XII.
THE INSTINCT OF THE COWBIRD
THE books on ornithology tell us that the cowbird (Molothrus ater) is a common summer resident of New England, without regard to locality. However true this may be as to other parts, it is a fact that the bird was unknown to me in Penobscot County, Maine.
Cowbirds are summer residents of Cape Ann, and I have studied their habits for years. I commenced by requiring answers to the following questions:
Why do birds, when victimized, rear the young cowbird?
Why does the young cowbird desert its foster parents to associate with its own kind?
Why do young cowbirds lay eggs in other birds' nests, instead of building nests for themselves?
How did the cowbird acquire this unnatural habit?
Writers on the subject usually answer the first question by the term "stupidity," and the other three by the word "instinct."
In all my life I have never found the birds stupid. They are as intelligent as to the requirements of bird life as man is as to the requirements of human life.
The theory of instinct is only a dream of the uninitiated. Nature's children are never troubled by such nightmares.
The most of our bird books have the earmarks of the library. An author may be familiar with a few birds studied afield, but the greater number are studied in the library. Take the cowbird as an example. One author after another rings in the same old chestnut about the disreputable bird that lays its eggs in other birds' nests and deserts its offspring. These authors wind up by calling attention to the wonderful instinct that causes the young cowbird to desert its foster-parents to associate with its kind. I will say now, that long before I had the opportunity to study the bird, I did not believe it possible for a young bird, by its own knowledge, to hunt up and associate with birds of its kind. That would be a miracle, and the days of miracles are passed. In my study of birds I have found that old birds educate the young, and I knew that the young cowbird was piloted by its mother, or the foster-parents turned it over to its kind to be rid of incumbrances. Few writers have studied the cowbird through the nesting season. Mr. John Burroughs writes that he found small eggs in the path that had two pricks in the shell. Afterward he detected the cowbird removing an egg from a bird's nest. Mr. Burroughs intimates that the cowbird did this to deceive the owners of the nest. They, finding the proper number of eggs, would not detect the fraud. I was sincerely grieved that a delightful writer on natural history should make such a break. His interpretation would endow the cowbird with a keen reasoning power, and would make chumps of the others; too senseless to know their own eggs. In my observations, when the victimized birds return and find the alien egg, they exhibit great distress.
BLUE-WINGED YELLOW WARBLER.
My first study of the cowbird happened in an unexpected manner. I was watching the nest of a pair of yellow warblers (Dendroica æstiva) that contained two eggs. While the owners were absent I saw a cowbird flutter on to the nest and add her parasite egg to its contents. When the yellowbirds returned they at once discovered what had taken place, and acted as if wild with alarm and distress. For a half-hour the birds flew wildly about, uttering plaintive cries, after which they settled down on a twig, where they could overlook the nest. They now seemed less excited, and were evidently holding a consultation. After awhile they seemed to agree on a course of action, for the female went on to the nest and the male bird tried to sing away the trouble, but I thought his song less earnest than usual.
No more eggs were laid, which was somewhat remarkable, as the yellowbird's number is usually four.
I found the young cowbird hatched out just twelve days after the egg was laid. The next morning I found the two yellowbirds out of the shell. When the cowbird was two days old he crowded both the little birds out of the nest. When I found them, one was dead and the other gasping as if fatally hurt. While I was watching the latter, the mother-bird appeared with an insect. She offered the food to the dying bird, and appeared greatly troubled when it was not received. After awhile she seemed to comprehend that the little one could not eat, and she fed the insect to the cowbird. Before flying away, she returned to the gasping bird, and looked at it by turning her head from side to side, while she uttered a succession of low, plaintive notes.
After this, both yellowbirds had all they could do to supply the black giant with food. When he was old enough to fly, or, at least, was completely feathered, his foster-parents coaxed him out of the nest after the manner of all bird-kind. Birds know when their young are old enough to leave the nest, and withhold food until the little ones are downright hungry, and then tempt them out with a dainty morsel. While tempting the young cowbird from the nest, the yellowbirds made as much effort and appeared as joyous when successful as if the labor had been performed for their own bright-eyed, pretty birdlings.
The young cowbird, when once out, did not return to the nest for shelter. His growing appetite taxed the strength of both birds to the utmost. Every moment of daylight was occupied in catering to his wants. One day I missed the female yellowbird, and, after a long search, found her engaged in building a new nest. She had forsaken her former charge.
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.
However, the whole thing is mere speculation, and it is not worth while to follow it further.
A few years ago a cowbird laid an egg in a chewink's nest. The chewink visited my dooryard. I did not remove the egg, but watched for the cowbird. Before the egg was hatched I shot the mother. I wanted to see if a young cowbird, reared without his own mother, would go out to the cow-pasture where there was a flock of old cowbirds. The chewinks reared the cowbird and three of their own babies. This was the first brood. When the mother chewink made a new nest, the father took care of the four little ones. Before his mate hatched the second brood, he took his charge to a bird resort near a pond. This was near the cow-pasture, and the flock of cowbirds resorted to the pond for water. It gave the young cowbird a good chance to go with its kind. Several times I saw cowbirds approach the youngster, but he always fled as if he thought that his life was in danger. He acted just as young tame crows do when they see other crows near them. That fall all the chewinks, that is, the old ones and the first brood, with the cowbird, remained about the dooryard until migration. The second brood of chewinks was destroyed by a snake, after which the first family was brought back. The next spring the cowbird did not return with the chewinks. As a matter of fact, only two of the five chewinks returned. I suppose the others were killed in the rice-fields. I had wired the cowbird with copper wire, so looked for him in the different flocks in my locality. He was not to be found, and was probably shot because he was with the chewinks in the rice-fields.
Two years ago I found a cowbird's egg in the nest of a Maryland yellowthroat. This nest was under a tussock of cut grass, just over a stone wall that enclosed the cow-pasture. As usual, it was the first nest of the Maryland yellowthroats. The young birds, three besides the cowbird, were crowded out of the nest, but as luck would have it they fell into a cavity on one side of the nest, and 'were fed by the parents. I saw the mother cowbird feed her baby before he was out of the nest, and when he could hop about, his mother led him to the cow-pasture. Afterward I saw her carry flies from the cows to her baby, which was in the bushes near the wall. I think the Maryland yellowthroats covered their own little ones from the night air. Perhaps one of them protected the cowbird. I did not see the foster-parents feed the young cowbird after he was able to leave the nest. I watched one morning for two hours, and saw the birds make many trips with insects, which they fed to their own birds. The cowbird was near at hand, over the wall, but the birds did not go near him.
From my observations I am convinced that the cowbird does not desert its offspring, but; instead, keeps an eye to its welfare, and ends by assuming the whole care of its food, and leads it to associate with its kind after it is large, or old enough to fly.
I have a little bird friend, a chestnut-sided warbler, that nests near my cabin. Three springs running I found a cowbird's egg in my little friend's nest. The first two eggs I threw out, but the third year I thought to try an experiment, the same that was afterward tried on the chewinks, and shot the mother cowbird. The cowbird was out of the shell before the other eggs had hatched. There were three eggs in the nest, and the young cowbird managed to break them. The chestnut-sided warblers had begun to feed the alien, but when they found the broken eggs, they deserted the nest and left the young cowbird to starve. They made a new nest not over three rods from the old one. I was sorry that I had shot the mother cowbird. It would have proved whether a cowbird would leave her offspring to starve, if deserted by the foster-parents.
I have mentioned putting copper wire on the young cowbird's leg. This artifice was used on other birds as well. I could easily identify my birds when they returned in migration. I put two turns of wire around a young robin's leg one spring. This robin was brought up by catbirds, with my assistance. I had removed a catbird's egg to a robin's nest, and a robin's egg to a catbird's nest. The crows destroyed the robin's nest, but the catbirds reared their family. The young robin proved to be a male. He associated with the catbirds, and went South with them. He returned in the spring with the male catbirds. The females and young returned together about a week later. The young robin remained about the cabin and the little brook where the catbirds nested until the last of June. He had a favorite tree, an oak, where he would perch in the morning and attempt to sing. His song was made up from that of the robin and catbird. A curious medley. The last of June I missed the bird, and looked for him in his favorite oak. I found his body lodged in a small hemlock beneath the oak. He had been shot while singing in his favorite tree.
