Heretofore I have neglected to state that I often saw the mother cowbird. I think she visited the nest several times a day after the egg was laid. Her frequent visits had accustomed the young bird to her presence, thus making possible what followed.
After discovering the new nest, I looked up the young cowbird, and found the male yellowbird feeding him as usual, but not alone. The old cowbird was acting as assistant, as if just aroused to the responsibility of maternal duties. For several days both birds fed the young cowbird, after which the yellowbird spent much of his time with his mate, gradually deserting his charge, to return no more when the second brood was out.
Thus my observations had answered two questions; my first and second. My first question, "Why the victimized birds rear the parasite?" was answered to my belief in this way: I believe that the yellowbirds had had experience with cowbirds before, and intelligently understood that they must sacrifice their first brood in order to raise a second brood unmolested. The actions of the birds when they discovered the parasite egg, their great distress, their consultation and prompt action, their neglect to lay the usual number of eggs can be construed in no other light. It is far beyond the province of instinct.
My second question, "Why the young cowbird deserts its foster-parents?" is already intelligently answered. It is no desertion. The foster parents turn over the parasite to its own mother, in a matter-of-fact way, and then go about their own affairs in peace.
My third question, "Why do young cowbirds lay eggs in other birds' nests instead of building nests for themselves?"
When the cowbird was out of the shell, it was big and black. It was my first young cowbird, and I thought it was a male. I made it a male in my note-book. While the bird was in the nest I fastened a bit of copper wire to its leg, and the next spring, when it returned, I found that the bird was a female. I saw her with another female, I think it was the mother, visiting birds' nests. So the young cowbird was educated to lay its eggs in other birds' nests. Nest-building is educational and not instinctive.
My fourth question could not be answered by observation.
How did the cowbird acquire this unnatural habit?
The answer to this question is not within the province of proof. It is fair to assume that the cowbird, in the distant past, reared its young in a nest of its own. It may have happened that some tragedy had deprived a family of young cowbirds of their parents. Other birds may have reared the young ones until they were capable of providing for themselves. In migration all would remain together, but when nesting begun the young cowbirds would not be tolerated near a nest. Not educated in nest building, the female would fly to other nests to drop her eggs. Other cowbirds may have adopted the same method, finding it pleasant to have the care of their babies shouldered on to servants, like some human mothers.