the nests were built comparatively open, so that the bird within was not concealed.1 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 open 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.

Professor Jeitteles of Vienna has described various forms of nests of Hirundo urbica adapted to different situations, some having the form of a semi-ellipsoid placed vertically, with the entrance at one side, others being three-quarters of a sphere, with the entrance in the centre. A nest of Hirundo rustica was also observed supported on an iron hook in a wall, but not itself touching the wall. It was quite hemispherical, like that of a blackbird, a form common in England, whereas the usual form on the Continent is that of a quarter of a sphere.2

The following case of a recent change of habit in nestbuilding was communicated to me by Mr. Henry Reeks in 1870 "Thirty years ago, and perhaps less, the herring-gulls used to breed on some inland rocks in a large lake called

1 Popular Science Monthly, vol. vi. p. 481. Quoted by Vice-President E. S. Morse, in Address to American Association for Advancement of Science at Buffalo, N. Y., August 1876.

2 Ornithologischer Verein in Wien. Mitthelungen des Ausschusses, No. 3, 12 Juli 1876. See also Seebohm's British Birds, vol. ii. p. 174.

'Parsons Pond,' in Newfoundland, which is separated from the sea only by a high pebbly beach. Within the period above stated high tides and heavy seas have shifted the course of the brook flowing from the lake into the sea, and caused a greater, and consequently a more rapid fall of fresh water, which has so shallowed that part of the lake where the gulls were in the habit of breeding that it was no longer safe to build on rocks easily accessible to their common enemy, the fox. They therefore betook themselves to some neighbouring spruce and balsam firs not much over a hundred yards distant from their old breeding station." Audubon also notes a similar change of habit, some herring-gulls building their nests in spruce-trees on an island in the Bay of Fundy, where they had formerly built on the ground.

A curious example of a recent change of habits has occurred in Jamaica. Previous to 1854 the palm swift (Tachornis phoenicobea) 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.

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 chief 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. То 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 not only innate ideas but innate knowledge 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.

VI

A THEORY OF BIRDS' NESTS, SHOWING THE RELATION OF CERTAIN DIFFERENCES OF COLOUR IN FEMALE BIRDS

TO THEIR MODE OF NIDIFICATION

1

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.

1 Published in the Journal of Travel and Natural History, No. 2; reprinted in Contributions, etc., with considerable additions and corrections.

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 arechanged 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