III. Multiple and Superimposed Nests
We have referred to the towering Eerie of the eagle and osprey as being, so far as instinct is concerned, a series of superimposed nests; indeed, any nest built on the site or over the ruins of a former abode, might be regarded in this light. When attachment to the site is strong, the bird, like the peasant in ancient Egypt and many of the earlier races of mankind, builds anew on the ruins of his former home, without taking care to clear the ground or raze such parts as still exist. The result is similar in either case—a series of superimposed structures of different ages, the height to which the pile may rise, depending upon the number of times the same site has been used.
The building of nest on nest, or of new nest on the site of the old, according to this interpretation, gives rise to the wonderful storied structures sometimes produced by the yellow warbler, or vireo, when plagued by the cowbird. That the intruding egg is buried out of sight is not due, however, to a feat of reason on the part of the suffering bird, but is the curious result of a nearly pure instinct, modified only by association. Fear breaks the cycle, but it is not always strong enough to break the habit of going to the old site. Instead of two or more supernumerary nests, more than one of which may contain eggs, and even stand side by side, as has been reported in the case of the phcebe, we have a series of superimposed nests, as is clearly illustrated iu the remarkable four-storied structure of the summer or yellow warbler, here shown (Fig 22).[1] Each section of this composite, moreover, is seen to contain an egg of the parasitic cowbird, that in the first story being partially concealed by the warbler's eggs present.
According to this view, the new nest is not built to conceal the cowbird's egg, although it does so perfectly, any more than the addition of new materials to an osprey's nest in the fall is of the nature of repairs, although it may answer such a purpose admirably. The nest is built or "repaired" because the bird is at the opening of a new cycle, and is impelled to action by the rise of the building instinct. Whether the new nest is built upon the remains of the old, or close beside it, or half a mile away, must be attributed to the ordinary workings of instinct, modified by association and fear, when for some cause the normal cycle has been disturbed.
The so-called "cock nests" of the little marsh wrens may prove to be only another illustration of the supernumerary nests given above, but no opportunity has yet been offered to study these interesting structures. The fact that they may be used secondarily as sleeping apartments, if this is really the case, has no special significance. I have seen the abortive hole of a kingfisher so used, but a few rods from the
- ↑ For the use of this photograph I am indebted to the courtesy of Mr. F. J. V. Skiff, director of the Field Museum of Natural History, Chicago.
