Take the case of the little yellow warbler when the cowbird drops her egg into its nest--does anything like a process of thought or reflection pass in the bird's mind then? The warbler is much disturbed when she discovers the strange egg, and her mate appears to share her agitation. Then after a time, and after the two have apparently considered the matter together, the mother bird proceeds to bury the egg by building another nest on top of the old one. If another cowbird's egg is dropped in this one, she will proceed to get rid of this in the same way. This all looks very like reflection. But let us consider the matter a moment. This thing between the cowbird and the warbler has been going on for innumerable generations. The yellow warbler seems to be the favorite host of this parasite, and something like a special instinct may have grown up in the warbler with reference to this strange egg. The bird reacts, as the psychologists say, at sight of it, then she proceeds to dispose of it in the way above described. _All yellow warblers act in the same manner_, which is the way of instinct. Now if this procedure was the result of an individual thought or calculation on the part of the birds, they would not all do the same thing; different lines of conduct would be hit upon. How much simpler and easier it would be to throw the egg out--how much more like an act of rational intelligence. So far as I know, no bird does eject this parasitical egg, and no other bird besides the yellow warbler gets rid of it in the way I have described. I have found a deserted phoebe's nest with one egg of the phoebe and one of the cowbird in it. Some of our wild birds have changed their habits of nesting, coming from the woods and the rocks to the protection of our buildings. The phoebe-bird and the cliff swallow are marked examples. We ascribe the change to the birds' intelligence, but to my mind it shows only their natural adaptiveness. Take the cliff swallow, for instance; it has largely left the cliffs for the eaves of our buildings. How naturally and instinctively this change has come about! In an open farming country insect life is much more varied and abundant than in a wild, unsettled country. This greater food supply naturally attracts the swallows. Then the protecting eaves of the buildings would stimulate their nesting-instincts. The abundance of mud along the highways and about the farm would also no doubt have its effect, and the birds would adopt the new sites as a matter of course. Or take the phoebe, which originally built its nest under ledges, and does so still to some extent. It, too, would find a more abundant food supply in the vicinity of farm-buildings and bridges. The protected nesting-sites afforded by sheds and porches would likewise stimulate its nesting-instincts, and attract the bird as we see it attracted each spring.