baton
Rouge. It has as yet been impossible to reconcile the early drop-off
at this station with the idea that birds are still mounting into the
air at eleven o'clock, as is implied by the ideal time curves.
C. MIGRATION IN RELATION TO TOPOGRAPHY
To this point we have considered the horizontal distribution of birds
in the sky only on a very narrow scale and mainly in terms of the
chance element in observations. Various considerations have supported
the premise that the spread of nocturnal migration is rather even, at
least within restricted spacial limits and short intervals of time.
This means that in general the flow of birds from hour to hour at a
single station exhibits a smooth continuity. It does not mean that it
is a uniform flow in the sense that approximately the same numbers of
birds are passing at all hours, or at all localities, or even on all
one-mile fronts in the same locality. On the contrary, there is
evidence of a pronounced but orderly change through the night in the
intensity of the flight, corresponding to a basic and definitely timed
cycle of activity. Other influences may interfere with the direct
expression of this temporal rhythm as it is exhibited by observations
at a particular geographical location. Among these, as we have just
seen, is the disposition of the areas that offer suitable resting
places for transient birds and hence contribute directly and
immediately to the flight overhead. A second possible geographical
effect is linked with the question of the tendency of night migrants
to follow topographical features.
_General Aspects of the Topographical Problem_
That many diurnal migrants tend to fly along shorelines, rivers, and
mountain ridges is well known, but this fact provides no assurance
that night migrants do the same thing. Many of the obvious advantages
of specialized routes in daylight, such as feeding opportunities, the
lift provided by thermal updrafts, and the possible aid of certain
landmarks in navigation, assume less importance after night falls.
Therefore, it would not be safe to conclude that _all_ nocturnal
migrants operate as do _some_ diurnal migrants. For instance, the
passage of great numbers of certain species of birds along the Texas
coast in daylight hours cannot be regarded as certain proof that the
larger part of the nocturnal flight uses the same route. Neither can
we assume that birds follow the Mississippi River at night simply
because we frequently find migrants concentrated along its course in
the day. Fortunately we shall not need to speculate indefinitely on
this problem; for the telescopic method offers a means of study based
on what night migrants are doing _at night_. Two lines of attack may
be pursued. First we may compare flight densities obtained when the
field of the telescope lies over some outstanding topographical
feature, such as a river, with the recorded volume of flight when the
cone of observation is directed away from that feature. Secondly, we
may inquire how the major flight directions at a certain station are
oriented with respect to the terrain. If the flight is concentrated
along a river, for instance, the flight density curve should climb
upward as the cone of observation swings over the river, _regardless
of the hour at which it does so_. The effect should be most pronounced
if the observer were situated on the river bank, so that the cone
would eventually come to a position directly along the watercourse.
Though in that event birds coming up the river route would be flying
across the short axis of an elliptical section of the cone, the fact
that the whole field of observation would be in their path should
insure their being seen in maximum proportions. If, on the other hand,
the telescope were set up some distance away from the river so that
the cone merely moved _across_ its course, only a section of the
observation field would be interposed on the main flight lane.
The interaction of these possibilities with the activity rhythm should
have a variety of effects on the flight density curves. If the cone
comes to lie over the favored topographical feature in the hour of
greatest migrational activity, the results would be a simple sharp
peak of doubtful meaning. However, since the moon rises at a different
time each evening, the cone likewise would reach the immediate
vicinity of the terrain feature at a different time each night. As a
result, the terrain peak would move away from its position of
coincidence with the time peak on successive dates, producing first,
perhaps, a sustention of peak and later a definitely bimodal curve.
Since other hypotheses explain double peaks equally well, their mere
existence does not necessarily imply that migrants actually do travel
along narrow topographical lanes. Real proof requires that we
demonstrate a moving peak, based on properly corrected density
computations, corresponding always with the position of the cone over
the most favored terrain, and that the flight vectors be consistent
with the picture thus engendered.
_The Work of Winkenwerder_
To date, none of the evidence in favor of the topographical hypothesis
completely fills these requirements. Winkenwerder (_loc. cit._), in
analyzing the results of telescopic counts of birds at Madison and
Beloit, Wisconsin, Detroit and Ann Arbor, Michigan, and at Lake
Forest, Illinois, between 1898 and 1900, plotted the number of birds
seen at fifteen-minute intervals as a function of the time of the
night. He believed that the high points in the resulting frequency
histograms represented intervals when the field of the telescope was
moving over certain topographically determined flight lanes, though he
did not specify in all cases just what he assumed the critical
physiographic features to be. Especially convincing to him were
results obtained at Beloit, where the telescope was situated on the
east bank of the Rock River, on the south side of the city.
Immediately below Beloit the river turns southwestward and continues
in this direction about five miles before turning again to flow in a
southeastward course for approximately another five miles. In this
setting, on two consecutive nights of observation in May, the number
of birds observed increased tremendously in the 2 to 3 A. M. interval,
when, according to Winkenwerder's interpretation of the data (he did
not make the original observations at Beloit himself), the telescope
was pointing directly down the course of the river. This conclusion is
weakened, however, by notable inconsistencies. Since the moon rises
later each evening, it could not have reached the same position over
the Rock River at the same time on both May 12-13 and May 13-14, and
therefore, if the peaks in the graph were really due to a greater
volume of migration along the watercourse, they should not have so
nearly coincided. As a matter of fact the incidence of the peak on
May 12-13 should have preceded that of the peak on May 13-14; whereas
his figure shows the reverse to have been true. Singularly enough,
Winkenwerder recognized this difficulty in his treatment of the data
from Madison, Wisconsin. Unable to correlate the peak period with the
Madison terrain by the approach used for Beloit, he plotted the
observations in terms of hours after moonrise instead of standard
time. This procedure was entirely correct; the moon does reach
approximately the same position at each hour after its rise on
successive nights. The surprising thing is that Winkenwerder did not
seem to realize the incompatibility of his two approaches or to
realize that he was simply choosing the method to suit the desired
results.
Furthermore, as shown in Part I of this paper, the number of birds
seen through the telescope often has only an indirect connection with
the actual number of birds passing over. My computations reveal that
the highest counts of birds at Beloit on May 12-13 were recorded when
the moon was at an altitude of only 8 deg. to 15 deg. and, that when
appropriate allowance is made for the immense size of the field of
observation at this time, the partially corrected flight density for
the period is not materially greater than at some other intervals in
the night when the telescope was not directed over the course of the
Rock River. These allowances do not take the direction factor into
consideration. Had the birds been flying at right angles to the short
axis of an elliptical section of the cone throughout the night, the
flight density in the period Winkenwerder considered the peak would
have been about twice as high as in any previous interval. On the
other hand, if they had been flying across the long axis at all times,
the supposed peak would be decidedly inferior to the flight density at
10 to 11:00 P. M., before the cone came near the river.
Admittedly, these considerations contain a tremendous element of
uncertainty. They are of value only because they expose the equal
uncertainty in Winkenwerder's basic evidence. Since the cooerdinates of
the birds' apparent pathways at Beloit were given, I at first
entertained the hope of computing the flight densities rigorously, by
the method herein employed. Unfortunately, Winkenwerder was apparently
dealing with telescopes that gave inverted images, and he used a
system for recording cooerdinates so ambiguously described that I am
not certain I have deciphered its true meaning. When, however, his
birds are plotted according to the instructions as he stated them, the
prevailing direction of flight indicated by the projection formula
falls close to west-northwest, not along the course of the Rock River,
but _at direct right angles to it_.
[Illustration: FIG. 31. Directional components in the flight
at Tampico on three nights in 1948. The lengths of the
sector vectors are determined by their respective densities
expressed as a percentage of the station density for that
night; the vector resultants are plotted from them by
standard procedure. Thus, the nightly diagrams are not on the
same scale with respect to the actual number of birds involved.]
[Illustration: FIG. 32. Hourly station density curve at
Tampico, Tamaulipas, on the night of April 21-22, 1948 (CST).]
_Interpretation of Recent Data_
I am in a position to establish more exact correlations between flight
density and terrain features in the case of current sets of
observations. Some of these data seem at first glance to fit the idea
of narrow topographically-oriented flight lanes rather nicely. At
Tampico, where six excellent sets of observations were made in March
and April, 1948, the telescope was set up on the beach within a few
yards of the Gulf of Mexico. As can be seen from Figure 25 (_ante_),
the slant of the coastline at this point is definitely west of north,
as is also the general trend of the entire coast from southern
Veracruz to southern Tamaulipas (see Figure 34, beyond). The over-all
vector resultant of all bird flights at this station was N 11 deg. W, and,
as will be seen from Figure 31, none of the nightly vector resultants
in April deviates more than one degree from this average. Thus the
prevailing direction of flight, as computed, agrees with the trend of
the coast at the precise point of the observations, at least to the
extent that both are west of north. To be sure, the individual sector
vectors indicate that not all birds were following this course;
indeed, some appear to have been flying east of north, heading for a
landfall in the region of Brownsville, Texas, and a very few to have
been traveling northeastward toward the central Gulf coast. But it
must be remembered that a certain amount of computational deviation
and of localized zigzagging in flight must be anticipated. Perhaps
none of these eastward vectors represents an actual extended flight
path. The nightly vector resultants, on the other hand, are so
consistent that they have the appearance of remarkable accuracy and
tempt one to draw close correlations with the terrain. When this is
done, it is found that, while the prevailing flight direction is 11 deg.
west of north, the exact slant of the coastline at the location of the
station is about 30 deg. west of north, a difference of around 19 deg.. It
appears, therefore, that the birds were not following the shoreline
precisely but cutting a chord about ten miles long across an
indentation of the coast. If it be argued that the method of
calculation is not accurate enough to make a 19 deg. difference
significant, and that most of the birds might have been traveling
along the beach after all, it can be pointed out with equal
justification that, if this be so, the 11 deg. divergence from north does
not mean anything either and that perhaps the majority of the birds
were going due north. We are obliged to conclude either that the main
avenue of flight paralleled the disposition of the major topographical
features only in a general way or that the angle between the line of
the coast and true north is not great enough to warrant any inference
at all.
Consideration of the Tampico density curves leads to similarly
ambiguous results. On the night of April 21-22, as is evident from a
comparison of Figures 25 and 32, the highest flight density occurred
when the projection of the cone on the terrain was wholly included
within the beach. This is very nearly the case on the night of April
23-24 also, the positions of the cone during the peak period of
density being only about 16 deg. apart. (On the intervening date, clouds
prevented continuous observation during the critical part of the
night.) These correlations would seem to be good evidence that most of
these night migrants were following the coastline of the Gulf of
Mexico. However, the problem is much more complicated. The estimated
point of maximum flight density fell at 10:45 P. M. on April 21-22
and 11:00 P. M. on April 23-24, both less than an hour from the peak
in the ideal time curve (Figure 26, _ante_). We cannot be sure,
therefore, that the increase in density coinciding with the position
of the moon over the beach is not an increase which would have
occurred anyway. Observations conducted several nights before or after
the second quarter, when the moon is not on or near its zenith at the
time of the predictable peak in the density curve, would be of
considerable value in the study of this particular problem.
The situation at Tampico has been dealt with at length because, among
all the locations for which data are available, it is the one that
most strongly supports the topographical hypothesis. In none of the
other cases have I been able to find a definite relation between the
direction of migration and the features of the terrain. Studies of
data from some of these stations disclose directional patterns that
vary from night to night only slightly more than does the flight at
Tampico. In three nights of observation at Lawrence, Kansas, marked by
very high densities, the directional trend was north by
north-northeast with a variation of less than 8 deg., yet Lawrence is so
situated that there seems to be no feature of the landscape locally or
in the whole of eastern Kansas or of western Missouri that coincides
with this heading. At Mansfield, Louisiana, in twelve nights of
observation, the strong east by northeast trend varied less than 15 deg.,
but again there appears to be no correlation over a wide area between
this direction and any landmarks. And, at Progreso, Yucatan, where the
vector resultants were 21 deg. and 27 deg. on successive nights, most of the
birds seen had left the land and were beginning their flight northward
over the trackless waters of the Gulf of Mexico. Furthermore, as I
have elsewhere pointed out (1946: 205), the whole northern part of the
Yucatan Peninsula itself is a flat terrain, unmarked by rivers,
mountains, or any other strong physiographic features that conceivably
might be followed by birds.
[Illustration: FIG. 33. The nightly net trend of migrations
at three stations in 1948. Each arrow is the vector resultant
for a particular night, its length expressing the nightly
density as a percentage of the total station density for the
nights represented. Thus, the various station diagrams are
not to the same scale.]
In Figure 33 I have shown the directional patterns at certain stations
where, unlike the cases noted above, there is considerable change on
successive nights. Each vector shown is the vector resultant for one
particular night. The lengths of the vectors have been determined by
their respective percentages of the total computed density, or total
station magnitude, for all the nights in question. In other words, the
lengths of the individual vectors denote the percentile role that each
night played in the total density. From the directional spread at
these stations it becomes apparent that if most of the birds were
traveling along a certain topographic feature on one night, they
could not have been traveling along the same feature on other nights.
The possibility should be borne in mind, however, that there may be
more than one potential topographic feature for birds to follow at
some stations. Moreover, it is conceivable that certain species might
follow one feature that would lead them in the direction of their
ultimate goal, whereas other species, wishing to go in an entirely
different direction, might follow another feature that would lead them
toward their respective destination. It would seem unlikely, however,
that the species composition of the nocturnal flights would change
materially from night to night, although there is a strong likelihood
that it might do so from week to week and certainly from month to
month.
By amassing such data as records of flight direction along the same
coast from points where the local slant of the shoreline is materially
different, and comparisons of the volume of migration at night along
specialized routes favored during the day with the flight densities at
progressive distances from the critical terrain feature involved, we
shall eventually be able to decide definitely the role topography
plays in bird migration. We cannot say on the basis of the present
ambiguous evidence that it is not a factor in determining which way
birds fly, but, if I had to hazard a guess one way or the other, I
would be inclined to discount the likelihood of its proving a major
factor.
D. GEOGRAPHICAL FACTORS AND THE CONTINENTAL DENSITY PATTERN
A study of the total nightly or seasonal densities at the various
stations brings forth some extremely interesting factors, many of
which, however, cannot be fully interpreted at this time. A complete
picture of the magnitude of migration at a given station cannot be
obtained from the number of birds that pass the station on only a few
nights in one spring. Many years of study may be required before hard
and fast principles are justifiable. Nevertheless, certain salient
features stand out in the continental density pattern in the spring of
1948. (The general results are summarized in Tables 2-5; the location
of the stations is shown in Figure 34.) These features will be
discussed now on a geographical basis.
TABLE 2.--Extent of Observations and Seasonal Station
Densities at Major Stations in 1948
==========================================================================
|Nights of observation| Hours of observation|
OBSERVATION STATION |---------------------+---------------------|Season
|March|April|May|Total|March|April|May|Total|density
---------------------+-----+-----+---+-----+-----+-----+---+-----+--------
CANADA | | | | | | | | |
Pt. Pelee | | | 1 | 1 | | | 6 | 6 | 2,500
| | | | | | | | |
MEXICO | | | | | | | | |
S. L. P.: Ebano | 1 | | | 1 | 3 | | | 3 | 1,300
Tamps.: Tampico | 3 | 3 | | 6 | 20 | 20 | | 40 | 140,300
Yuc.: Progreso | | 3 | | 3 | | 18 | | 18 | 60,500
| | | | | | | | |
UNITED STATES | | | | | | | | |
Fla.: Pensacola | | 2 | 2 | 4 | | 8 | 7 | 15 | 1,500
Winter Park | | 5 | 6 | 11 | | 39 |38 | 77 | 21,700
Ga.: Athens | | 2 | | 2 | | 10 | | 10 | 4,000
Thomasville | | 1 | 1 | 2 | | 8 | 8 | 16 | 4,700
Iowa: Ottumwa | | 5 | 5 | 10 | | 16 |28 | 44 | 134,400
Kans.: Lawrence | 2 | 1 | | 3 | 16 | 4 | | 20 | 68,700
Ky.: Louisville | | 3 | 2 | 5 | | 20 |14 | 34 | 49,300
Murray | | 2 | | 2 | | 13 | | 13 | 26,200
La.: Baton Rouge | | 3 | | 3 | | 15 | | 15 | 11,000
Lafayette | | 1 | | 1 | | 5 | | 5 | 2,800
Mansfield | 1 | 5 | 4 | 10 | 2 | 16 |22 | 40 | 22,400
New Orleans | 1 | 1 | | 2 | 5 | 2 | | 7 | 1,900
Oak Grove | | 2 | 2 | 4 | | 16 |15 | 31 | 33,900
Mich.: Albion | | 1 | | 1 | | 3 | | 3 | 1,100
Minn.: Hopkins | | | 1 | 1 | | | 4 | 4 | 2,000
Miss.: Rosedale | | 1 | 1 | 2 | | 6 | 8 | 14 | 12,600
Mo.: Columbia | | 2 | 1 | 3 | | 8 | 6 | 14 | 13,100
Liberty | | 1 | 1 | 2 | | 7 | 7 | 14 | 4,800
Okla.: Stillwater | 1 | 2 | 1 | 4 | 5 | 11 | 3 | 19 | 8,400
S. Car.: Charleston| 1 | 1 | 1 | 3 | 5 | 8 | 9 | 22 | 3,000
Tenn.: Knoxville | | 2 | 2 | 4 | | 18 |14 | 32 | 35,400
Memphis | 2 | 3 | 2 | 7 | 13 | 20 |12 | 45 | 29,700
Tex.: College | | 3 | 1 | 4 | | 19 | 8 | 27 | 32,200
Station Rockport | | 1 | | 1 | | 4 | | 4 | 6,200
--------------------------------------------------------------------------
TABLE 3.--Average Hourly Station Densities in 1948
========================================================
OBSERVATION STATION | March | April | May | Season
------------------------+-------+-------+-------+-------
CANADA | | | |
Pt. Pelee | | | 400 | 400
| | | |
MEXICO | | | |
S. L. P.: Ebano | 400 | | | 400
Tamps.: Tampico | 700 | 6,300 | | 3,500
Yuc.: Progreso | | 2,800 | | 2,800
| | | |
UNITED STATES | | | |
Fla.: Pensacola | | 0+| 200 | 100
Winter Park | | 300 | 200 | 300
Ga.: Athens | | 400 | | 400
Thomasville | | 500 | 100 | 300
Iowa: Ottumwa | | 1,700 | 3,800 | 3,100
Kans.: Lawrence | 4,000 | 1,400 | | 3,400
Ky.: Louisville | | 2,000 | 700 | 1,500
Murray | | 2,000 | | 2,000
La.: Baton Rouge | | 700 | | 700
Lafayette | | 600 | | 600
Mansfield | 0 | 700 | 800 | 600
New Orleans | 60 | 800 | | 300
Oak Grove | | 1,400 | 800 | 1,100
Mich.: Albion | | 400 | | 400
Minn.: Hopkins | | | 500 | 500
Miss.: Rosedale | | 1,100 | 700 | 900
Mo.: Columbia | | 400 | 1,700 | 900
Liberty | | 500 | 200 | 300
Okla.: Stillwater | 500 | 200 | 1,000 | 400
S. Car.: Charleston | 200 | 200 | 0+| 100
Tenn.: Knoxville | | 1,300 | 800 | 1,100
Memphis | 300 | 800 | 900 | 700
Tex.: College Station | | 1,100 | 1,500 | 1,200
Rockport | | 1,600 | | 1,600
--------------------------------------------------------
TABLE 4.--Maximum Hourly Station Densities in 1948
======================================================
OBSERVATION STATION | March | April | May
------------------------+---------+---------+---------
CANADA | | |
Pt. Pelee | | | 1,400
| | |
MEXICO | | |
S. L. P.: Ebano | 600 | |
Tamps.: Tampico | 3,100 | 21,200 |
Yuc.: Progreso | | 11,900 |
| | |
UNITED STATES | | |
Fla.: Pensacola | | 100 | 700
Winter Park | | 2,300 | 1,000
Ga.: Athens | | 900 |
Thomasville | | 1,500 | 200
Iowa: Ottumwa | | 3,800 | 12,500
Kans.: Lawrence | 14,500 | 2,200 |
Ky.: Louisville | | 5,000 | 1,400
Murray | | 3,700 |
La.: Baton Rouge | | 3,400 |
Lafayette | | 1,800 |
Mansfield | | 2,100 | 1,600
New Orleans | 200 | 1,100 |
Oak Grove | | 2,700 | 2,500
Mich.: Albion | | 700 |
Minn.: Hopkins | | | 1,100
Miss.: Rosedale | | 2,200 | 1,400
Mo.: Columbia | | 800 | 3,400
Liberty | | 800 | 800
Okla.: Stillwater | 900 | 700 | 1,400
S. Car.: Charleston | 400 | 600 | 200
Tenn.: Knoxville | | 5,800 | 1,900
Memphis | 1,200 | 3,400 | 2,100
Tex.: College Station | | 3,400 | 3,100
Rockport | | 2,400 |
------------------------------------------------------
TABLE 5.--Maximum Nightly Densities at Stations with More
Than One Night of Observation
======================================================
OBSERVATION STATION | March | April | May
------------------------+---------+---------+---------
| | |
MEXICO | | |
Tamps.: Tampico | 5,500 | 63,600 |
Yuc.: Progreso | | 31,600 |
| | |
UNITED STATES | | |
Fla.: Winter Park | | 6,200 |
Ga.: Athens | | 2,600 |
Thomasville | | 3,900 |
Iowa: Ottumwa | | 15,300 | 54,600
Kans.: Lawrence | 51,600 | 5,400 |
Ky.: Louisville | | 17,000 | 8,400
Murray | | 16,400 |
La.: Baton Rouge | | 6,200 |
Mansfield | | 4,900 | 5,200
Oak Grove | | 13,600 | 5,800
Miss.: Rosedale | | 6,800 | 5,800
Mo.: Columbia | | 1,400 | 10,300
Okla.: Stillwater | 2,700 | 1,900 | 3,000
Tenn.: Knoxville | | 15,200 | 9,000
Memphis | 3,600 | 7,900 | 7,000
Tex.: College Station | | 6,200 | 13,200
------------------------------------------------------
[Illustration: FIG. 34. Stations at which telescopic
observations were made in 1948.]
_Gulf Migration: A Review of the Problem_
In view of the controversy in recent years pertaining to migration
routes in the region of the Gulf of Mexico (Williams, 1945 and 1947;
Lowery, 1945 and 1946), the bearing of the new data on the problem is
of especial interest. While recent investigations have lent further
support to many of the ideas expressed in my previous papers on the
subject, they have suggested alternative explanations in the case of
others. In the three years that have elapsed since my last paper
dealing with Gulf migration, some confusion seems to have arisen
regarding the concepts therein set forth. Therefore, I shall briefly
re-state them.
It was my opinion that evidence then available proved conclusively
that birds traverse the Gulf frequently and intentionally; that the
same evidence suggested trans-Gulf flights of sufficient magnitude to
come within the meaning of migration; that great numbers of birds move
overland around the eastern and western edges of the Gulf; that it was
too early to say whether the coastal or trans-Gulf route was the more
important, but that enough birds cross the water from Yucatan to
account for transient migration in the extreme lower Mississippi
Valley; and, that, in fair weather, most trans-Gulf migrants continue
on inland for some distance before coming to land, creating an area of
"hiatus" that is usually devoid of transient species. I tried to make
it emphatically clear that I realized that many birds come into Texas
from Mexico overland, that I did not think the hordes of migrants
normally seen on the Texas coast in spring were by any means all
trans-Gulf migrants. I stated (1946: 206): "Proving that birds migrate
in numbers across the Gulf does not prove that others do not make the
journey by the coastal routes. But that is exactly the point. No one
has ever pretended that it does." Although some ornithologists seem to
have gained the impression that I endorse only the trans-Gulf route,
this is far from the truth. I have long held that the migrations
overland through eastern Mexico and southern Texas on one hand, and
the over-water flights on the other, are each part of the broad
movement of transients northward into the United States. There are
three avenues of approach by which birds making up the tremendous
concentrations on the Texas coast may have reached there: by a
continental pathway from a wintering ground in eastern and southern
Mexico; by the over-water route from Yucatan and points to the
southward; and, finally, by an overland route from Central America via
the western edge of the Gulf. As a result of Louisiana State
University's four-year study of the avifauna in eastern Mexico, I
know that migrants reach Texas from the first source. As a consequence
of my studies in Yucatan of nocturnal flight densities and their
directional trends, I strongly believe that migrants reach Texas from
this second source. As for the third source, I have never expressed an
opinion. I am not prepared to do so now, for the reason that today, as
three years ago, there is no dependable evidence on which to base a
judgment one way or another.
TABLE 6.--Computed Hourly Densities at Tampico, Tamps.,
in Spring of 1948
=========================================================================
| Average hour of observation
DATE |-----+------+-------+-------+------+------+------+------+----
| 8:30| 9:30 | 10:30 | 11:30 |12:30 | 1:30 | 2:30 | 3:30 |4:30
-----------|-----+------+-------+-------+------+------+------+------+----
22-23 March| 600| 700 | 1,000 | 800 | 100 | 100 | 0 | 100 | ..
23-24 March| 0| 400 | 1,200 | 3,100 | 800 | .. | .. | .. | ..
24-25 March| 300| 700 | 800 | 1,600 |1,100 | .. | .. | .. | ..
21-22 April|1,100|7,000 |14,900 |12,900 |8,100 |3,800 |3,500 | 200 | ..
22-23 April| 700|2,900 | 7,500 | .. | .. | .. | .. | .. | ..
23-24 April| 600|4,700 |19,100 |21,200 |5,500 |5,900 |4,000 |2,000 |200
-------------------------------------------------------------------------
_Western Gulf Area_
Among the present flight density data bearing on the above issues, are
the six sets of observations from the vicinity of Tampico, Tamaulipas,
already referred to. These were secured in the spring of 1948 by a
telescope set up on the Gulf beach just north of the Miramar pavilion
and only a hundred feet from the surf (see Figure 25, _ante_). The
beach here is approximately 400 feet wide and is backed by
scrub-covered dunes, which rapidly give way toward the west to a
rather dense growth of low shrubs and trees. One might have expected
that station densities at Tampico in March would be rather high.
Actually, though they are the second highest recorded for the month,
they are not impressive and afford a striking contrast with the record
flights there in April (Table 6). Unfortunately, only a few stations
were operating in March and thus adequate comparisons are impossible;
but the indications are that, in March, migration activity on the
western edges of the Gulf is slight. It fails even to approach the
volume that may be observed elsewhere at the same time, as for
example, in eastern Kansas where, however, the migration is not
necessarily correlated with the migration in the lower Gulf area.
Strangely enough, on the night of March 22-23, at Tampico,
approximately 85 per cent of the birds were flying from north of an
east-west line to south of it, opposite to the normal trend of spring
migration. This phenomenon, inexplicable in the present instance, will
be discussed below. On the other two nights in March, the directional
trend at Tampico was northward with few or no aberrant components.
Observations made approximately thirty-five miles inland from the
Gulf, at Ebano, San Luis Potosi, on the night of March 25-26, show
lower station densities than the poorest night at Tampico, but since
they cover only a three-hour watch, they reveal little or nothing
concerning the breadth of the so-called coastal flyway.
April flight densities at Tampico are the highest recorded in the
course of this study. The maximum hourly density of 21,200 birds is 46
per cent higher than the maximum hourly density anywhere else. The
average hourly density of 6,300 in April is more than twice as great
as the next highest average for that month. These figures would seem
to satisfy certain hypotheses regarding a coastwise flight of birds
around the western edge of the Gulf. Other aspects of the observations
made at that time do not satisfy these hypotheses. Texas
ornithologists have found that in periods of heavy spring migration,
great numbers of birds are invariably precipitated by rainy weather.
On April 23, in the midst of the record-breaking telescopic studies at
Tampico, Mr. Robert J. Newman made a daytime census immediately
following four hours of rain. He made an intensive search of a small
area of brush and low growth back of the beach for traces of North
American migrants. In his best hour, only thirteen individual birds
out of seventy-five seen were of species that do not breed there. The
transient species were the Ruby-throated Hummingbird (1),
Scissor-tailed Flycatcher (1), Western Wood Pewee (1), Black-throated
Green Warbler (2) Orchard Oriole (7), and Baltimore Oriole (1), all of
which winter extensively in southern Mexico. Perhaps, however, the
apparent scarcity of transients on this occasion is not surprising in
the light of the analysis of flight density in terms of bird density
on the ground which I shall develop beyond. My only point here is to
demonstrate that rain along the coast does not always produce birds.
As large as the nocturnal flights at Tampico have so far proved to be,
they are not commensurate with the idea that nearly all birds follow a
narrow coastwise route around the Gulf. To establish the latter idea,
one must be prepared to show that the migrant species returning to the
United States pass along two flyways a few miles wide in the immense
volume necessary to account for their later abundance on a 1500-mile
front extending across eastern North America. One might expect at
least ten to twenty fold the number observable at any point in the
interior of the United States. In actuality, the highest nightly
density of 63,600 birds at Tampico is barely sufficient to account for
the highest nightly density of 54,600 at Ottumwa, Iowa, alone.
Of course, there is no way of knowing how closely a ratio of anywhere
from ten to one through twenty to one, employed in this comparison,
expresses the true situation. It may be too high. It could be too low,
particularly considering that preliminary studies of flight density in
Florida indicate that the western shores of the Gulf of Mexico must
carry the major part of the traffic if migratory flights back to the
United States in spring take place only along coastwise routes.
Consideration of the data obtained in Florida in 1948 will serve to
emphasize the point.
_Eastern Gulf Area_