has been supposed to prove a nest-building instinct, we may, therefore,
fairly impute to the uniformity of the conditions under which each
species lives. Their range is often very limited, and they very seldom
permanently change their country, so as to be placed in new conditions.
When, however, new conditions do occur, they take advantage of them just
as freely and wisely as man could do. The chimney and house-swallows are
a standing proof of a change of habit since chimneys and houses were
built, and in America this change has taken place within about three
hundred years. Thread and worsted are now used in many nests instead of
wool and horsehair, and the jackdaw shows an affection for the church
steeple which can hardly be explained by instinct. In the more thickly
populated parts of the United States, the Baltimore oriole uses all
sorts of pieces of string, skeins of silk, or the gardener's bass, to
weave into its fine pensile nest, instead of the single hairs and
vegetable fibres it has painfully to seek in wilder regions; and Wilson,
a most careful observer, believes that it improves in nest-building by
practice--the older birds making the best nests. The purple martin takes
possession of empty gourds or small boxes, stuck up for its reception in
almost every village and farm in America; and several of the American
wrens will also build in cigar boxes, with a small hole cut in them, if
placed in a suitable situation. The orchard oriole of the United States
offers us an excellent example of a bird which modifies its nest
according to circumstances. When built among firm and stiff branches the
nest is very shallow, but if, as is often the case, it is suspended from
the slender twigs of the weeping willow, it is made much deeper, so that
when swayed about violently by the wind the young may not tumble out. It
has been observed also, that the nests built in the warm Southern States
are much slighter and more porous in texture than those in the colder
regions of the north. Our own house-sparrow equally well adapts himself
to circumstances. When he builds in trees, as he, no doubt, always did
originally, he constructs a well-made domed nest, perfectly fitted to
protect his young ones; but when he can find a convenient hole in a
building or among thatch, or in any well-sheltered place, he takes much
less trouble, and forms a very loosely-built nest.
A curious example of a recent change of habits has occurred in Jamaica.
Previous to 1854, the palm swift (Tachornis phaenicobea) inhabited
exclusively the palm trees in a few districts in the island. A colony
then established themselves in two cocoa-nut palms in Spanish Town, and
remained there till 1857, when one tree was blown down, and the other
stripped of its foliage. Instead of now seeking out other palm trees,
the swifts drove out the swallows who built in the Piazza of the House
of Assembly, and took possession of it, building their nests on the tops
of the end walls and at the angles formed by the beams and joists, a
place which they continue to occupy in considerable numbers. It is
remarked that here they form their nest with much less elaboration than
when built in the palms, probably from being less exposed.
A still more curious example of change and improvement in nest building
was published by Mr. F. A. Pouchet, in the tenth number of the _Comptes
Rendus_ for 1870, just as the first edition of this work appeared. Forty
years ago M. Pouchet had himself collected nests of the House-Martin or
Window-Swallow (_Hirundo urbica_) from old buildings at Rouen, and
deposited them in the museum of that city. On recently obtaining some
more nests he was surprised, on comparing them with the old ones, to
find that they exhibited a decided change of form and structure. This
led him to investigate the matter more closely. The changed nests had
been obtained from houses in a newly erected quarter of the city, and he
found that all the nests in the newly-built streets were of the new
form. But on visiting the churches and older buildings, and some rocks
where these birds build, he found many nests of the old type along with
some of the new pattern. He then examined all the figures and
descriptions of the older naturalists, and found that they invariably
represented the older form only.
The difference between the two forms he states to be as follows. In the
old form the nest is a portion of a globe--when situated in the upper
angle of a window one-fourth of a hemisphere--and the opening is very
small and circular, being of a size just sufficient to allow the body of
the bird to pass. In the new form the nest is much wider in proportion
to its height, being a segment of a depressed spheroid, and the aperture
is very wide and shallow, and close to the horizontal surface to which
the nest is attached above.
M. Pouchet thinks that the new form is an undoubted improvement on the
old. The nest has a wider bottom and must allow the young ones to have
more freedom of motion than in the old narrower, and deeper nests, and
its wide aperture allows the young birds to peep out and breathe the
fresh air. This is so wide as to serve as a sort of balcony for them,
and two young ones can often be seen on it without interfering with the
passage in and out of the old birds. At the same time, by being so close
to the roof, it is a better protection against rain, against cold, and
against enemies, than the small round hole of the old nests. Here, then,
we have an improvement in nest building, as well marked as any
improvement that takes place in human dwellings in so short a time.
But perfection of structure and adaptation to purpose, are not universal
characteristics of birds' nests, since there are decided imperfections
in the nesting of many birds which are quite compatible with our present
theory, but are hardly so with that of instinct, which is supposed to be
infallible. The Passenger pigeon of America often crowds the branches
with its nests till they break, and the ground is strewn with shattered
nests, eggs, and young birds. Rooks' nests are often so imperfect that
during high winds the eggs fall out; but the Window-Swallow is the most
unfortunate in this respect, for White, of Selborne, informs us that he
has seen them build, year after year, in places where their nests are
liable to be washed away by a heavy rain and their young ones destroyed.
_Conclusion._
A fair consideration of all these facts will, I think, fully support the
statement with which I commenced, and show, that the mental faculties
exhibited by birds in the construction of their nests, are the same in
kind as those manifested by mankind in the formation of their dwellings.
These are, essentially, imitation, and a slow and partial adaptation to
new conditions. To compare the work of birds with the highest
manifestations of human art and science, is totally beside the question.
I do not maintain that birds are gifted with reasoning faculties at all
approaching in variety and extent to those of man. I simply hold that
the phenomena presented by their mode of building their nests, when
fairly compared with those exhibited by the great mass of mankind in
building their houses, indicate no essential difference in the kind or
nature of the mental faculties employed. If instinct means anything, it
means the capacity to perform some complex act without teaching or
experience. It implies innate ideas of a very definite kind, and, if
established, would overthrow Mr. Mill's sensationalism and all the
modern philosophy of experience. That the existence of true instinct may
be established in other cases is not impossible, but in the particular
instance of birds' nests, which is usually considered one of its
strongholds, I cannot find a particle of evidence to show the existence
of anything beyond those lower reasoning and imitative powers, which
animals are universally admitted to possess.
VII.
A THEORY OF BIRDS' NESTS;
SHOWING THE RELATION OF CERTAIN DIFFERENCES OF COLOUR IN FEMALE
BIRDS, TO THEIR MODE OF NIDIFICATION.
The habit of forming a more or less elaborate structure for the
reception of their eggs and young, must undoubtedly be looked upon as
one of the most remarkable and interesting characteristics of the class
of birds. In other classes of vertebrate animals, such structures are
few and exceptional, and never attain to the same degree of completeness
and beauty. Birds' nests have, accordingly, attracted much attention,
and have furnished one of the stock arguments to prove the existence of
a blind but unerring instinct in the lower animals. The very general
belief that every bird is enabled to build its nest, not by the ordinary
faculties of observation, memory, and imitation, but by means of some
innate and mysterious impulse, has had the bad effect of withdrawing
attention from the very evident relation that exists between the
structure, habits, and intelligence of birds, and the kind of nests they
construct.
In the preceding essay I have detailed several of these relations, and
they teach us, that a consideration of the structure, the food, and
other specialities of a bird's existence, will give a clue, and
sometimes a very complete one, to the reason why it builds its nest of
certain materials, in a definite situation, and in a more or less
elaborate manner.
I now propose to consider the question from a more general point of
view, and to discuss its application to some important problems in the
natural history of birds.
_Changed Conditions and persistent Habits as influencing Nidification._
Besides the causes above alluded to, there are two other factors whose
effect in any particular case we can only vaguely guess at, but which
must have had an important influence in determining the existing details
of nidification. These are--changed conditions of existence, whether
internal or external, and the influence of hereditary or imitative
habit; the first inducing alterations in accordance with changes of
organic structure, of climate, or of the surrounding fauna and flora;
the other preserving the peculiarities so produced, even when changed
conditions render them no longer necessary. Many facts have been already
given which show that birds do adapt their nests to the situations in
which they place them, and the adoption of eaves, chimneys, and boxes,
by swallows, wrens, and many other birds, shows that they are always
ready to take advantage of changed conditions. It is probable,
therefore, that a permanent change of climate would cause many birds to
modify the form or materials of their abodes, so as better to protect
their young. The introduction of new enemies to eggs or young birds,
might introduce many alterations tending to their better concealment. A
change in the vegetation of a country, would often necessitate the use
of new materials. So, also, we may be sure, that as a species slowly
became modified in any external or internal characters, it would
necessarily change in some degree its mode of building. This effect
would be produced by modifications of the most varied nature; such as
the power and rapidity of flight, which must often determine the
distance to which a bird will go to obtain materials for its nest; the
capacity of sustaining itself almost motionless in the air, which must
sometimes determine the position in which a nest can be built; the
strength and grasping power of the foot in relation to the weight of the
bird, a power absolutely essential to the constructor of a
delicately-woven and well-finished nest; the length and fineness of the
beak, which has to be used like a needle in building the best textile
nests; the length and mobility of the neck, which is needful for the
same purpose; the possession of a salivary secretion like that used in
the nests of many of the swifts and swallows, as well as that of the
song-thrush--peculiarities of habits, which ultimately depend on
structure, and which often determine the material most frequently met
with or most easily to be obtained. Modifications in any of these
characters would necessarily lead, either to a change in the materials
of the nest, or in the mode of combining them in the finished
structure, or in the form or position of that structure.
During all these changes, however, certain specialities of nest-building
would continue, for a shorter or a longer time after the causes which
had necessitated them had passed away. Such records of a vanished past
meet us everywhere, even in man's works, notwithstanding his boasted
reason. Not only are the main features of Greek architecture, mere
reproductions in stone of what were originally parts of a wooden
building, but our modern copyists of Gothic architecture often build
solid buttresses capped with weighty pinnacles, to support a wooden roof
which has no outward thrust to render them necessary; and even think
they ornament their buildings by adding sham spouts of carved stone,
while modern waterpipes, stuck on without any attempt at harmony, do the
real duty. So, when railways superseded coaches, it was thought
necessary to build the first-class carriages to imitate a number of
coach-bodies joined together; and the arm-loops for each passenger to
hold on by, which were useful when bad roads made every journey a
succession of jolts and lurches, were continued on our smooth
macadamised mail-routes, and, still more absurdly, remain to this day in
our railway carriages, the relic of a kind of locomotion we can now
hardly realize. Another good example is to be seen in our boots. When
elastic sides came into fashion we had been so long used to fasten them
with buttons or laces, that a boot without either looked bare and
unfinished, and accordingly the makers often put on a row of useless
buttons or imitation laces, because habit rendered the appearance of
them necessary to us. It is universally admitted that the habits of
children and of savages give us the best clue to the habits and mode of
thought of animals; and every one must have observed how children at
first imitate the actions of their elders, without any regard to the use
or applicability of the particular acts. So, in savages, many customs
peculiar to each tribe are handed down from father to son merely by the
force of habit, and are continued long after the purpose which they
originally served has ceased to exist. With these and a hundred similar
facts everywhere around us, we may fairly impute much of what we cannot
understand in the details of Bird-Architecture to an analogous cause. If
we do not do so, we must assume, either that birds are guided in every
action by pure reason to a far greater extent than men are, or that an
infallible instinct leads them to the same result by a different road.
The first theory has never, that I am aware of, been maintained by any
author, and I have already shown that the second, although constantly
assumed, has never been proved, and that a large body of facts is
entirely opposed to it. One of my critics has, indeed, maintained that I
admit "instinct" under the term "hereditary habit;" but the whole course
of my argument shows that I do not do so. Hereditary habit is, indeed,
the same as instinct when the term is applied to some simple action
dependent upon a peculiarity of structure which is hereditary; as when
the descendants of tumbler pigeons tumble, and the descendants of pouter
pigeons pout. In the present case, however, I compare it strictly to the
hereditary, or more properly, persistent or imitative, habits of
savages, in building their houses as their fathers did. Imitation is a
lower faculty than invention. Children and savages imitate before they
originate; birds, as well as all other animals, do the same.
The preceding observations are intended to show, that the exact mode of
nidification of each species of bird is probably the result of a variety
of causes, which have been continually inducing changes in accordance
with changed organic or physical conditions. The most important of these
causes seem to be, in the first place, the structure of the species,
and, in the second, its environment or conditions of existence. Now we
know, that every one of the characters or conditions included under
these two heads is variable. We have seen that, on the large scale, the
main features of the nest built by each group of birds, bears a relation
to the organic structure of that group, and we have, therefore, a right
to infer, that as structure varies, the nest will vary also in some
particular corresponding to the changes of structure. We have seen also,
that birds change the position, the form, and the construction of their
nest, whenever the available materials or the available situations, vary
naturally or have been altered by man; and we have, therefore, a right
to infer that similar changes have taken place, when, by a natural
process, external conditions have become in any way permanently altered.
We must remember, however, that all these factors are very stable during
many generations, and only change at a rate commensurate with those of
the great physical features of the earth as revealed to us by geology;
and we may, therefore, infer that the form and construction of nests,
which we have shown to be dependent on them, are equally stable. If,
therefore, we find less important and more easily modified characters
than these, so correlated with peculiarities of nidification as to
indicate that one is probably the cause of the other, we shall be
justified in concluding that these variable characters are dependent on
the mode of nidification, and not that the form of the nest has been
determined by these variable characters. Such a correlation I am now
about to point out.
_Classification of Nests._
For the purpose of this inquiry it is necessary to group nests into two
great classes, without any regard to their most obvious differences or
resemblances, but solely looking to the fact of whether the contents
(eggs, young, or sitting bird) are hidden or exposed to view. In the
first class we place all those in which the eggs and young are
completely hidden, no matter whether this is effected by an elaborate
covered structure, or by depositing the eggs in some hollow tree or
burrow underground. In the second, we group all in which the eggs,
young, and sitting bird are exposed to view, no matter whether there is
the most beautifully formed nest, or none at all. Kingfishers, which
build almost invariably in holes in banks; Woodpeckers and Parrots,
which build in hollow trees; the Icteridae of America, which all make
beautiful covered and suspended nests; and our own Wren, which builds a
domed nest, are examples of the former; while our Thrushes, Warblers,
and Finches, as well as the Crowshrikes, Chatterers, and Tanagers of the
tropics, together with all Raptorial birds and Pigeons, and a vast
number of others in every part of the world, all adopt the latter mode
of building.
It will be seen that this division of birds according to their
nidification, bears little relation to the character of the nest itself.
It is a functional not a structural classification. The most rude and
the most perfect specimens of bird-architecture are to be found in both
sections. It has, however, a certain relation to natural affinities, for
large groups of birds, undoubtedly allied, fall into one or the other
division exclusively. The species of a genus or of a family are rarely
divided between the two primary classes, although they are frequently
divided between the two very distinct modes of nidification that exist
in the first of them.
All the Scansorial or climbing, and most of the Fissirostral or
wide-gaped birds, for example, build concealed nests; and, in the latter
group, the two families which build open nests, the Swifts and the
Goat-suckers, are undoubtedly very widely separated from the other
families with which they are associated in our classifications. The
Tits vary much in their mode of nesting, some making open nests
concealed in a hole, while others build domed or even pendulous covered
nests, but they all come under the same class. Starlings vary in a
similar way. The talking Mynahs, like our own starlings, build in holes,
the glossy starlings of the East (of the genus Calornis) form a hanging
covered nest, while the genus Sturnopastor builds in a hollow tree. One
of the most striking cases in which one family of birds is divided
between the two classes, is that of the Finches; for while most of the
European species build exposed nests, many of the Australian finches
make them dome-shaped.
_Sexual differences of Colour in Birds._
Turning now from the nests to the creatures who make them, let us
consider birds themselves from a somewhat unusual point of view, and
form them into separate groups, according as both sexes, or the males
only, are adorned with conspicuous colours.
The sexual differences of colour and plumage in birds are very
remarkable, and have attracted much attention; and, in the case of
polygamous birds, have been well explained by Mr. Darwin's principle of
sexual selection. We can, to a great extent, understand how male
Pheasants and Grouse have acquired their more brilliant plumage and
greater size, by the continual rivalry of the males both in strength and
beauty; but this theory does not throw any light on the causes which
have made the female Toucan, Bee-eater, Parroquet, Macaw and Tit, in
almost every case as gay and brilliant as the male, while the gorgeous
Chatterers, Manakins, Tanagers, and Birds of Paradise, as well as our
own Blackbird, have mates so dull and inconspicuous that they can hardly
be recognised as belonging to the same species.
_The Law which connects the Colours of Female Birds with the mode of
Nidification._
The above-stated anomaly can, however, now be explained by the influence
of the mode of nidification, since I find that, with but very few
exceptions, it is the rule--_that when both sexes are of strikingly gay
and conspicuous colours, the nest is of the first class, or such as to
conceal the sitting bird; while, whenever there is a striking contrast
of colours, the male being gay and conspicuous, the female dull and
obscure, the nest is open and the sitting bird exposed to view_. I will
now proceed to indicate the chief facts that support this statement, and
will afterwards explain the manner in which I conceive the relation has
been brought about.
We will first consider those groups of birds in which the female is
gaily or at least conspicuously coloured, and is in most cases exactly
like the male.
1. Kingfishers (Alcedinidae). In some of the most brilliant species of
this family the female exactly resembles the male; in others there is a
sexual difference, but it rarely tends to make the female less
conspicuous. In some, the female has a band across the breast, which is
wanting in the male, as in the beautiful Halcyon diops of Ternate. In
others the band is rufous in the female, as in several of the American
species; while in Dacelo gaudichaudii, and others of the same genus, the
tail of the female is rufous, while that of the male is blue. In most
kingfishers the nest is in a deep hole in the ground; in Tanysiptera it
is said to be in a hole in the nests of termites, or sometimes in
crevices under overhanging rocks.
2. Motmots (Momotidae). In these showy birds the sexes are exactly alike,
and the nest in a hole under ground.
3. Puff-birds (Bucconidae). These birds are often gaily coloured; some
have coral-red bills; the sexes are exactly alike, and the nest is in a
hole in sloping ground.
4. Trogons (Trogonidae). In these magnificent birds the females are
generally less brightly coloured than the males, but are yet often gay
and conspicuous. The nest is in a hole of a tree.
5. Hoopoes (Upupidae). The barred plumage and long crests of these birds
render them conspicuous. The sexes are exactly alike, and the nest is in
a hollow tree.
6. Hornbills (Bucerotidae). These large birds have enormous coloured
bills, which are generally quite as well coloured and conspicuous in the
females. Their nests are always in hollow trees, where the female is
entirely concealed.
7. Barbets (Capitonidae). These birds are all very gaily-coloured, and,
what is remarkable, the most brilliant patches of colour are disposed
about the head and neck, and are very conspicuous. The sexes are
exactly alike, and the nest is in a hole of a tree.
8. Toucans (Rhamphastidae). These fine birds are coloured in the most
conspicuous parts of their body, especially on the large bill, and on
the upper and lower tail coverts, which are crimson, white, or yellow.
The sexes are exactly alike, and they always build in a hollow tree.