Contributions to the Theory of Natural Selection Part 12

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.

_Cla.s.sification of Nests._

For the purpose of this inquiry it is necessary to group nests into two great cla.s.ses, 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 cla.s.s 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; Woodp.e.c.k.e.rs 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 cla.s.sification. 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 cla.s.ses, 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 a.s.sociated in our cla.s.sifications. The t.i.ts 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 cla.s.s. 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 cla.s.ses, is that of the Finches; for while most of the European species build exposed nests, many of the Australian finches make them dome-shaped.

_s.e.xual 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 s.e.xes, or the males only, are adorned with conspicuous colours.

The s.e.xual 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 s.e.xual 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 t.i.t, 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 s.e.xes are of strikingly gay and conspicuous colours, the nest is of the first cla.s.s, 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 s.e.xual 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 s.e.xes 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 s.e.xes 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 s.e.xes 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 s.e.xes 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 s.e.xes are exactly alike, and they always build in a hollow tree.

9. Plaintain-eaters (Musophagidae). Here again the head and bill are most brilliantly coloured in both s.e.xes, and the nest is in a hole of a tree.

10. Ground cuckoos (Centropus). These birds are often of conspicuous colours, and are alike in both s.e.xes. They build a domed nest.

11. Woodp.e.c.k.e.rs (Picidae). In this family the females often differ from the males, in having a yellow or white, instead of a crimson crest, but are almost as conspicuous. They all nest in holes in trees.

12. Parrots (Psittaci). In this great tribe, adorned with the most brilliant and varied colours, the rule is, that the s.e.xes are precisely alike, and this is the case in the most gorgeous families, the lories, the c.o.c.katoos, and the macaws; but in some there is a s.e.xual difference of colour to a slight extent. All build in holes, mostly in trees, but sometimes in the ground, or in white ants' nests. In the single case in which the nest is exposed, that of the Australian ground parrot, Pezoporus formosus, the bird has lost the gay colouring of its allies, and is clothed in sombre and completely protective tints of dusky green and black.

13. Gapers (Eurylaemidae). In these beautiful Eastern birds, somewhat allied to the American chatterers, the s.e.xes are exactly alike, and are adorned with the most gay and conspicuous markings. The nest is a woven structure, _covered over_, and suspended from the extremities of branches over water.

14. Pardalotus (Ampelidae). In these Australian birds the females differ from the males, but are often very conspicuous, having brightly-spotted heads. Their nests are sometimes dome-shaped, sometimes in holes of trees, or in burrows in the ground.

15. t.i.ts (Paridae). These little birds are always pretty, and many (especially among the Indian species) are very conspicuous. They always have the s.e.xes alike, a circ.u.mstance very unusual among the smaller gaily-coloured birds of our own country. The nest is always covered over or concealed in a hole.

16. Nuthatches (Sitta). Often very pretty birds, the s.e.xes alike, and the nest in a hole.

17.---- (Sittella). The female of these Australian nuthatches is often the most conspicuous, being white-and black-marked. The nest is, according to Gould, "completely concealed among upright twigs connected together."

18. Creepers (Climacteris). In these Australian creepers the s.e.xes are alike, or the female most conspicuous, and the nest is in a hole of a tree.

19. Estrelda, Amadina. In these genera of Eastern and Australian finches the females, although more or less different from the males, are still very conspicuous having a red rump, or being white spotted. They differ from most others of the family in building domed nests.

20. Certhiola. In these pretty little American creepers the s.e.xes are alike, and they build a domed nest.

21. Mynahs (Sturnidae). These showy Eastern starlings have the s.e.xes exactly alike. They build in holes of trees.

22. Calornis (Sturnidae). These brilliant metallic starlings have no s.e.xual differences. They build a pensile covered nest.

23. Hangnests (Icteridae). The red or yellow and black plumage of most of these birds is very conspicuous, and is exactly alike in both s.e.xes.

They are celebrated for their fine purse-shaped pensile nests.

It will be seen that this list comprehends six important families of Fissirostres, four of Scansores, the Psittaci, and several genera, with three entire families of Pa.s.seres, comprising about twelve hundred species, or about one-seventh of all known birds.

The cases in which, whenever the male is gaily coloured, the female is much less gay or quite inconspicuous, are exceedingly numerous, comprising, in fact, almost all the bright-coloured Pa.s.seres, except those enumerated in the preceding cla.s.s. The following are the most remarkable:--

1. Chatterers (Cotingidae). These comprise some of the most gorgeous birds in the world, vivid blues, rich purples, and bright reds, being the most characteristic colours. The females are always obscurely tinted, and are often of a greenish hue, not easily visible among the foliage.

2. Manakins (Pipridae). These elegant birds, whose caps or crests are of the most brilliant colours, are usually of a sombre green in the female s.e.x.

3. Tanagers (Tanagridae). These rival the chatterers in the brilliancy of their colours, and are even more varied. The females are generally of plain and sombre hues, and always less conspicuous than the males.

In the extensive families of the warblers (Sylviadae), thrushes (t.u.r.didae), flycatchers (Muscicapidae), and shrikes (Laniadae), a considerable proportion of the species are beautifully marked with gay and conspicuous tints, as is also the case in the Pheasants and Grouse; but in every case the females are less gay, and are most frequently of the very plainest and least conspicuous hues. Now, throughout _the whole of these families the nest is open_, and I am not aware of a single instance in which any one of these birds builds a _domed nest_, or places it in a _hole of a tree_, or _underground_, or in any place where it is effectually concealed.

In considering the question we are now investigating, it is not necessary to take into account the larger and more powerful birds, because these seldom depend much on concealment to secure their safety.

In the raptorial birds bright colours are as a rule absent; and their structure and habits are such as not to require any special protection for the female. The larger waders are sometimes very brightly coloured in both s.e.xes; but they are probably little subject to the attacks of enemies, since the scarlet ibis, the most conspicuous of birds, exists in immense quant.i.ties in South America. In game birds and water-fowl, however, the females are often very plainly coloured, when the males are adorned with brilliant hues; and the abnormal family of the Megapodidae offers us the interesting fact of an ident.i.ty in the colours of the s.e.xes (which in Megacephalon and Talegalla are somewhat conspicuous), in conjunction with the habit of not sitting on the eggs at all.

_What the Facts Teach us._

Taking the whole body of evidence here brought forward, embracing as it does almost every group of bright-coloured birds, it will, I think, be admitted that the relation between the two series of facts in the colouring and nidification of birds has been sufficiently established.

There are, it is true, a few apparent and some real exceptions, which I shall consider presently; but they are too few and unimportant to weigh much against the ma.s.s of evidence on the other side, and may for the present be neglected. Let us then consider what we are to do with this unexpected set of correspondences between groups of phenomena which, at first sight, appear so disconnected. Do they fall in with any other groups of natural phenomena? Do they teach us anything of the way in which nature works, and give us any insight into the causes which have brought about the marvellous variety, and beauty, and harmony of living things? I believe we can answer these questions in the affirmative; and I may mention, as a sufficient proof that these are not isolated facts, that I was first led to see their relation to each other by the study of an a.n.a.logous though distinct set of phenomena among insects, that of protective resemblance and "mimicry."

On considering this remarkable series of corresponding facts, the first thing we are taught by them seems to be, that there is no incapacity in the female s.e.x among birds, to receive the same bright hues and strongly contrasted tints with which their partners are so often decorated, since whenever they are _protected and concealed_ during the period of incubation _they are similarly adorned_. The fair inference is, that it is chiefly due to the absence of protection or concealment during this important epoch, that gay and conspicuous tints are withheld or left undeveloped. The mode in which this has been effected is very intelligible, if we admit the action of natural and s.e.xual selection. It would appear from the numerous cases in which both s.e.xes are adorned with equally brilliant colours (while both s.e.xes are rarely armed with equally developed offensive and defensive weapons when not required for individual safety), that the normal action of "s.e.xual selection" is to develop colour and beauty in both s.e.xes, by the preservation and multiplication of all varieties of colour in either s.e.x which are pleasing to the other. Several very close observers of the habits of animals have a.s.sured me, that male birds and quadrupeds do often take very strong likes and dislikes to individual females, and we can hardly believe that the one s.e.x (the female) can have a general taste for colour while the other has no such taste. However this may be, the fact remains, that in a vast number of cases the female acquires as brilliant and as varied colours as the male, and therefore most probably acquires them in the same way as the male does; that is, either because the colour is useful to it, or is correlated with some useful variation, or is pleasing to the other s.e.x. The only remaining supposition is that it is transmitted from the other s.e.x, without being of any use. From the number of examples above adduced of bright colours in the female, this would imply that colour-characters acquired by one s.e.x are generally (but not necessarily) transmitted to the other. If this be the case it will, I think, enable us to explain the phenomena, even if we do not admit that the male bird is ever influenced in the choice of a mate by her more gay or perfect plumage.