telescope in April and May. This was 71 per cent more hours of actual
observation than at the next highest station. Nevertheless, the total
seasonal density amounted to only 21,700 birds. The average hourly
density was only 300 birds, with the maximum for any one hour being
2,300 birds. In contrast, forty-five hours of observation at Tampico,
Tamaulipas, in March and April, yielded a total station density of
140,300 birds. At the latter place, on the night of April 23-24,
almost as many birds passed _in a single hour_ as passed Winter Park
in all of its seventy-seven hours of observation.
Should future telescopic studies at Florida stations fail to produce
densities appreciably higher than did Winter Park in 1948, the
currently-held ideas that the Florida Peninsula is a major flyway will
be seriously shaken. But one consideration must be kept in mind
regarding the present picture. No observations were made at Winter
Park in March, when it is conceivable that densities may have been
materially higher. We know, for instance, that many of the early
migrants to the southern United States are species whose winter homes
are in the West Indies. Numbers of Vireonidae and Parulidae (notably
the genera _Vireo_, _Parula_, _Protonotaria_, _Mniotilta_, _Seiurus_,
_Geothlypis_, _Setophaga_, and certain _Dendroica_ and _Vermivora_)
winter extensively in this region and are among the first birds to
return to the southern states in the spring. Many of them often reach
Louisiana and other states on the Gulf coastal plain by mid-March. In
the same connection, it may be mentioned that many of the outstanding
instances of birds striking lighthouses in southern Florida occurred
in March and early April (Howell, 1932).
_Yucatan Area_
I have long felt that the answers to many of the questions which beset
us in our study of Gulf migration are to be found on the open waters
of the Gulf of Mexico itself or on the northern tip of the Yucatan
Peninsula. Accordingly, in the spring of 1945 I crossed the Gulf by
slow freighter for the purpose of determining how many and what kinds
of birds might be seen between the mouth of the Mississippi River and
the Yucatan Peninsula in fair weather, when it could not be argued
that the birds had been blown there by inclement weather. To my own
observations I was able to add those of other ornithologists who
likewise had been aboard ship in the Gulf.
The summary of results proved that birds of many species cross the
Gulf and do so frequently. It failed to demonstrate beyond all doubt
that they do so in large numbers. Nor had I expected it to do so. The
consensus of Gulf coast ornithologists seemed to be that transient
migration in their respective regions is often performed at too high
an elevation to be detected unless the birds are forced to earth by
bad weather. I saw no reason to anticipate that the results would be
otherwise over the waters of the Gulf of Mexico.
The application of the telescopic method held promise of supplying
definite data on the numbers of trans-Gulf migrants, however high
their flight levels. The roll and vibration of the ship had prevented
me in 1945 from making telescopic observations at sea. Since no
immediate solution to the technical difficulties involved presented
itself, I undertook to reach one of the small cays in Alacran Reef,
lying seventy-five miles north of Yucatan and in line with the coast
of southern Louisiana. Because of transportation difficulties, my
plans to place a telescopic station in this strategic location failed.
Consequently, I returned in 1948 by freighter to Progreso, Yucatan,
where telescopic counts were made for three nights, one of which was
rendered almost valueless by the cloud cover.
[Illustration: FIG. 35. Positions of the cone of
observation at Progreso, Yucatan, on the night of April
23-24, 1948, from 8:53 P. M. to 3:53 A. M. Essential
features of this map are drawn to scale. The telescope was
set up on the end of a one-mile long wharf that extends
northward from the shore over the waters of the Gulf of
Mexico. The triangular (white) lines represent the
projections of the cone of visibility on the earth at the
mid-point of each hour of observation. Only briefly, in the
first two hours, did the cone lie even in part over the
adjacent mainland. Hence, nearly all of the birds seen in the
course of the night had actually left the land behind.]
The observation station at Progreso was situated on the northern
end of the new wharf which projects northward from the beach to
a point one mile over the Gulf. As will be seen from Figure 35, the
entire cone of observation lay at nearly all times over the intervening
water between the telescope on the end of the wharf and the
beach. Therefore, nearly all of the birds seen were actually observed
leaving the coast and passing out over the open waters of the
Gulf. The hourly station densities are shown in Table 7 and Figures
24 and 36. In the seventeen hours of observation on the nights of
April 23-24 and April 24-25, a total computed density of 59,200 birds
passed within one-half mile of each side of Progreso. This is the
third highest density recorded in the course of this study. The
maximum for one hour was a computed density of 11,900 birds. This
is the fourth highest hourly density recorded in 1948.
[Illustration: FIG. 36. Hourly station density curve for
night of April 23-24, 1948, at Progreso, Yucatan.]
TABLE 7.--Computed Hourly Densities at Progreso, Yuc.,
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
-----------+-----+------+------+-------+------+------+------+-----+-----
23-24 April| 400 |3,000 |5,100 |10,000 |9,000 |2,800 | 900 | 400 |....
24-25 April| 0 | 500 |3,700 |11,900 |7,900 |1,900 |1,100 | 400 | 200
-----------+-----+------+------+-------+------+------+------+-----+-----
It is not my contention that this many birds leave the northern coast
of Yucatan every night in spring. Indeed, further studies may show
negligible flight densities on some nights and even greater densities
on others. As a matter of fact several hours of observation on the
night of April 25-26, at Merida, Yucatan, approximately twenty-five
miles inland from Progreso, indicated that on this night the density
overhead was notably low, a condition possibly accounted for by a
north wind of 10 mph blowing at 2,000 feet. I merely submit that on
the nights of April 23-24 and 24-25, birds were leaving the coast of
Yucatan _at Progreso_ at the rate indicated. But, as I have emphasized
in this paper and elsewhere (1946: 205-206), the northern part of the
Yucatan Peninsula is notably unmarked by streams or any other
physiographic features which birds might follow. The uniformity of the
topography for many miles on either side of Progreso, if not indeed
for the entire breadth of the Peninsula, makes it probable that
Progreso is not a particularly favored spot for observing migration,
and that it is not the only point along the northern coast of Yucatan
where high flight densities can be recorded. This probability must be
considered when comparisons are made between Progreso densities and
those at Tampico. The argument could be advanced that the present
densities from Tampico do not sufficiently exceed those at Progreso to
establish the coastal route as the main avenue of traffic in spring,
since there is every reason to suspect topography of exerting some
influence to produce a channeling effect in eastern Mexico. Here the
coast parallels the directional trend of the migratory movement for
more than 600 miles. Likewise the Sierra Madre Oriental of eastern
Mexico, situated approximately 100 miles inland (sometimes less), lies
roughly parallel to the coast. Because of the slant of the Mexican
land mass, many winter residents in southern Mexico, by short
northward movements, would sooner or later filter into the coastal
plain. Once birds are shunted into this lowland area, it would seem
unlikely that they would again ascend to the top of the Sierra Madre
to the west. In this way the great north-south cordillera of mountains
may act as a western barrier to the horizontal dispersion of
transients bound for eastern North America. Similarly, the Gulf itself
may serve as an eastern barrier; for, as long as migrants may progress
northward in the seasonal direction of migration and still remain over
land, I believe they would do so.
To put the matter in a slightly different way, the idea of a very
narrow flight lane is inherent in the idea of coastwise migration.
For, as soon as we begin to visualize flights of great volume over
fronts extending back more than fifty miles from the shore line, we
are approaching, if indeed we have not already passed, the point where
the phenomenon is no longer coastwise in essence, but merely overland
(as indeed my own unprocessed, telescopic data for 1949 indicate may
be the case). In actuality, those who have reported on the migration
along the western edge of the Gulf of Mexico have never estimated the
width of the main flight at more than fifty miles and have intimated
that under some circumstances it may be as narrow as two miles. No
evidence of such restrictions can be discerned in the case of the
trans-Gulf flights. If it cannot be said that they may be assumed to
be as wide as the Gulf itself, they at least have the potential
breadth of the whole 260-mile northern coast of the Yucatan Peninsula.
On these premises, to be merely equal in total magnitude, the
coastwise flights must exhibit, depending on the particular situation,
from five to 130 times the concentrations observable among trans-Gulf
migrants. This point seems almost too elementary to mention, but I
have yet to find anyone who, in comparing the two situations, takes it
into consideration.
Judged in this light, the average hourly density of 2,800 birds at
Progreso in April would appear to be indicative of many more migrants
on the entire potential front than the 6,300 birds representing the
average hourly density for the same month at Tampico.
That the Progreso birds were actually beginning a trans-Gulf flight
seems inevitable. The Yucatan Peninsula projects 200 miles or more
northward into the vast open expanses of the Gulf of Mexico and the
Caribbean Sea, with wide stretches of water on either side. The great
majority of the birds were observed _after_ they had proceeded beyond
the northern edge of this land mass. Had they later veered either to
the east or the west, they would have been obliged to travel several
hundred miles before again reaching land, almost as far as the
distance straight across the Gulf. Had they turned southward, some
individuals should have been detected flying in that direction. As can
be seen from Figures 23, 42, and 44, not one bird observed was heading
south of east or south of west on either night. No other single piece
of evidence so conclusively demonstrates that birds cross the Gulf of
Mexico in spring in considerable numbers as do flight density data
recorded from Progreso in 1948.
_Northern Gulf Area_
Unfortunately only a few data on flight density are available from
critical localities on the northern shores of the Gulf in spring. As
the density curves in Figure 30 demonstrate, several sets of
observation, including some phenomenal flights, have been recorded at
Baton Rouge. This locality, however, lies sixty-four miles from the
closest point on the Gulf coast, and the point due southward on the
coast is eighty-four miles distant. Since all of the birds seen at
Baton Rouge on any one night may have come from the heavily forested
area between Baton Rouge and the coast of the Gulf, we cannot use data
from Baton Rouge as certainly representative of incoming trans-Gulf
flights. Data from repeated observations at stations on the coast
itself are needed to judge the degree of trans-Gulf migration
northward. On the few nights of observation at such localities
(Cameron and Grand Isle, Louisiana, and Pensacola, Florida), flight
densities have been zero or negligible. To be sure, negative results
have been obtained at stations in the interior of the United States,
and flights of low density have been recorded on occasion at stations
where the flight densities are otherwise high. Nevertheless, in view
of the volume of migration departing from Progreso, Yucatan, it would
appear, upon first consideration, that we should at times record on
the coast of Louisiana enough birds arriving in a night of continuous
observation to yield a high density figure.
Upon further consideration, however, there are factors mitigating
against heavy densities of birds in northern flight on the northern
coast of the Gulf. In the first place, presuming the main trans-Gulf
flight to originate from northern Yucatan, and that there is a
directional fanning to the northward, the birds leave on a 260-mile
front, and arrive on a front 400 miles or more wide. Consequently,
other factors remaining the same, there would be only approximately
half the number of birds on the coast of arrival, at a given time and
place, as there was on the coast of departure. Secondly, we may now
presume on the basis of the telescopic studies at Progreso, that most
migrants leaving northern Yucatan do so in the few hours centering
about midnight. The varying speeds of the birds making the 580-mile
flight across the Gulf distribute them still more sparsely on the
north coast of the Gulf both in time and in space. Also we can see
only that segment of the flight, which arrives in that part of a
twenty-four hour period when the moon is up. This circumstance further
reduces the interceptive potential because the hours after dark, to
which the present telescopic studies have been restricted, comprise
the period in which the fewest migrants arrive from over the water. To
illustrate: it is a mathematical certainty that _none_ of the birds
leaving Yucatan in the hours of heaviest flight, before 12 P. M.,
and flying on a straight course at a speed of approximately 33 mph
will reach the northern Gulf coast after nightfall; they arrive in the
daytime. It will be useful to devise a technique for employing the sun
as a background for telescopic observation of birds, thereby making
observations possible on a twenty-four hour basis, so as to test these
inferences by objective data.
When a whole night's observation (1949 data not yet processed) at Port
Aransas, on the southern coast of Texas, on the great overland route
from eastern Mexico, yields in one night in April only seven birds,
the recording of no birds at a station near the mouth of the
Mississippi River becomes less significant.
As I have previously remarked in this paper, the new data obtained
since 1946, when I last wrote on the subject of migration in the
region of Gulf of Mexico, requires that I alter materially some of my
previously held views. As more and more facts come to light, I may be
compelled to alter them still further. For one thing, I have come to
doubt seriously the rigidity of the coastal hiatus as I envisioned it
in 1945. I believe instead that the scarcity of records of transient
migrants on the Gulf coastal plain in fair weather is to a very large
extent the result of a wide dispersion of birds in the dense cover
that characterizes this general region. I now question if appreciable
bird densities on the ground ever materialize anywhere except when the
sparseness of suitable habitat for resting or feeding tends to
concentrate birds in one place, or when certain meteorological
conditions erect a barrier in the path of an oncoming migratory
flight, precipitating many birds in one place.
This retrenchment of ideas is a direct consequence of the present
study, for time and again, as discussed in the case of Tampico
densities, maximal nightly flights have failed to produce a visible
abundance of transients on land the following day. A simple example
may serve to illustrate why. The highest one-hour density recorded in
the course of this study is 21,200 birds. That means that this many
birds crossed a line one mile long on the earth's surface and at right
angles to the direction of flight. Let us further assume that the
average flight speed of all birds comprising this flight was 30 mph.
Had the entire flight descended simultaneously, it would have been
dispersed over an area one mile wide and thirty miles long, and the
precipitated density on the ground would have been only 1.1 birds per
acre. Moreover, if as many as ten species had been involved in the
flight, this would have meant an average per species of less than one
bird per nine acres. This would have failed, of course, to show
appreciable concentrations to the observer in the field the following
day. If, however, on the other hand, the same flight of 21,200 birds
had encountered at one point a weather barrier, such as a cold-front
storm, all 21,200 birds might have been precipitated in one place and
the field observer would have recorded an "inundation of migrants."
This would be especially true if the locality were one with a high
percentage of open fields or prairies and if the flight were mainly of
woodland dwelling species, or conversely, if the locality were densely
forested with few open situations and the flight consisted mainly of
open-country birds. As explained on page 389, the density formula may
be too conservative in its expression of actual bird densities. Even
if the densities computed for birds in the air are only half as high
as the actual densities in the air, the corresponding ground density
of 2.2 birds per acre that results if all the birds descended
simultaneously would hardly be any more impressive than the 1.1 bird
per acre.
This consideration is doubtless highly modified by local
circumstances, but, in general, it seems to suggest a working
hypothesis that provides an explanation for many of the facts that we
now have. For example, on the coast of Texas there are great expanses
of terrain unattractive to such birds as warblers, vireos, tanagers,
and thrushes. The precipitation there by bad weather of even a
mediocre nightly flight composed of birds of the kinds mentioned would
surely produce an overwhelming concentration of birds in the scattered
woods and shrubs.
In spite of all that has been written about the great concentrations
of transient migrants on the coast of Texas in spring, I am not convinced
that they are of a different order of magnitude than those concentrations
that sometimes occur along the cheniers and coastal islands
of Louisiana and Mississippi. I have read over and over the
highly informative accounts of Professor Williams (_loci cit._) and the
seasonal summaries by Davis (1936-1940) and Williams (1941-1945).
I have conversed at length with Mrs. Jack Hagar, whom I
regard as one of the leading authorities on the bird life of the
Texas coast, and she has even permitted me access to her voluminous
records covering a period of fifteen years residence at Rockport.
Finally, I have spent a limited amount of time myself on the Texas
coast studying first-hand the situation that obtains there in order
that I might be in a position to compare it with what I have learned
from observations elsewhere in the region of the Gulf of Mexico,
Louisiana, Florida, Yucatan, and eastern Mexico.
Although the concentrations of birds on some days near the mouth of
the Mississippi River are almost incalculable, the fact remains that
in Texas the densities of transient species on the ground are more
consistently high from day to day. The reason for this may be simple.
As birds move up daily from Mexico overland, a certain percentage
would be destined to come down at all points along the route but so
dispersed in the inland forest that they might pass unnoticed.
However, that part of the same flight settling down in coastal areas,
where trees are scarce, would produce visible concentrations of
woodland species. With the advent of a cold-front storm, two
diametrically opposite effects of the same meteorological phenomenon
would tend to pile up great concentrations of migrants of two
classes--the overland and the trans-Gulf flights. During the
prepolar-front weather the strong southerly (from the south) and
southeasterly winds would tend to displace much of the trans-Gulf
segment to the western part of the Gulf. With the shift of the winds
to the north and northwest, which always occurs as the front passes,
the overland flight still in the air would tend to be banked up
against the coast, and the incoming trans-Gulf flight would be
confronted with a barrier, resulting in the precipitation of birds on
the first available land.
These postulated conditions are duplicated in part in autumn along the
Atlantic coast of the eastern United States. There, as a result of the
excellent work of Allen and Peterson (1936) and Stone (1937), a
similar effect has been demonstrated when northwest winds shove the
south-bound flights up against the coast of New Jersey and concentrate
large aggregations of migrants there.
_Interior of the United States_
Attention has been drawn already to the nature of the nightly flights
at stations immediately inland from the Gulf coast, where densities
decline abruptly well before midnight. I have suggested that this
early drop-off is mainly a result of the small amount of terrain south
of these stations from which birds may be contributed to a night's
flight. At Oak Grove, Louisiana, the flight exhibited a strong
directional trend with no significant aberrant components. Therefore,
one may infer that a considerable part of the flight was derived from
regions to the south of the station.
At Mansfield, Louisiana, thirty-eight hours of observation in April
and May resulted in flight densities that are surprisingly low--much
lower, in fact, than at Oak Grove. In eleven of the hours of
observation no birds at all were seen. A possible explanation for
these low densities lies in the fact that eastern Texas and western
Louisiana, where, probably, the Mansfield flights originated, is not
an especially attractive region to migrants because of the great
amount of deforested and second growth pine land. Oak Grove, in
contrast, is in the great Tensas-Mississippi River flood plain,
characterized by an almost solid stand of deciduous forest extending
over thousands of square miles in the lower Mississippi valley.
[Illustration: FIG. 37. Sector density representation on
two nights at Rosedale, Mississippi, in 1948. The white lines
are the vector resultants.]
In further contrast to the considerable flight densities and
pronounced directional trend at Oak Grove, we have the results from
Rosedale, Mississippi, only seventy miles to the north and slightly to
the east. At Rosedale the densities were mediocre and the flight
directions were extremely divergent. Many of the nights of observation
at this locality were seriously interrupted by clouds, but such counts
as were made on those dates indicated little migration taking place.
On two nights, however, April 21-22 and May 20-21, visibility was
almost continuous and densities were moderately high. In Figure 37 I
have shown the flight directions for these two nights. The lengths of
the individual sector vectors are plotted as a percentage of the total
station density for each of the two nights (5,800 and 6,800 birds,
respectively). Although the vector resultants show a net movement of
birds to the northeast, there are important divergent components of
the flights. This "round-the-compass" pattern is characteristic of
stations on the edge of meteorological disturbances, as was Rosedale
on April 21-22, but not on the night of May 20-21. If bats are
presumed to have played a role in these latter observations, their
random flights would tend to cancel out and the vector resultant
would emerge as a graphic representation of the actual net trend
density of the birds and its prevailing direction of flow. Although I
do not believe that bats are the real reason for the diverse
directional patterns at Rosedale, I can offer no alternative
explanation consistent with data from other stations.
Moving northward in the valley of the Mississippi and its tributaries,
we find a number of stations that yielded significantly high densities
on most nights when weather conditions were favorable for migration.
Louisville and Murray, Kentucky, and Knoxville, Tennessee, each show
several nights with many birds flying, but only Lawrence, Kansas, and
Ottumwa, Iowa, had migrations that approach in magnitude the record
station densities at Tampico. Indeed, these were the only two stations
in the United States that produced flights exceeding the densities at
Progreso, Yucatan. The densities at Lawrence are unique in one
respect, in that they were extremely high in the month of March. Since
there were very few stations in operation then, these high densities
would be of little significance were it not for the fact that at no
time in the course of this study from 1945 to the present have
comparable densities been obtained this early in the migration period.
Examination of the "Remarks" section of the original data sheets from
Lawrence show frequent mention of "duck-like" birds passing before the
moon. We may infer from these notations that a considerable part of
the overhead flight was composed of ducks and other aquatic birds that
normally leave the southern United States before the main body of
transient species reach there. The heavy flight densities at Lawrence
may likewise have contained certain Fringillidae, Motacillidae,
Sylviidae, and other passerine birds that winter mainly in the
southern United States and which are known to begin their return
northward in March or even earlier. Observations in 1948 at Lawrence
in April were hindered by clouds, and in May no studies were
attempted. However, we do have at hand two excellent sets of data
recorded at Lawrence on the nights of May 3-4 and May 5-6, 1947, when
the density was also extremely high.
At Ottumwa, Iowa, where a splendid cooperative effort on the part of
the local ornithologists resulted in forty-four hours of observation
in April and May, densities were near the maximum for all stations.
Considering this fact along with results at Lawrence and other
mid-western stations where cloud cover did not interfere at the
critical periods of observation, we have here evidence supporting the
generally held thesis that eastern Kansas, Missouri, and Iowa lie on a
principal migratory flyway. Stations in Minnesota, Illinois,
Michigan, Massachusetts, and Ontario were either operated for only
parts of one or two nights, or else clouds seriously interfered with
observations, resulting in discontinuous counts. It may be hoped that
future studies will include an adequate representation of stations in
these states and that observations will be extensive enough to permit
conclusions regarding the density and direction of migration.
Charleston, South Carolina, which does not conveniently fall in any of
the geographic regions so far discussed, had, to me, a surprisingly
low flight density; twenty-two hours of observation there in March,
April, and May yielded a total flight density of only 3,000 birds.
This is less, for example, than the number of birds computed to have
passed Lawrence, Kansas, in one hour, or to have passed Progreso,
Yucatan, in one twenty-minute interval! Possibly observations at
Charleston merely chanced to fall on nights of inexplicably low
densities; further observations will be required to clear up this
uncertainty.
E. MIGRATION AND METEOROLOGICAL CONDITIONS
The belief that winds affect the migration of birds is an old one. The
extent to which winds do so, and the precise manner in which they
operate, have not until rather recently been the subject of real
investigation. With modern advances in aerodynamics and the
development of the pressure-pattern system of flying in aviation,
attention of ornithologists has been directed anew to the part that
air currents may play in the normal migrations of birds. In America, a
brief article by Bagg (1948), correlating the observed abundance of
migrants in New England with the pressure pattern obtaining at the
time, has been supplemented by the unpublished work of Winnifred
Smith. Also Landsberg (1948) has pointed out the close correspondence
between the routes of certain long-distance migrants and prevailing
wind trajectories. All of this is basis for the hypothesis that most
birds travel along definite air currents, riding with the wind. Since
the flow of the air moves clockwise around a high pressure area and
counterclockwise around a low pressure area, the birds are directed
away from the "high" and toward the center of the "low." The arrival
of birds in a particular area can be predicted from a study of the
surrounding meteorological conditions, and the evidence in support of
the hypothesis rests mainly upon the success of these predictions in
terms of observations in the field.
From some points of view, this hypothesis is an attractive one. It
explains how long distances involved in many migrations may be
accomplished with a minimum of effort. But the ways in which winds
affect migration need analysis on a broader scale than can be made
from purely local vantage points. Studies of the problem must be
implemented by data accumulated from a study of the process in action,
not merely from evidence inferred from the visible results that follow
it. Although several hundred stations operating simultaneously would
surely yield more definite results, the telescopic observations in
1948 offer a splendid opportunity to test the theory on a continental
scale.
The approach employed has been to plot on maps sector vectors and
vector resultants that express the directional trends of migration in
the eastern United States and the Gulf region, and to compare the data
on these maps with data supplied by the U. S. Weather Bureau regarding
the directions and velocities of the winds, the location of high and
low pressure areas, the movement of cold and warm fronts, and the
disposition of isobars or lines of equal pressure. It should be borne
in mind when interpreting these vectors that they are intended to
represent the directions of flight only at the proximal ends, or
junction points, of the arrows. The tendency of the eye to follow a
vector to its distal extremity should not be allowed to create the
misapprehension that the actual flight is supposed to have continued
on in a straight line to the map location occupied by the arrowhead.
A fundamental difficulty in the pressure-pattern theory of migration
has no doubt already suggested itself to the reader. The difficulty to
which I refer is made clear by asking two questions. How can the birds
ever get where they are going if they are dependent upon the whim of
the winds? How can pressure-pattern flying be reconciled with the
precision birds are supposed to show in returning year after year to
the same nesting area? The answer is, in part, that, if the wind is a
major controlling influence on the routes birds follow, there must be
a rather stable pattern of air currents prevailing from year to year.
Such a situation does in fact exist. There are maps showing wind roses
at 750 and 1,500 meters above mean sea level during April and May
(Stevens, 1933, figs. 13-14, 17-18). Similarly, the "Airway
Meteorological Atlas for the United States" (Anonymous, 1941) gives
surface wind roses for April (Chart 6) and upper wind roses at 500 and
1,000 meters above mean sea level for the combined months of March,
April, and May (Charts 81 and 82). The same publication shows wind
resultants at 500 and 1,000 meters above mean sea level (Charts 108
and 109). Further information permitting a description in general
terms of conditions prevailing in April and May is found in the
"Monthly Weather Review" covering these months (_cf._ Anonymous,
1948 _a_, Charts 6 and 8; 1948 _b_, Charts 6 and 8).
[Illustration: FIG. 38. Over-all sector vectors at major
stations in the spring 1948. See text for explanation of
system used in determining the length of vectors. For
identification of stations, see Figure 34.]
[Illustration: FIG. 39. Over-all net trend of flight
directions at stations shown in Figure 38. The arrows
indicate direction only and their slants were obtained by
vector analysis of the over-all sector densities.]
First, however, it is helpful as a starting point to consider the
over-all picture created by the flight trends computed from this
study. In Figure 38, the individual sector vectors are mapped for the
season for all stations with sufficient data. The length of each
sector vector is determined as follows: the over-all seasonal density
for the station is regarded as 100 percent, and the total for the
season of the densities in each individual sector is then expressed as
a percentage. The results show the directional spread at each station.
In Figure 39, the direction of the over-all vector resultant, obtained
from the sector vectors on the preceding map, is plotted to show the
net trend at each station.
As is evident from the latter figure, the direction of the net trend
at Progreso, Yucatan, is decidedly west of north (N 26 deg. W). At Tampico
this trend is west of north (N 11 deg. W), but not nearly so much so as at
Progreso. In Texas, Louisiana, Georgia, Tennessee, and Kentucky, it is
decidedly east of north. In the upper Mississippi Valley and in the
eastern part of the Great Plains, the flow appears to be northward or
slightly west of north. At Winter Park, Florida, migration follows in
general the slant of the Florida Peninsula, but, the meager data from
Thomasville, Georgia, do not indicate a continuation of this trend.
It might appear, on the basis of the foregoing data, that birds
migrate along or parallel to the southeast-northwest extension of the
land masses of Central America and southern Mexico. This would carry
many of them west of the meridian of their ultimate goal, obliging
them to turn back eastward along the lines of net trend in the Gulf
states and beyond. This curved trajectory is undoubtedly one of the
factors--but certainly not the only factor--contributing to the effect
known as the "coastal hiatus." The question arises as to whether this
northwestward trend in the southern part of the hemisphere is a
consequence of birds following the land masses or whether instead it
is the result of some other natural cause such as a response to
prevailing winds. I am inclined to the opinion that both factors are
important. Facts pertinent to this opinion are given below.
In April and May a high pressure area prevails over the region of the
Gulf of Mexico. As the season progresses, fewer and fewer cold-front
storms reach the Gulf area, and as a result the high pressure area
over the Gulf is more stable. Since the winds move clockwise around a
"high," this gives a general northwesterly trajectory to the air
currents in the vicinity of the Yucatan Peninsula. In the western area
of the Gulf, the movement of the air mass is in general only slightly
west of north, but in the central Gulf states and lower Mississippi
Valley the trend is on the average northeasterly. In the eastern part
of the Great Plains, however, the average circulation veers again
slightly west of north. The over-all vector resultants of bird
migration at stations in 1948, as mapped in Figure 39, correspond
closely to this general pattern.
Meteorological data are available for drawing a visual comparison
between the weather pattern and the fight pattern on individual
nights. I have plotted the directional results of four nights of
observation on the Daily Weather Maps for those dates, showing surface
conditions (Figures 40, 42, 44 and 46). Each sector vector is drawn in
proportion to its percentage of the corresponding nightly station
density; hence the vectors at each station are on an independent
scale. The vector resultants, distinguished by the large arrowheads,
are all assigned the same length, but the nightly and average hourly
station densities are tabulated in the legends under each figure. For
each map showing the directions of flight, there is on the facing page
another map showing the directions of winds aloft at 2,000 and 4,000
feet above mean sea level on the same date (see Figures 41-47). The
maps of the wind direction show also the velocities.
Unfortunately, since there is no way of analyzing the sector trends in
terms of the elevations of the birds involved, we have no certain way
of deciding whether to compare a given trend with the winds at 2,000,
1,000, or 0 feet. Nor do we know exactly what wind corresponds to the
average or median flight level, which would otherwise be a good
altitude at which to study the net trend or vector resultant.
Furthermore, the Daily Weather Map illustrates conditions that
obtained at 12:30 A. M. (CST); the winds aloft are based on
observations made at 10:00 P. M. (CST); and the data on birds covers
in most cases the better part of the whole night. Add to all this the
fact that the flight vectors, their resultants, and the wind
representations themselves are all approximations, and it becomes
apparent that only the roughest sort of correlations are to be
expected.
However, as will be seen from a study of the accompanying maps
(Figures 40-47), the shifts in wind direction from the surface up to
4,000 feet above sea level are not pronounced in most of the
instances at issue, and such variations as do occur are usually in a
clockwise direction. All in all, except for regions where frontal
activity is occurring, the weather maps give a workable approximation
to the average meteorological conditions on a given night.
The maps (Figures 40-47) permit, first, study of the number of
instances in which the main trend of flight, as shown by the vector
resultant, parallels the direction of wind at a reasonable potential
mean flight elevation, and, second, comparison of the larger
individual sector vectors and the wind currents at any elevation below
the tenable flight ceiling--one mile.
On the whole, inspection of the trend of bird-flight and wind
direction on specific nights supports the principle that the flow of
migration is in general coincident with the flow of air. It might be
argued that when the flow of air is toward the north, and when birds
in spring are proceeding normally in that direction, no significance
can be attached to the agreement of the two trends. However, the same
coincidence of wind directions and bird flights seems to be maintained
when the wind currents deviate markedly from a northward trajectory.
Figures 46 and 47, particularly in regard to the unusual slants of the
flight vectors at Ottumwa, Knoxville, and Memphis, illustrate that
this coincidence holds even when the wind is proceeding obliquely
eastward or westward. On the night of May 22-23, when a high pressure
area prevailed from southern Iowa to the Atlantic coast, and the
trajectory of the winds was northward, migration activity at Knoxville
and Ottumwa was greatly increased and the flow of birds was again
northward in the normal seasonal direction of migration.
Further study of the data shows fairly conclusively that maximum
migration activity occurs in the regions of high barometric pressure
and that the volume of migration is either low or negligible in
regions of low pressure. The passage of a cold-front storm may almost
halt migration in spring. This was demonstrated first to me by the
telescopic method at Baton Rouge, on April 12, 1946, following a
strong cold front that pushed southeastward across the Gulf coastal
plain and over the eastern Gulf of Mexico. The winds, as usual,
shifted and became strong northerly. On this night, following the
shift of the wind, only three birds were seen in seven hours of
continuous observation. Three nights later, however, on April 15, when
the warm air of the Gulf was again flowing from the south, I saw 104
birds through the telescope in two hours. Apropos of this
consideration in the 1948 data are the nights of May 21-22 and 22-23.
[Illustration: FIG. 40. Comparison of flight trends and
surface weather conditions on April 22-23, 1948. The
meteorological data were taken from the U. S. Weather Bureau
Daily Weather Map for 12:30 A. M. (CST) on April 23. The
nightly station densities and the average hourly station
density (shown in parentheses) are as follows:
5. Louisville: 9,100 (1,100)
6. Murray: 16,300 (2,700)
8. Stillwater: 1,900 (500)
9. Knoxville: 15,200 (1,700)
13. Oak Grove: 13,600 (1,700)
16. College Station: 13,300 (1,900)
17. Baton Rouge: 6,200 (1,000)
19. Lafayette: 2,800 (600)
21. Winter Park: 6,200 (700)
23. Tampico: 11,100 (3,700)]
[Illustration: FIG. 41. Winds aloft at 10:00 P. M. on
April 22 (CST). Winds at 2,000 feet above mean sea level are
shown in black; those at 4,000 feet, in white. Velocities are
indicated by standard Beaufort Scale of Wind Force. The
numbers in circles refer to the stations shown in Figure 40.]
Correction: Figures 41 and 45 were inadvertently transposed.
[Illustration: FIG. 42. Comparison of flight trends and
surface weather conditions on April 23-24, 1948. The
meteorological data were taken from the U. S. Weather Bureau
Daily Weather Map for 12:30 A. M. (CST) on April 24. The
nightly station densities and the average hourly station
density (shown in parentheses) are as follows:
1. Albion: 1,100 (300)
2. Ottumwa: 5,500 (900)
4. Lawrence: 5,400 (1,400)
5. Louisville: 13,300 (2,700)
6. Murray: 9,800 (1,400)
8. Stillwater: 800 (100)
9. Knoxville: 8,000 (900)
10. Memphis: 7,900 (1,000)
14. Mansfield: 4,900 (1,200)
16. College Station: 700 (100)
17. Baton Rouge: 1,700 (400)
18. Pensacola: migration negligible
20. New Orleans: 1,600 (800)
21. Winter Park: 2,700 (300)
23. Tampico: 63,600 (6,300)
24. Progreso: 31,300 (3,900)]
[Illustration: FIG. 43. Winds aloft at 10:00 P. M. on
April 23 (CST). Winds at 2,000 feet above mean sea level are
shown in black; those at 4,000 feet, in white. Velocities are
indicated by standard Beaufort Scale of Wind Force. The
numbers in circles refer to the stations shown in Figure 42.]
[Illustration: FIG. 44. Comparison of flight trends and
surface weather conditions on April 24-25, 1948. The
meteorological data were taken from the U. S. Weather Bureau
Daily Weather Map for 12:30 A. M. (CST) on April 25. The
nightly station densities and the average hourly station
density (shown in parentheses) are as follows:
1. Albion: migration negligible
2. Ottumwa: 4,600 (1,500)
3. Columbia: 1,400 (400)
5. Louisville: 1,700 (200)
10. Memphis: 6,600 (900)
12. Rosedale: 1,100 (100)
14. Mansfield: 1,700 (400)
18. Pensacola: migration negligible
21. Winter Park: 600 (100)
24. Progreso: 27,300 (3,000)]
[Illustration: FIG. 45. Winds aloft at 10:00 P. M. on
April 24 (CST). Winds at 2,000 feet above mean sea level are
shown in black; those at 4,000 feet, in white. Velocities are
indicated by standard Beaufort Scale of Wind Force. The
numbers in circles refer to the stations shown in Figure 44.]
Correction: Figures 41 and 45 were inadvertently transposed.
[Illustration: FIG. 46. Comparison of flight trends and
surface weather conditions on May 21-22, 1948. The
meteorological data were taken from the U. S. Weather Bureau
Daily Weather Map for 12:30 A. M. (CST) on May 22. The
nightly station densities and the average hourly station
density (shown in parentheses) are as follows:
2. Ottumwa: 6,900 (1,400)
5. Louisville: 1,500 (200)
9. Knoxville: 3,200 (500)
10. Memphis: 7,000 (1,200)
13. Oak Grove: 5,800 (800)
14. Mansfield: 2,500 (800)
18. Pensacola: migration negligible
21. Winter Park: 1,200 (200)]
[Illustration: FIG. 47. Winds aloft at 10:00 P. M. on May
21 (CST). Winds at 2,000 feet above mean sea level are shown.
Velocities are indicated by standard Beaufort Scale of Wind
Force. The numbers in circles refer to the stations shown in
Figure 46.]
On the first night, following the passage of a cold front, migration
at Ottumwa was comparatively low (6,900 birds in five hours). On the
following night, when the trajectory of the winds was toward the
north, the volume of migration was roughly twice as high (22,300 birds
in eight hours). At Louisville, on May 21-22, the nightly station
density was only 1,500 birds in seven hours, whereas on the following
night, it was 8,400 birds in the same length of time, or about six
times greater.
The evidence adduced from the present study gives support to the
hypothesis that the continental pattern of spring migration in eastern
North America is regulated by the movement of air masses. The
clockwise circulation of warm air around an area of high pressure
provides, on its western edge, tail winds which are apparently
favorable to northward migration. High pressure areas exhibit a
centrifugal force outward from the center, which may tend to disperse
the migratory flight originating at any given point. In contrast, the
circulation of air in the vicinity of a low pressure area is
counterclockwise with the force tending to be directed inward toward
the center. Since the general movement of the air is from the high
pressure area toward a low pressure area, birds starting their
migrations with favorable tail winds, are often ultimately carried to
a region where conditions are decidedly less favorable. In the
vicinity of an area of low pressure the greater turbulence and high
wind velocities, combined with the possibly slightly less buoyant
property of the air, cause birds to descend. Since low pressure areas
in spring generally precede cold fronts, with an attending shift of
the wind to the north, an additional barrier to the northward
migration of birds is imposed. The extreme manifestation of low
pressure conditions and the manner in which they operate against bird
flight, are associated with tropical hurricanes. There, the
centripetal force of the wind is so great that it appears to draw
birds into the "eye" of the hurricane. A classic example of this
effect is seen in the case of the birds that came aboard the "West
Quechee" when this vessel passed through the "eye" of a hurricane in
the Gulf of Mexico in August, 1927. I have already discussed the
details of this incident in a previous paper (1946:192). There is also
the interesting observation of Mayhew (1949), in which a similar
observation was made of large numbers of birds aboard a ship passing
through one of these intense low-pressure areas.
Although the forces associated with an ordinary low-pressure area are
by no means as intense as those associated with a tropical hurricane,
the forces operating are much the same. Consequently birds conceivably
might tend to be drawn toward a focal point near the center of the
low, where the other factors already mentioned would tend to
precipitate the entire overhead flight. Visible evidence of migration
would then manifest itself to the field ornithologists.
CONCLUSIONS
1. Telescopic counts of birds passing before the moon may be used
to determine reliable statistical expressions of the volume of
migration in terms of direction and of definite units of time
and space.
2. Night migrants fly singly more often than in flocks, creating a
remarkably uniform dispersion on a local scale throughout the
sky, quite unlike the scattered distributions observable in the
daytime.
3. The nocturnal migration of birds is apparently preceded by a
resting or feeding pause during which there are few migrants in
the air. It is not to an important degree a non-stop continuation
of flights begun in the daylight.
4. Nightly migrational activity in North America varies from hour to
hour according to a definite temporal pattern, corresponding to
the _Zugunruhe_ of caged European birds, and expressed by
increasingly heavy flights up until the hour before midnight,
followed by a pronounced decline.
5. The visible effects of the time pattern are subject to
modification at a particular station by its location with respect
to the resting areas from which the night's flight originates.
6. Quantitative and directional studies have so far failed to prove
that nocturnal migrants favor narrow, topographically-determined
flight lanes to an important degree.
7. Flight densities on the east coast of Mexico, though of first
magnitude, have not yet been demonstrated in the volume demanded
by the premise that almost all migrants returning to the
United States from regions to the south do so by coastal routes.
8. Heavy flights have been recorded from the northern coast of
Yucatan under circumstances leading inevitably to the conclusion
that birds migrate across the Gulf of Mexico in considerable
numbers.
9. There is reason to believe that the importance of the Florida
Peninsula as an April and May flyway has been over-estimated,
as regards the numbers of birds using it in comparison with the
numbers of birds using the Mexican and Gulf routes.
10. The amount of migration is apparently seldom sufficient to produce
heavy densities of transient species on the ground without
the operation of concentrative factors such as ecological patterns
and meteorological forces.
11. The absence or scarcity of transients in some areas in fine
weather may be explained by this consideration.
12. A striking correlation exists between air currents and the
directional flight trends of birds, suggesting that most night
migrants travel by a system of pressure-pattern flying.
LITERATURE CITED
ALLEN, R. P., AND R. T. PETERSON
1936. The hawk migrations at Cape May Point, New Jersey. Auk,
53:393-404.
ANONYMOUS
1936-1941. Tables of computed altitude and azimuth. U. S. Navy
Department Hydrographic Office. U. S. Govt. Printing
Office, Washington, D. C., vols. 3-5.
1941. Airway meteorological atlas for the United States.
Weather Bureau Publ. 1314. U. S. Dept. Commerce,
Washington, D. C.
1945-1948. The American air almanac. U. S. Naval Observatory.
U. S. Govt. Printing Office, Washington, D. C., 3 vols.,
issued annually.
1948_a_. Meteorological and climatological data for April 1948.
Monthly Weather Review, April 1948, 76:65-84, 10 charts.
1948_b_. Meteorological and climatological data for May 1948.
Monthly Weather Review, May 1948, 76:85-103, 11 charts.
BAGG, A. M.
1948. Barometric pressure-patterns and spring migration.
Auk, 65:147.
BERGMAN, G.
1941. Der Fruhlingszug von _Clangula hyemalis_ (L.) und
_Oidemia nigra_ (L.) bei Helsingfors. Eine Studie ueber
Zugverlauf und Witterung sowie Tagesrhythmus und Flughoehe.
Ornis Fennica, 18:1-26.
BRAY, R. A.
1895. A remarkable flight of birds. Nature (London), 52:415.
CARPENTER, F. W.
1906. An astronomical determination of the height of birds
during nocturnal migration. Auk, 23:210-217.
CHAPMAN, F. M.
1888. Observations on the nocturnal migration of birds.
Auk, 5:37-39.
DAVIS, L. I.
1936-1940. The season: lower Rio Grande Valley region. Bird-Lore
(now Audubon Mag.), 38-42.
F. [ARNER], D. [ONALD] S.
1947. Studies on daily rhythm of caged migrant birds (review of
Palmgren article). Bird-Banding, 18:83-84.
GATES, W. H.
1933. Hailstone damage to birds. Science, 78:263-264.
HOWELL, A. H.
1932. Florida bird life. Florida Department Game and Fresh Water
Fish, Tallahassee, 1-579 + 14 pp., 58 pls., 72 text figs.
LANSBERG, H.
1948. Bird migration and pressure patterns. Science, 108:708-709.
LIBBY, O. G.
1899. The nocturnal flight of migratory birds. Auk, 16:140-146.
LOWERY, G. H., JR.
1945. Trans-Gulf spring migration of birds and the coastal
hiatus. Wilson Bull., 57:92-121.
1946. Evidence of trans-Gulf migration. Auk, 63:175-211.
MAYHEW, D. F.
1949. Atmospheric pressure and bird flight. Science, 109:403.
OVERING, R.
1938. High mortality at the Washington Monument. Auk, 55:679.
PALMGREN, P.
1944. Studien ueber die Tagesrhythmik gekaefigter Zugvoegel.
Zeitschrift fuer Tierpsychologie, 6:44-86.
POUGH, R. H.
1948. Out of the night sky. Audubon Mag., 50:354-355.
PUTKONEN, T. A.
1942. Kevaetmuutosta Viipurinlakdella. Ornis Fennica, 19:33-44.
RENSE, W. A.
1946. Astronomy and ornithology. Popular Astronomy, 54:55-73.
SCOTT, W. E. D.
1881_a._ Some observations on the migration of birds. Bull. Nuttall
Orni. Club, 6:97-100.
1881_b._ Migration of birds at night. Bull. Nuttall Orni. Club,
6:188.
SIIVONEN, L.
1936. Die Staerkevariation des Naechtlichen Zuges bei _Turdus ph.
philomelos_ Brehn und _T. musicus_ L. auf Grund der
Zuglaute geschaetz und mit der Zugunruhe einer gekaefigten
Singdrossel Verglichen. Ornis Fennica, 13:59-63.
SPOFFORD, W. R.
1949. Mortality of birds at the ceilometer of the Nashville
airport. Wilson Bull., 61:86-90.
STEBBINS, J.
1906. A method of determining height of migrating birds.
Popular Astronomy, 14:65-70.
STEVENS, LLOYD A.
1933. Upper-air wind roses and resultant winds for the eastern
United States. Monthly Weather Review, Supplement No. 35,
November 13, pp. 1-3, 65 figs.
STONE, W.
1906. Some light on night migration. Auk, 23:249-252.
1937. Bird studies at Old Cape May. Delaware Valley Orni. Club,
Philadelphia, Vol. 1, 1-520 + 15 pp., pls. 1-46 and frontis.
THOMSON, A. L.
1926. Problems of bird migration. Houghton Mifflin Company,
Boston.
VAN OORDT, G.
1943. Vogeltrek. E. J. Brill, Leiden, xii + 145 pp.
VERY, F. W.
1897. Observations of the passage of migrating birds across the
lunar disc on the nights of September 23 and 24, 1896.
Science, 6:409-411.
WALTERS, W.
1927. Migration and the telescope. Emu, 26:220-222.
WEST, R. H.
1896. Flight of birds across the moon's disc. Nature (London),
53:131.
WILLIAMS, G. G.
1941-1948. The season: Texas coastal region. Audubon Mag., 43-50.
1945. Do birds cross the Gulf of Mexico in spring? Auk,
62:98-111.
1947. Lowery on trans-Gulf migration. Auk, 64:217-238.
WINKENWERDER, H. A.
1902_a_. The migration of birds with special reference to nocturnal
flight. Bull. Wisconsin Nat. Hist. Soc., 2:177-263.
1902_b_. Some recent observations on the migration of birds. Bull.
Wisconsin Nat. Hist. Soc., 2:97-107.
Transmitted June 1, 1949.
[]
23-1020
UNIVERSITY OF KANSAS PUBLICATIONS
The University of Kansas Publications, Museum of Natural History, are
offered in exchange for the publications of learned societies and
institutions, universities and libraries. For exchanges and
information, address the Exchange Desk, University of Kansas Library,
Lawrence, Kansas, U. S. A.
MUSEUM OF NATURAL HISTORY.--E. Raymond Hall, Chairman, Editorial
Committee.
This series contains contributions from the Museum of Natural History.
Cited as Univ. Kans. Publ., Mus. Nat. Hist.
Vol. 1. (Complete) Nos. 1-26. Pp. 1-638. August 15, 1946-January 20,
1951.
Vol. 2. (Complete) Mammals of Washington. By Walter W. Dalquest.
Pp. 1-444, 140 figures in text. April 9, 1948.
Vol. 3. 1. The avifauna of Micronesia, its origin, evolution, and
distribution. By Rollin H. Baker. Pp. 1-359, 16 figures
in text. June 12, 1951.
2. A quantitative study of the nocturnal migration of birds.
By George H. Lowery, Jr. Pp. 361-472, 47 figures in text.
June 29, 1951.
Transcriber's Notes
With the exception of the typographical corrections detailed below
and some minor corrections for missing periods or extra punctuation
(item 28 in List of Figures), the text presented here is that
contained in the original printed version. A transcription of the
Data presented in Figure 12 was added to illustrate the information
contained on that sheet. Some text was moved to rejoin paragraphs.
The list of UK publications was moved to the end of the document.
In writing variables for formulae, superscripted characters are
shown using a caret (^). So, X squared would be X^2. Subscripts are
shown using an underscore. Carbon dioxide is CO_2. Where several
superscript or subscript character(s) are required or to aid in
clarity, they are placed in braces (ex., H_{2}O for water and
[theta]_{Npt.} for theta degrees from the North point).
Emphasis Notation
_Text_ = Italics
Typographical Corrections
Page Correction
385 flght => flight
394 diargrams => diagrams
404 Determinaton => Determination
411 obsever => observer
419 Morover => Moreover
425 Mississippii => Mississippi
425 a => as
430 at => and
431 inserted "a"
("...traveling along a certain topographic feature...")
442 concensus => consensus
472 Stephens, Loyd A. => Stevens, Lloyd A.