Only a few moments before the cowbird appeared I had seen the happy pair leave the tree together, flying to a clump of trees lower down the slope of the hill. The female had evidently just deposited her egg, the cowbird had probably been watching near by, and had seized it the moment the nest was vacated. Her plan was of course to deposit one of her own in its place.
I now made a more thorough search for the nest, and soon found it, but it was beyond my reach on an outer branch, and whether or not the cowbird dropped one of her own eggs in place of the one she had removed I do not know. Certain am I that the vireos soon abandoned the nest, though they do not always do this when hoodwinked in this way.
I once met a gentleman on the train who told me about a brood of quails that had hatched out under his observation. He was convinced that the mother quail had broken the shells for the young birds. He sent me one of the shells to convince me that it had been broken from the outside. At first glance it did appear so. It had been cut around near the large end, with the exception of a small space, as if by regular thrusts or taps from a bird's beak, so that this end opened like the lid of a box on a hinge, and let the imprisoned bird escape. What convinced the gentleman that the force had been applied from the outside was that the edges of the cut or break were bent in.
If we wish rightly to interpret nature, to get at the exact truth of her ways and doings, we must cultivate what is called the critical habit of mind; that is, the habit of mind that does not rest with mere appearances. One must sift the evidence, must cross-question the facts. This gentleman was a lawyer, but he laid aside the cunning of his craft in dealing with this question of these egg-shells.
The bending in, or the indented appearance of the edge of the shells was owing to the fact that the thin paper-like skin that lines the interior of the shell had dried and shrunken, and had thus drawn the edges of the shell inward. The cut was made by the beak of the young bird, probably by turning its head from right to left; one little point it could not reach, and this formed the hinge of the lid I have spoken of. Is it at all probable that if the mother bird had done this work she would have left this hinge, and left it upon every egg, since the hinge was of no use? The complete removal of the cap would have been just as well.