THE RELATION OF THE TERRITORY TO THE SYSTEM OF REPRODUCTION
In the first two chapters I tried to show that the inherited nature of
the male leads it to remain in a definite place at a definite season and
to become intolerant of the approach of members of its own sex, and that
a result is thus attained which the word "territory" in some measure
describes. But the use of this word is nevertheless open to criticism,
for it denotes a human end upon which the highest faculties have been
brought to bear, and consequently we have to be on our guard lest our
conception of the "territory" should tend to soar upwards into regions
which require a level of mental development not attained by the bird. It
is necessary to bear this in mind now we have come to consider the
meaning of the territory, or rather the position that it occupies in the
whole scheme of reproduction.
Relationship to a territory within the interrelated whole of a bird's
life serves more than one purpose, and not always the same purpose in
the case of every species. We have only to glance at the life-histories
of divergent forms to see that the territory has been gradually adjusted
to suit their respective needs--limited in size here, expanded there, to
meet new conditions as they arose. Now some may think that the theory
would be more likely to be true if the territory had but one purpose to
fulfil, and that one the same for every species; and they may see
nothing but weakness in the multiplication of ways in which I shall
suggest it may be serviceable. But such an objection, if it were raised,
would arise from a mistaken conception, a conception which, instead of
starting with a relationship and working up to the "territory," sees in
the "territory" something of the bird's own selection and thence works
back to its origin. Holding the view that it is nothing but a term in a
complex relationship which has gradually become interwoven in the
history of the individual, I see no reason why the fact of its serving a
double or a treble purpose should not be a stronger argument for its
survival. I now propose to examine the various ways in which the
territory may have been of use in furthering the life of the individual,
and the circumstances in the inorganic world which have helped to
determine its survival.
The purpose that it serves depends largely upon the conditions in the
external environment--the climate, the supply of food, the supply of
breeding-stations, and the presence of enemies. Hence its purpose varies
with varying conditions of existence. But before we proceed to examine
the particular ways in which it has been modified to suit the needs of
particular classes of species, and the reason for such modifications, we
must inquire whether there is not some way in which it has been
serviceable alike to every species, or at least to a large majority of
them.
Success in the attainment of reproduction depends upon the successful
discharge of the sexual function; and the discharge of the sexual
function depends primarily upon an individual of one sex coming into
contact with one of the opposite sex at the appropriate season and when
its appropriate organic condition arises. Now the power of locomotion is
so highly developed in birds that it may seem unreasonable to suppose
that males and females would have any difficulty in meeting when their
inherited nature required that they should do so, still less reasonable
to suggest that this power might even act as a hindrance to successful
mating. Nevertheless, if we try to picture to ourselves the conditions
which would obtain if the movements of both sexes were in no wise
controlled, and mating were solely dependent upon fortuitous gatherings,
we shall come, I fancy, to no other conclusion than that much loss of
valuable time and needless waste of energy would often be incurred in
the search, and that many an individual would fail to breed just because
its wanderings took it into districts in which, at the time, there
happened to be too many of this sex or too few of that. And as the power
of locomotion increased and the distribution of the sexes became more
and more irregular, so the opportunity would be afforded for the
development of any variation which would have tended to facilitate the
process of pairing, and by so doing have conferred upon the individuals
possessing it, some slight advantage over their fellows.
What would have been the most likely direction for variation to have
taken? Any restriction upon the freedom of movement of both sexes would
only have added to the difficulties of mating; but if restriction had
been imposed upon one sex, whilst the other had been left free to
wander, some order would have been introduced into the process. That the
territory serves to restrict the movements of the males and to
distribute them uniformly throughout all suitable localities, there can
be no question; and since the instinctive behaviour in relation to it is
timed to appear at a very early stage in the seasonal sexual process,
the males are in a position to receive mates before the impulse to mate
begins to assert itself in the female.
We will take the Ruff as an example. According to Mr. Edmund Selous,
pairing, in this species, is promiscuous--the Ruffs are polygamous, the
Reeves polyandrous. Suppose, then, that upon this island of some few
miles in circumference, whereon his investigations were made, the
movements of neither Ruff nor Reeve were subject to control, that the
birds wandered in all directions, and that the union of the sexes were
fortuitous, would the result have been satisfactory? We must remember
that the Reeve requires more than one Ruff to satisfy her sexual
instinct; we must also bear in mind the possibility that the functioning
of her instinct may be subject to some periodicity, and we ask whether,
under these circumstances, accidental gatherings would meet all the
requirements of the situation. Now, manifestly, she must be in a
position to find males when her appropriate organic condition arises.
But in the absence of any system in the distribution of the sexes, how
could delay be avoided, or how could a uniform discharge of the sexual
function be assured? There is, however, a system. In the first place,
there are the assembly grounds to which the birds repair season after
season; and then, on the assembly grounds, there are the territories,
represented, as Mr. Selous tells us, by depressions where the grass by
long use has been worn away, and each depression is owned by one
particular Ruff. The assembly grounds have the effect of splitting up
and scattering the birds, and the number of Ruffs at any one particular
meeting place is limited by the territories; with the result that Ruffs
fit to breed are evenly distributed and always to be found in certain
definite places, and the Reeves know by experience where to find them.
The advantage of this territorial system is therefore apparent. Instead
of this district being overcrowded and that one deserted; instead of
there being too many of one sex here and too few of the other sex there;
instead of a high percentage of individuals failing to procreate their
kind, just because circumstances over which they have no control prevent
their discovering one another at the appropriate time--each sex has its
allotted part to play, each district has its allotted number of
inhabitants, and the waste of energy and the loss of time incurred in
the process of mating is reduced to a minimum.
Let us return again to the question of fortuitous mating, and consider
the position of a male and female that have discovered one another by
accident and have paired; what will be the subsequent course of their
behaviour? We are assuming, of course, that a territory forms no part of
their life-history. If the discharge of the sexual function takes place
immediately and the ovaries of the female are in an advanced state of
seasonal development, the construction of the nest will proceed without
delay--and the nest will answer the same purpose as the territory in so
far as it serves to restrict the movements of the birds and tends to
make them remain in, or return to, its vicinity; but if not, there will
be an interval during which both sexes will continue to wander as
before, guided only by the scarcity or abundance of food. In the first
case, there will be the attraction of the nest to prevent any untimely
separation; in the second, there will be nothing in the external
environment to induce them to remain in any particular spot. Now if we
turn to any common species and observe the sequence of events in the
life of different pairs, we shall find that pairing is seldom followed
by an immediate attempt to build; that an interval of inactivity is the
rule rather than the exception, and that this interval varies in
different species, in different individuals, and in different seasons.
Our imaginary male and female will therefore be faced with considerable
difficulty; for with nothing in the external environment to attract them
and with no restriction imposed upon the direction or extent of their
flight, their union will continue to be, as it began by being,
fortuitous. Next, let us consider their position were a disposition to
establish a territory to form part of the inherited nature of the male.
Each one will then be free to seek food when and where it wills and to
associate with other individuals without the risk of permanent
separation from its mate; and, no matter how long an interval may elapse
between mating and nest-building, each one will be in a position to find
the other when the appropriate moment for doing so arrives. Hence, while
preserving freedom of movement for each individual, the territory will
render their future, as a pair, secure.
No doubt the course of behaviour, as we observe it to-day in the lives
of many species, is the outcome of, rather than the condition which has
led to, the evolution of the territory. Thus, in many cases, we find
that early mating is the rule rather than the exception; we find that
the sexes frequently separate to seek their food, and fly away
temporarily in different directions; and, under exceptional climatic
conditions, we find that they even revert to their winter routine and
form flocks; only, however, to return to their territories, as pairs,
under more congenial conditions. Yellow Buntings, for example, pair
comparatively early in the season--some in the latter part of February,
others in March, and others again in April; and some build their nests
in April, others in May. There is a gorse-covered common which I have in
mind, a favourite breeding resort of this species. Between this common
and the surrounding country, the birds constantly pass to and fro. If
you watch a particular male you will observe that it sings for a while
in its territory, that it then rises in the air and disappears from
view, and finally that it returns to the tree, bush, or mound which
constitutes its headquarters, where it again sings. Meanwhile the
female, with which there is every reason to believe that this male has
paired, behaves similarly; she, too, flies to the surrounding country
and in time returns with equal certainty. Sometimes male and female
accompany one another--that is, they leave simultaneously and likewise
return; at other times, though they depart together, the male returns
alone; or the male may disappear in one direction whilst the female does
so in another--and, on the whole, there is a sameness in the direction
of flight taken by the same pairs on different occasions. An interval of
nearly two months may thus elapse between mating and nest-building,
during which the sexes are not only often apart but often separated by a
considerable distance.
What does this species gain by the individuals belonging to it mating so
early in the season? If the appropriate condition which leads the
females to seek males were to arise in each individual at a late date,
the first stage in the process--mating--would not be completed before
the second--the discharge of the sexual function--were due to begin.
Thus, instead of having ample time, the females would have but a short
period in which to discover males; and this in some cases might lead to
delay, in others to failure, and in others again to needlessly severe
competition, entailing physical exhaustion at a critical moment in
their lives. Hence those females in which the appropriate organic
condition developed early in the season would not only be more likely to
find males, but would be in a position to rear more broods than those in
which it developed late; and they would have a better chance of leaving
offspring, which, in their turn, would reproduce the peculiarities of
their parents. Moreover, within certain limitations, the more these
successful females varied in the date of their development, the less
severe would be the competition, and the more uniformly successful would
the mating of all the individuals in a given district tend to become.
But all of this renders an interval of sexual inactivity unavoidable; an
interval which must constitute a danger unless there were something in
the external environment to prevent the male and female from drifting
apart. Inasmuch, then, as the occupation of a territory serves to remove
all possibility of permanent separation, I suggest that its evolution
has afforded the condition under which this beneficial procedure has
developed--free to mate when they will, free to seek food where they
will, free to pursue their normal routine of existence, and to meet all
exigencies as they arise in their ordinary daily life--whilst free to do
this, their future, as a pair, is nevertheless secure.
Thus far we have considered the territory in its relation to the
discharge of the sexual function. In many of the lower forms of life,
the success or the failure of reproduction, so far as the individual is
concerned, may be said to end with the completion of the sexual act--the
female has but to deposit her eggs in a suitable environment and then
her work is done, because in due course and under normal conditions of
temperature the young hatch out, and from the first are able to fend for
themselves. And so, when we come to consider the question of
reproduction in the higher forms of life, we are apt to focus attention
too much upon the sexual function and too little upon the contributory
factors, the failure of any one of which would mean failure of the
whole. For a bird, success in the attainment of reproduction does not
merely imply the successful discharge of the sexual function; much more
is demanded; it must find somewhere to build its nest and to lay its
eggs, it must shield its young from extremes of temperature and protect
them from enemies, and it must be in a position to supply them with food
at regular intervals. And, consequently, every situation is not equally
favourable for rearing young; there must be a plentiful supply of food
of the right kind in the immediate vicinity of the nest, and it must be
in greatest abundance just at the moment when it is most urgently
needed--that is to say, during the first few weeks after the birth of
the young. Success, therefore, depends upon manifold relationships which
centre in the station, and these relationships vary in intensity with
the conditions of existence.
First, then, let us examine the problem from the point of view of the
food-supply. There are many species whose success in rearing offspring
is largely dependent upon the rapidity with which they can obtain food;
and it makes but little difference which species we choose out of
many--Finch, Bunting, Warbler, or Chat. I shall choose the Buntings, as
their life-history in broad outline conforms to the general type, and,
moreover, their behaviour is fresh in my mind. The young are born in a
very helpless state; they are without covering--fragile organisms,
ill-fitted, one would think, to withstand extremes of temperature, and
wholly incapable of protecting themselves from enemies of any
description. For the first three days after they are hatched the female
spends much of her time in brooding them, and, when she is thus
occupied, the male sometimes brings food to her, which she proceeds to
distribute or swallows. But all the young cannot be fed, neither are
they ready to be fed, at the same moment; and the parents have besides
to find food for themselves, and the nest has to be cleaned--all of
which necessitates the young being exposed to the elements at frequent
intervals. Now it is impossible to observe the instinctive routine of
the parents, when the young need attention, without being impressed with
the conative aspect of their behaviour. Why, we ask, are the movements
of the female so brisk; why does she seek food and clean the nest so
hurriedly; why, if her instinctive routine is interrupted, do her
actions and her attitude betray such bewilderment? I take it that the
only answer we can give to these questions is that the part of her
inherited nature which predominates just at this particular time is to
brood. But why is brooding of such importance? Partly to maintain the
young at the proper temperature, and thereby to induce sleep--and sleep
for offspring newly hatched is as important as food--and partly to
protect them from the risk of exposure to extremes of temperature. This
latter danger is no imaginary one. Examine a young bird that has
recently left the egg; observe its nakedness; and consider what it has
to withstand--a temperature that may rise to 70° F. or may fall to 40°
F., the tropical rain of a thunderstorm or the persistent drizzle of
many hours' duration, the scorching effect of a summer sun or the
chilling effect of a cold north-easterly wind, and, constantly, the
sudden change of temperature each time that the parent leaves the nest.
One marvels that it ever does survive; one marvels at the evolution of a
constitution sufficiently elastic to withstand such changes. But,
however much the constitution may give us cause to wonder, it is clear
that much depends upon the parents. A slight inefficiency of the
instinctive response which the presence of the young evokes, a little
slowness in searching for food or sluggishness in returning to the nest,
might lead to exposure and prove fatal. And, however much is demanded of
the parents, it is clear that much also depends upon the relationships
in the external environment; for no matter how sensitive or how well
attuned the instinctive response of the parent may be, it will avail but
little in the presence of unfavourable conditions in the environment.
Everything turns upon the question of the effect of exposure. And in
order to ascertain how far extremes of temperature are injurious, I
removed the nests of various species containing newly hatched young,
and, placing them in surroundings that afforded the customary amount of
protection from the elements, I made a note of the temperature and the
atmospheric conditions and then observed the condition of the young at
frequent intervals. Details of these experiments will be found at the
end of the chapter.
The experiments with the Blackbirds and the Whitethroats gave the most
interesting results. Both broods of each species were respectively of
much the same age, yet one brood of Blackbirds survived for five, and
the other only for two and a half hours, and one brood of Whitethroats
lived for twelve hours whilst the other succumbed in a little over an
hour. This difference is rather remarkable; and it seems clear that the
power of resistance of the young diminishes rapidly when the temperature
falls below 52° F. It must be borne in mind, however, that the
conditions under which the experiments were made were, on the whole,
favourable--the weather was dry, the temperature was not unusually low,
nor was the wind exceptionally strong or cold; and even in those cases
in which the young succumbed so rapidly, the atmospheric conditions
could by no means be regarded as abnormal.
What, then, would happen in an unusually wet or cold breeding season?
For how long would the young then survive? In the spring and early
summer of the year 1916, I was fortunate in observing the effect of
exposure under natural but inclement conditions. I happened to be
watching the Yellow Buntings on Hartlebury Common--200 acres of Upper
Soft Red Sandstone, profusely overgrown with cross-leaved heath (_Erica
tetralix_), ling (_Calluna vulgaris_), and furze (_Ulex_)--in one corner
of which eight males had established adjoining territories covering some
fifteen acres of ground. The males obtained mates towards the end of
March or at the beginning of April; nests were built in the middle of
May, and the successful pairs hatched out their young in June. On the
10th June the weather became exceptionally cold, and during the next ten
days the temperature fell at times to 40° F. during the daytime. Slight
frosts were registered at night in the district, and the young bracken,
which covered the Common in places, had the appearance of having been
scorched and eventually withered away. At the coldest period of this
cold spell the young were hatched in two of the nests--in the first one
on the 10th June, and in the second a day or so later; and on the
morning of the 10th June, having found a suitable position near the
first nest, I began to watch the movements of the parents, with the
intention of keeping some record of their behaviour each day so long as
the young needed attention. An hour passed without their appearing, and
on examining the young I found that they were cold, feeble, and
unresponsive, but the female presently arrived and went to the nest.
Later in the day the young were lively and responded freely when the
nest was approached, but nevertheless I was impressed with the length of
time during which the parents were absent; for, judging by the
experience of previous experiment, there seemed to be every likelihood
of their losing their offspring in such abnormally cold weather, unless
they brooded them more persistently. On the 11th June at 5.50 A.M.
neither parent was to be seen and the young could scarcely be made to
respond; but shortly afterwards both male and female appeared, and,
after remaining a few minutes, again disappeared without even
approaching the nest. At 6.45 A.M. no attempt had been made to brood and
the young were then so feeble that they were scarcely able to open their
mouths, and at 6 P.M. one was still alive but the remaining three were
dead. Yet the parents returned and the female went to the nest; and,
from a distance of a few feet, I watched her brooding the living and the
dead. At 5.45 A.M. the following day the remaining young bird had
succumbed, the temperature then being 49° F.
At the second nest, I was unable to watch the behaviour of the parents
so closely. On the 15th June the nest contained three young from three
to four days old, and during the morning of that and the succeeding day
nothing unusual occurred, with the exception that the period of exposure
seemed, as in the former case, to be too long. On the 17th June at 3.10
A.M. the young had collapsed and were stiff, but the parents were in
their territory and anxious apparently to attend to their brood. At 9.15
A.M. only two of the young were left in the nest, and though I searched
amongst the undergrowth and in the gorse bush in which the nest was
placed, no trace of the third bird was to be found. Of the two remaining
young, one was alive and responsive but the other was dead, and though
the female attended assiduously to the sole surviving offspring, yet it
too had succumbed by the following morning.
In a third territory, there was a nest containing four eggs. These eggs
were due to hatch at much the same time as those in the two nests just
referred to, but they failed to do so, and an examination showed that
they contained well developed but dead chicks.
To what can the death of the young and of the chicks in the eggs be
attributed? Not to any failure in the instinctive response of the
females, for they fed their young, they brooded them, they even brooded
the dead as well as the living, and probably did all that racial
preparation had fitted them to do. Yet the fact that the young in the
second nest were lifeless and exposed at 3 A.M. seems to betoken absence
on the part of the parents during the night, and may be interpreted as a
failure of the parental instinctive response. Let us return for a moment
to the experiments. These showed, it will be remembered, that a rise or
fall in the temperature of but a few degrees was sufficient to make an
astonishing difference in the length of time that the young were able to
survive without their parents; that when the temperature reached 58° F.
the bodies of the young retained their warmth, and that under such
conditions even a night's exposure had little, if any, effect; so that
even supposing that the parents were absent during the night, the death
of the young cannot be said to have been due to a failure of the
parental instinct, because under normal conditions--and under such has
their instinctive routine been evolved--their absence would not have
prejudiced the existence of the offspring. I attribute the collapse of
the young solely to the exceptional cold that prevailed at just the most
critical time, and I base this conclusion partly on the experience
gained from experiment, but mainly on their condition observed at
different intervals; for during exposure they collapsed rapidly, their
flesh became cold and their movements sluggish, their response grew
weak, and gradually they became more and more feeble until they could
scarcely close their bills after the mandibles had been forced asunder.
Yet, even after having reached so acute a stage of collapse, the warmth
from the body of the brooding bird was sufficient to restore them
temporarily; once more they would become lively and responsive, only,
however, to revert to the previous condition soon after the parent had
again abandoned them. Doubtless their power of resistance grew less and
less during each successive period of exposure.
If the nestling Bunting is to be freed from the risk of exposure, it is
evident that there must be, in the vicinity of the nest, an adequate
supply of food upon which the parents can draw liberally. Hence those
pairs that exercise dominion over the few acres surrounding the nest,
and are thus able to obtain food rapidly, will stand a better chance of
rearing their offspring than others which have no certain supply to draw
upon--and this, I believe, is one of the biological ends for which the
territory has been evolved. But it must not be supposed that each pair
finds, or even attempts to find, the whole of the food within its
territory, or that it is necessary for the theory that it should do so;
all that is required is that such overcrowding as might lead to
prolonged absence on the part of the parents and inordinate exposure of
the young shall be avoided. So that the problem has to be considered not
merely from the point of view of the individual, but from the larger
point of view of all the pairs inhabiting a given area.
Now there were eight pairs of Yellow Buntings occupying the one corner
of Hartlebury Common, and their territories in the aggregate covered
some fifteen acres. The birds obtained part of their food-supply amongst
the gorse and in some young scattered oak-trees, and part in an
adjoining coppice and on the surrounding arable land. But they were not
the sole occupants of this corner of the Common; other insectivorous
species had territories there also--amongst which were Whitethroats,
Grasshopper-Warblers, Willow-Warblers, Whinchats, Stonechats,
Meadow-Pipits, Tree-Pipits, and Skylarks. Suppose then that there had
been sixteen pairs of Yellow Buntings instead of eight; that there had
been other pairs, which assuredly there were, inhabiting the locality;
that they had also resorted, which assuredly they did, to the coppice
and arable ground for the purpose of securing food; and that their
numbers had also been increased in a similar ratio--would a supply of
food for all have been forthcoming with the necessary regularity and
promptitude? Well, the parents might have had to travel a little
farther; but even if they had been compelled to do so, their absence
would only have been prolonged by so many minutes the more, and under
normal conditions what harmful result to the offspring could possibly
have followed? The question for us, however, is not what might have
occurred under normal conditions, but whether the life behaviour is so
adjusted as to meet the exigencies of diverse, and in this case of
abnormal, circumstances. Now the capacity of the young to resist
exposure diminishes very rapidly when the temperature falls below the
normal--the danger zone seems to be reached at approximately 52° F., and
the length of time during which they survive then becomes astonishingly
short--and moreover the fall in the temperature would tend to decrease
the supply of insect life upon which they depend, so that if the size of
the territories had been reduced by one half, and the parents in
consequence had been compelled to seek their food at a greater distance,
can it be doubted that the cumulative effect of even a few minutes of
additional exposure would have been detrimental, if not disastrous, to
the offspring?
We speak, however, of the parents extending their journeys a little
farther in this direction or a little farther in that, as though they
could do so with impunity except in so far as it affected themselves, or
their offspring, or the other Yellow Buntings inhabiting that particular
area. But, most certainly, any extension would have meant so much
encroachment upon the available means of support of other members of the
species inhabiting adjoining areas, whose young in turn would have been
liable to have been affected; and, with even greater certainty, the
Whitethroats, the Stonechats, the Tree-Pipits, and the Willow-Warblers
that had also established themselves in that one corner of the Common
would have been hard pressed to find sufficient food with sufficient
rapidity.
Let me give another illustration of a somewhat different kind. Lapwings,
as we saw in the previous chapters, establish territories and guard them
from intrusion with scrupulous care. The young are able to leave the
nest soon after they are hatched, and consequently the parents are not
necessarily obliged to bring food _to_ them--they can, if they so
choose, lead them _to_ the food. Whether each pair limits its search for
food to its territory, I do not know. But even supposing that all
ownership of territory were to lapse directly the young were hatched,
that the boundaries were to cease to exist, and that the birds were free
to wander at will without fear of molestation, the end for which the
territory had been evolved would none the less have been obtained; for
inasmuch as the parents are accompanied by their young, it matters not
in what part of the meadow they seek their food; all that matters is
that the number of families shall not exceed the available supply of
food. So far, then, as the Lapwing is concerned, the territory fulfils
its purpose when once it limits the number of males, since, by doing so,
it limits the number of families and prevents undue pressure upon the
means of support.
Nevertheless, there are many birds that seem to rely entirely upon the
territory to supply them with all that is necessary. Each Warbler seeks
its food within the precincts of its own particular domain, and, except
in occasional instances, neither resorts to neutral ground nor makes
excursions into the locality immediately surrounding the territory, as
does the Bunting. Probably it would be disastrous if it attempted to do
so, for since its young at birth are so delicate and so susceptible to
changes of temperature, it cannot afford to be absent from them for
long. Of the two experiments made with young Whitethroats, one was made
under favourable and the other under unfavourable conditions. In this
latter case the temperature was 50° F., and the young, it may be
remembered, only survived for a little over one hour. Now exposure at
that temperature is evidently dangerous, but it would be still more
dangerous if the weather were wet instead of dry, and the temperature
46° F. instead of 50° F.; and it is, I imagine, on this account that the
impulse to brood is so strongly implanted in the female. No sooner, it
seems, does she depart than she returns with a small quantity of food
which she hurriedly distributes and immediately settles down to brood;
and if forcibly prevented from returning, her attitude betrays symptoms
of what, humanly speaking, we should term great distress. If, then, the
conditions in the external environment were such as would make it
difficult for the female to obtain food rapidly, what advantage would
she derive from so strongly developed an impulse? Might it not be a
disadvantage? Might it not mean that she would abandon the search too
readily and be content to return with an insufficient supply, and might
not that be as injurious to the young as prolonged exposure? Manifestly
the impulse to brood could only have developed strength in so far as it
fitted in with all the other factors that make for survival; and the
principal factor in the external environment seems to be the territory.
How could the young have been freed from the risk of exposure if the
impulse to brood had not been so strongly implanted in the parent? How
could the impulse to brood have been free to develop if a supply of food
had not been first insured? How could the supply of food have been
insured if numbers of the same species had been allowed to breed in
close proximity?
From the foregoing facts it is clear that the young of many species are
at birth susceptible to cold and unable to withstand prolonged exposure.
The parents must therefore be in a position to obtain food rapidly, and
consequently it is important that there should be an ample supply in the
vicinity of the nest. This end the territory certainly serves to
promote; it roughly insures that the bird population of a given area is
in proportion to the available means of subsistence, and it thus reduces
the risk of prolonged exposure to which the young are always liable.
This leads on to a consideration of those cases in which the question of
securing food is subordinate to the question of securing a station
suitable for reproduction.
I take the Guillemot as an example. In principle its behaviour is
similar to that of the Bunting; the male repairs to a definite place,
isolates itself, and becomes pugnacious. But the Guillemot is generally
surrounded by other Guillemots, and the birds are often so densely
packed along the ledges that there is scarcely standing room, so it
seems, for all of them. Nevertheless the isolation of the individual is,
in a sense, just as complete as that of the individual Bunting, for each
one is just as vigilant in resisting intrusion upon its few square feet
as the Bunting is in guarding its many square yards, so that the
evidence seems to show that that part of the inherited nature which is
the basis of the territory is much the same in both species. What we
have then to consider is, What is the biological value to the Guillemot
of an inherited nature which, for the Bunting, has utility in relation
to the supply of food for the young? Up to a point, the act of securing
a territory has like value for each respective species, whether the area
occupied be large or small--that is to say, it enables the one sex to
discover the other with reasonable promptitude.
For the greater part of the year, Guillemots live at sea; singly, in
twos or threes, or in small parties, they move upon the face of the
waters, extending their wanderings far away from land, out into the
broad ocean, where for weeks together they face the gales and heavy seas
of the Atlantic. But in due course and in response to internal organic
changes, they return, like the Warbler, to their breeding grounds--rocky
headlands or islands appropriately situated and affording the
appropriate rock formation. During all these months of wandering, the
majority seem to ignore the land, to pass away from it altogether, and
to spread themselves over the surface of the ocean regardless of
mainland or island. Some useful observations, which throw some light on
the distance that Guillemots are accustomed to wander from land, were
made by Lieut. B. R. Stewart during a number of voyages between various
ports in Great Britain and Ireland and ports in North America,
principally New York and Quebec. Thus, on the 24th March, large numbers
were seen in lat. 55° N., long. 24° W., five hundred miles approximately
from land, though on the following day--four hundred miles off Tory
Island--they were not so plentiful. Again, on the 1st October, in lat.
53° N., long. 27° W., seven hundred miles or so from land, one bird was
seen, whilst on the following day, in lat. 52° N., long. 21° W., a
single individual was washed on board by the heavy seas and seemed
little the worse for the adventure. Within two hundred miles of the west
coast of Ireland, he found them plentiful on various occasions. From
this it is clear that the circumstances under which the bird lives for
many months in succession must impose a considerable strain upon its
constitution; and how it is able to withstand the buffeting of wind and
water, to secure its food, and to endure, is a mystery. It is important,
therefore, that the young bird should be properly nourished and
protected from anything that might harm its constitution, and important,
too, that the parents should be freed from any undue strain during the
course of reproduction.
The conditions which the breeding station has to fulfil are threefold:
in the first place, it must be in proximity to the food-supply;
secondly, it must provide the necessary shelter for the egg and for the
helpless offspring; and, in the third place, it must be so situated that
the young can reach the water in safety. We will examine these
conditions one by one.
The proximity to the food-supply is a consideration of some importance.
The life of the Guillemot during the winter is a strenuous one; we know
that large numbers succumb in stormy weather, and we can infer that
slight constitutional defects might make all the difference between
failure and success; and, therefore, the less severely the constitution
of the parent is taxed during reproduction, and the more securely the
constitution of the offspring is built up, the greater prospect will
both have of resisting the hardships of the winter successfully. Much,
then, will depend upon the distance the parents have to travel in order
to obtain food. The farther the breeding station is removed from the
feeding ground the greater the physical strain which will be imposed
upon the birds, and the greater the chance will there be of the
offspring being improperly nourished. Now the food consists of small
fish, largely of sand-eels, which are secured in deep water, and the
abundance of which varies, possibly according to the nature of the
currents. Hence cliffs which are situated away from the water, or from
which the water recedes at low tide, or which are surrounded by an area
of shallow water, and are thus not in proximity to the feeding ground,
even though they may fulfil the second and third condition, will not
answer the requirements of a breeding station.
Of no less importance is the type of rock-formation. Not every formation
affords the necessary ledges upon which the egg can be deposited with
safety--the face of the cliff may be too smooth, or too jagged, or the
shelves may run at too acute an angle. Many of the large assemblages of
Guillemots in the British Islands are found where the rock is quartzite,
mica-schist, limestone, or chalk. The reason of this is that such rocks
are weathered along the planes of stratification, of jointing, of
cleavage, or of foliation--the strata being probably of unequal
durability--with the result that innumerable shelves, ledges, and
caverns, which are taken advantage of by the birds, form a network over
the face of the cliff. But only those ledges can be made use of which
are placed at a considerable height above the water, because, when the
cliff faces the open sea, the lower ones are liable to be washed in
stormy weather by the incoming swell and thus become untenable. There is
a small cove in the midst of the most precipitous part of the breeding
station at Horn Head, wherein the shingly shore shelves rapidly to the
Atlantic and faces to the west. Here, towards the end of July, young
Kittiwake Gulls can sometimes be found washed up on the beach--some
living, but in every stage of exhaustion, others dead, and in every
stage of decomposition; here is the young bird, recently caught by the
swell and thrown upon the shore, lying side by side with the remains of
others that had previously succumbed to starvation--on every side
evidence of the devastation wrought by the Atlantic. May not some of
this destruction have been brought about by the nests having been placed
upon the lower ledges within reach of an exceptionally heavy sea? Hence
much depends upon the nature of the rock-formation, and many a mighty
precipice, even though it may fulfil the first and third condition, is
nevertheless valueless as a breeding station.
Finally, the young bird must occupy a ledge from which it can reach the
water in safety. There is much difference of opinion as to the manner in
which it leaves the ledge, but all agree that it does so before it is
capable of sustained flight. If, then, the face of the cliffs were made
up of a series of broken precipices, or if the rocks at the base
projected out into the water, or if detached rocks abounded in the
waters beneath, the mortality amongst the chicks would no doubt be
considerable.
The coast-line of Co. Donegal will illustrate the foregoing remarks. On
the southern and western side of the Slieve League promontory there is
no real Guillemot station; only on the northern side--the quartzite in
the vicinity of Tormore--are the birds to be found in large numbers.
Northwards from here, a wild and rugged coast is passed over before
other stations are reached--at the eastern end of Tory Island and on
Horn Head; and beyond this, to the east, there are none, not even on the
old rocks that form the promontory of Inishowen. Why, we ask, do
countless numbers crowd the ledges of Horn Head, whilst they are absent
from the precipices of Slieve League; why, too, are they absent from
the granite cliffs of Owey? The reason is not far to seek. Either the
face of the cliff is made up of a series of broken precipices, or the
face of the precipices is too smooth, or the otherwise suitable ledges
are situated too near the water, or the water recedes from the base of
the cliff at low tide. Many miles of rock-bound coast are thus useless
for the purpose of reproduction.
Now when we bear in mind how large an expanse of coast is formed of
blown sand or of rocks of low altitude, and how many miles of cliff fail
to supply the three essential conditions that we have been considering,
we can see that suitable breeding stations must be limited both in
number and extent. From a wide expanse of ocean hosts of individuals are
therefore obliged to converge at certain definite points; and hence,
each recurring season, there must arise a competition for positions at
the station, just as there is competition between individual Buntings
for positions in the marsh. And the ability to obtain a position upon a
suitable ledge involves, in the first place, an impulse to search for
it; in the second place, an impulse to dwell in it; and in the third
place, an impulse to resist intrusion upon it. It would be useless for
an individual to be pugnacious if it had no fixed abode; equally useless
for it to establish itself on a particular ledge if it had no power to
defend it--all of which implies an inherited nature similar to that of
the Bunting. But the proximate end to which the competition is directed
is not alike in the case of both species. In the case of the Guillemot
it has reference solely to the piece of rock whereon the egg is laid; in
the case of the Bunting to a piece of ground capable of furnishing an
adequate supply of food for the young; and the reason for the difference
is this, that there is always an abundance of food in the water beneath
the cliff, but breeding stations are scarce, whereas there is always an
abundance of situations in the marsh in which the Bunting can place its
nest, but the supply of food varies and at times can only be obtained
with difficulty.
If then the Guillemot were to behave after the manner of the Bunting and
assign to itself a portion of the face of the cliff, or if it were only
to occupy a few ledges, or an even lesser area--a single ledge--what
would be the result? That it would attain to reproduction is beyond
question; that the egg would be safely deposited there can be no manner
of doubt; neither is there any reason to suppose that the offspring
would not be successfully reared. But, indirectly, its behaviour would
affect the Guillemot race. For if it be true, as the crowded ledges
certainly seem to show, that there is a dearth of suitable breeding
ground, no greater calamity could befall the species than that some
members should exercise dominion over too large an area of the habitable
part of the cliff and thus prevent others from breeding. Under such
conditions the race could not endure, since in this, as in every case,
its survival must depend upon a close correspondence between the
behaviour of the individual and the circumstances in the external
environment.
Scarcity of suitable cliffs is the principal reason of the ledges being
so closely packed with Guillemots, just as it accounts for this part of
the precipice being crowded with Kittiwake Gulls, that part with
Herring-Gulls, and that part again with Razorbills and Puffins. Yet each
individual preserves its few square feet of rock or soil from
molestation, and the area each one occupies varies according to the
conditions of existence of the species. Thus the Herring-Gull occupies a
comparatively small area, although one many times larger than that of
the Guillemot. It requires more space than the latter, owing to the fact
that it not only builds a nest but rears four instead of a single
offspring, and it can be allowed this, because, since its young remain
in the nest until they are capable of sustained flight, it can make use
of many miles of cliff from which the tide recedes at the base, or which
have, at their base, rocks jutting out into the sea; but manifestly it
cannot be allowed so much space as the Bunting.
Martins build in close proximity to one another, owing probably to
shortage of accommodation, and, in their case, the nests have to be so
situated as to be sheltered from the wet. If water drips upon them for
any length of time, the mud, of which they are composed, crumbles and
large pieces fall away, with the result that the eggs or the young are
precipitated to the ground. Consequently, not every house or
perpendicular cliff will answer the purpose of a breeding station. A few
pairs build their nests beneath the eaves close against the walls of my
house, and year after year the result is much the same; after every
downfall of rain, the water collects into rivulets, trickles down over
the eaves, is absorbed by the mud and destroys the nests. Thereupon, the
birds set to work and rebuild; but again the nest is destroyed, and
again they rebuild, and so on throughout the summer, and only on rare
occasions do they succeed in rearing offspring at the proper season.
Similar conditions must prevail in many situations; but, clearly, the
more binding and plastic the building material, the longer the nest will
withstand the action of the dripping water and the greater chance will
there be of the young being reared in safety. Observe, therefore, how
far-reaching an effect so small a detail as the nature of the mud can
have upon the status of the species in any given locality. Where the
conditions are favourable, there the birds must congregate to breed,
and, like the Guillemot, if each individual exercised dominion over too
large an area, the species as a whole would suffer.
In all these examples, the fact of different individuals being in such
close proximity may afford some protection from enemies both as regards
the egg and the offspring, and in so far as there is a mutual advantage
such assemblages may be spoken of as communities. A community, however,
in the true sense of the word, is a collection of individuals brought
together, not primarily as a result of shortage of breeding ground, but
in consequence of advantages of communal ownership over individual
ownership. A rookery is an example of a true community. Neither shortage
of nesting accommodation nor scarcity of food can account for Rooks
assembling together to breed; for if the different pairs which go to
make up the rookery were to scatter throughout the surrounding
neighbourhood, they would, as a rule, find plenty of trees in which to
build their nests, and plenty of food.
How, then, can the theory apply to a species that breeds under such
conditions? What part can the territory play in furthering the life of
the individual when large numbers of nests are built closely together in
the same tree? There is much evidence to show that mutual protection is
a necessary condition of the Rook's existence; many cases are on record
of rookeries being destroyed by Carrion-Crows, Hooded Crows, and Ravens.
For instance, Mr. Ward Fowler records a case in which a pair of Crows
attacked a small rookery, ransacked the nests, and destroyed the eggs,
with the result that not a single pair of Rooks was left in the
settlement. Each Rook must therefore secure a position within the
precincts of the community if it is to have a chance of success in the
attainment of reproduction. But every locality cannot supply sufficient
trees of the right kind, appropriately situated and in suitable relation
to the food supply, in which numbers of nests can be built in close
proximity; so that if more than one community were to attempt to
establish itself in a limited area, the supply of food or the supply of
trees might become a pressing problem. Each community must therefore be
prepared to defend its own interests, and each must be regarded as one
unit and the area occupied as one territory within which are included a
number of lesser territories. The individual may fail to establish
itself within a community, but, even if it succeeds, the community may
fail to establish the rights of communal ownership; hence it has to face
a twofold possibility of failure, and if it lacked the inherited nature
which leads the Guillemot to secure a position upon the ledge, or the
Bunting to obtain a position in the marsh, the chances are that it would
fail in the attainment of reproduction.
The question now arises as to how it comes about that the area occupied
by each individual conforms in broad outline to that which has proved
beneficial for the welfare of the species as a whole. We shall find that
up to a point the answer is a simple one. No one could study the
behaviour of animals without observing the important part that habit
plays in the life of the individual; an action performed to-day is
liable to be repeated to-morrow and the following day until it becomes
ingrained in the life of the individual. This must not be taken to mean,
however, that a particular action has to be performed for many days in
succession before it becomes definitely fixed; if only it is repeated a
number of times, even within the space of a few hours, it will acquire
sufficient strength for its continuance; but continued repetition gives
increased fixity, and, as time goes by, it becomes increasingly
difficult for the creature to make a change unless the character of the
situation necessitates readjustment.
For example, when the organic condition which leads to nest-building
becomes active, the bird tentatively collects some of the necessary
material in its bill, flies round with it, and then drops it. After a
while it collects some more, and this time leaves it perhaps in a bush.
Later on it makes another attempt, and, meeting with a situation which
calls forth the appropriate response, it thereupon lays the foundation
of the structure. We will assume that the nest is placed in the midst of
a tangled bush. Well, the bird lays the first strands of the foundation
and then goes in search of more material. The next time it approaches
the nest from the opposite side of the bush, and presently it finds yet
a third entrance. But each entrance is not made use of in turn: one is
employed more frequently than the other two, and in the course of time
becomes the sole highway to and from the nest. Suppose now that, when
the young are hatched, I cut away the foliage from the bush on the
opposite side from that on which the bird customarily enters, and by so
doing leave the nest exposed, what is the result? The female arrives
with food, threads her way through the bush, and, when beside the nest,
pauses as if aware that some change had taken place, and then flies away
through the new opening. In a short time she returns, flits from twig to
twig on the outskirts of the bush, and comes upon the new opening--there
she hesitates. But though the nest is in full view and within a few
inches of her perch, and though the young stretch out their necks, yet
so strong is the former habit that she is compelled to return to the
opposite side and approach the nest by the usual circuitous route before
she distributes the food amongst her offspring.
Let us see how far this law of habit formation may have been effective
in defining the extent of the area occupied. When a male Warbler arrives
at its destination in the spring it seeks out a suitable environment,
and, having found a place unoccupied by any other male, settles in it
and remains there--its behaviour up to this point being determined by
racial preparation. After the fatigue of the journey its movements are
at first sluggish; hunger, however, asserts itself and a search is made
for food; wandering away from the position in which it first settled and
which acts as a headquarters, it hunts through certain trees here or
certain bushes there and returns, and presently it wanders away again,
perhaps in another direction, but, as before, works its way back again
to the headquarters. The journeys thus radiate outwards from the
headquarters, and according to the success with which the bird meets,
so, probably, it happens that some trees are searched more often than
others and certain directions are taken more frequently than others, and
by constant repetition a routine is established which limits the
direction and scope of its wanderings.
But in the case of the Guillemot the conditions of existence are
reversed: food can be had in abundance but suitable breeding stations
are scarce. The few square feet of ledge correspond to the tree or clump
of bushes which acts as a headquarters for the Warbler, and the
occupation of them is determined, as it is in the case of the Warbler,
by racial preparation. Since, however, the ledge is only made use of for
the immediate purpose of incubation and is in no way affected by
questions relating to food, there is no occasion for the bird to wander
along the ledge nor to encroach upon those adjoining. Hunger stimulates
the Warbler to search the surrounding trees, and so to extend its area;
but hunger takes the Guillemot down to the water, and hence the area
which it primarily occupied remains unmodified.
To sum up: the territory is useful in various ways, but not necessarily
in the same way for every species. Reproduction would always have
remained fortuitous, and the number of individuals that attained to it
would seldom have reached the possible maximum unless some provision had
been included in its system for insuring that the males and females
could meet at the proper moment and afterwards remain in touch with one
another, and that the number of pairs inhabiting a given area did not
exceed the available means of support. I have tried to show that the
inclusion of a disposition to secure a territory tends to remove these
difficulties. In the first place, the disposition which leads to its
occupation comes into functional activity (in the male) early in the
season; and so, by the time that the appropriate pairing condition
arises in the females, the process of acquiring territories is well
advanced, and the males being regularly distributed, each in its
respective position, are readily found by their prospective mates. The
behaviour of each sex is thus adjusted to further the end of mutual
discovery. Next, after mating has taken place, the position occupied by
the male acts as a headquarters to which the birds can always repair,
and becomes a bond of union which is serviceable in that it prevents any
possibility of their drifting apart. And in the third place, the males
become pugnacious and in this way secure for themselves areas which vary
in size according to the conditions of existence of the species, so that
there is no possibility of too many congregating in this locality, and
all the less likelihood of too few finding their way to that; and hence,
on the average, different pairs are distributed throughout all suitable
localities. Furthermore, owing to the fact of their having a
headquarters, the male and female are allowed a freedom of movement
which otherwise they would only possess when the construction of the
nest had actually begun; they can seek their food independently, and,
even though paired, they can if necessary continue their winter routine
without risk of separation. This means that the organic condition which
leads to pairing, is free to develop in the female earlier than would be
the case if there were nothing in the external environment to attract
the pair to a particular spot; and the longer the period over which the
process of pairing can be spread, the greater chance will females have
of discovering mates, the less severe will the competition tend to
become, and, consequently, the smaller the percentage of individuals
that fail to obtain suitable partners.
In these ways the territory has been serviceable alike to a number of
species. But much as the questions of mutual discovery and regular
distribution may have influenced the course of its development, there
can, I think, be little doubt that, on the one hand, the supply of the
necessary accommodation for rearing offspring, and on the other, the
necessity for an adequate supply of food in close proximity to the nest,
have been the main determining factors, and have led to a wide
divergence in its function. At the one extreme the function is to insure
a plentiful supply of food for the young; at the other, to insure a
station suitable for rearing offspring. I took the Bunting and the
Guillemot as types of the two extremes. The young of the former species
are born in a very helpless state. They are susceptible to cold and
unable to withstand prolonged exposure, and therefore it is essential
that there should be an ample supply of food, upon which the parents can
draw liberally, in the vicinity of the nest. But the nest is placed in a
variety of situations, and accommodation in this respect may be said to
be unlimited. The young of the latter species are not so susceptible to
exposure, and moreover there is always an abundance of food in the
waters beneath the cliff; but ledges of rock, upon which the egg can be
securely deposited and the young successfully reared, are limited both
in number and extent. The position then is as follows: there are
situations in plenty in which hosts of Buntings can build their nests
but the supply of food is a difficulty, and if the respective areas of
different individuals were insufficient to supply them with the
necessary food with the necessary rapidity, they would run the risk of
losing their offspring and the species would not endure; on the other
hand, cliffs upon which the Guillemot can rear its young are limited,
but the supply of food presents no difficulty, and consequently the
smaller the area over which each individual exercises dominion, the
greater the number that will attain to reproduction and the greater
prospect the species will have of survival. The emphasis in the one case
lies on the fact that the area occupied must be sufficiently large; on
the other, on its being just sufficient and no more to accommodate the
egg. Hence the difference in the function at the opposite extremes is
brought about, not by modifications of the instinctive behaviour which
leads to the establishment and defence of the territory, but solely by
modifications in the size of the area occupied, in accordance with the
conditions prevailing in the external environment. No doubt, if we had
the life-histories of a sufficient number of species worked out, we
should find that the gradations were complete from the one extreme to
the other. We are justified in thinking that this must be so because in
many directions we can not only observe differences in the size of the
area occupied, but can recognise a close correspondence between those
differences and the conditions of life of the species. Thus the
Herring-Gull occupies a comparatively small area, though one which is
many times larger than that of the Guillemot. It requires more space
because it not only builds a nest but rears four instead of a single
offspring, and it can be allowed more space because the young remain in
the nest until they are capable of sustained flight, and consequently it
can make use of many miles of cliff from which the tide recedes at the
base, and which on this account are denied to the Guillemot, but
manifestly it cannot be allowed so much space as the Bunting, for then
comparatively few individuals would attain to reproduction.
Again, the Reed-Warbler inhabits swamps overgrown with the common reed,
and in such places insect life is abundant just at the time when the
young are hatched. But these swamps cover a comparatively small acreage
in the breeding range of the bird, and if each pair were to attempt to
establish dominion over an area equal, let us say, to that of the
Willow-Warbler, the species would have but a poor chance in the struggle
for existence. So that, in a case of this description, the supply of
food and the comparative scarcity of breeding stations have been factors
of like importance in the evolution of the territory.
Finally we were led to inquire as to how it comes about that the extent
of the area occupied by each individual is adapted to the circumstances
in which the individual finds itself; and we came to the conclusion that
the movements of the bird, subsequent to the initial act of establishing
itself in a position, are regulated and defined by the law of habit
formation. For example, the Warbler, in response to its inherited
nature, takes up a position in an appropriate situation. It then
proceeds to search for food; it makes short journeys first in this
direction and then in that; it repeats these journeys, and gradually
forms a habit which compels it to remain within more or less
well-defined boundaries. But the actual distance that it traverses on
the occasion of its first attempt must be determined by the relative
abundance or scarcity of the particular kind of insect life which it
requires. So that, although habit defines and in some measure helps to
determine the boundaries of the territory, it is clear that in the last
resort they must depend upon the nature of the conditions in the
external environment.
We have, then, the congenital basis which leads to the occupation of a
position, and to the enmity shown by the owner of the position towards
other individuals; and this congenital basis is found alike in many
widely divergent forms, living under equally widely divergent
conditions; we have acquired accommodation; and we have relationships in
the organic and inorganic world--and the outcome of it all is a system
of behaviour which we, who can perceive the end to which such behaviour
is tending, are justified in speaking of as "a disposition to secure a
territory." In the development of this system a primary value must be
ascribed to the conditions in the external environment, for they
determine the direction of the variations of instinctive procedure and
of acquired habit which work towards the same goal--that of adjustment
to the conditions of life.
NOTE
The following are the experiments referred to on page 181:--
On the 14th May 1915, a nest of Blackbirds approximately four days old
was removed at 6.45 A.M. The temperature was considerably below the
normal, and snow lay on all the high ground in the neighbourhood. In a
short time the birds collapsed, and at 9.15 A.M. were dead. On the 29th
May, at 6 A.M., a second nest was removed, containing young of
approximately the same age, and although the conditions were more
normal, the temperature being 50° F., the birds collapsed at 8 A.M., and
an hour later one of the brood showed little signs of life. The wind,
however, then changed to the west, and the temperature rose one degree,
with the result that they were still living at 11 A.M. A further
experiment was made with Song-Thrushes on the 5th June. The wind was in
the south and the temperature 63° F. The young, approximately four days
old, were removed at 7.25 A.M., but as they showed no signs of collapse
at 1 P.M. I replaced the nest in the original site.
On the 30th May, a nest of Whitethroats three days old was removed at
7.15 A.M. The wind was northerly and the weather fine, but the
temperature low--50° F. At 8.15 A.M. the birds showed no sign of life. A
second experiment with this species was made on the 10th June under more
favourable circumstances, for although the sky was overcast and the wind
northerly, the temperature was 59° F. In this case the young survived
from 6.55 A.M. to 7 P.M.
On the 27th May 1915, a nest of Hedge-Sparrows hatched the previous day
was removed at 7 A.M. The temperature was below the normal, being 49° F.
At 8 A.M. the young were cold and in a state of collapse, but they
survived nevertheless until 3.20 P.M.
On the 7th June 1915, a nest of young Skylarks three days old was
removed at 7.15 A.M. The temperature was 62° F., and the birds survived
until 4 A.M. the next day.
On the 6th June 1916, a nest of Linnets just hatched was removed at 6.47
A.M. The temperature was 51° F. At 7.50 A.M. the birds were cold and in
a state of collapse, and only survived until 8.50 A.M.