by Carl Strang
Over the past 2-3 weeks I have been noticing cowbird fledglings at Mayslake Forest Preserve. These are the offspring of obligate nest parasites: brown-headed cowbirds cannot nest themselves, but rather lay their eggs in the nests of other birds. Foster parents that fail to recognize the foreign egg will raise the baby cowbird, often at the expense of their own young. The cowbird develops rapidly, and begs vigorously, gaining an advantage. Here is a fledgling cowbird raised by Mayslake’s eastern phoebes earlier this year.
I saw no phoebe fledglings. The cowbird’s begging call is distinctive. It sounds like a chorus of baby birds, and probably qualifies as a supernormal releaser. That term refers to an exaggerated stimulus that produces a particular instinctive response by an animal. In this case, the call draws disproportionate attention, in the form of food, from the foster parents. If you hear a chorus of baby birds that all start at once and all pause for breath at once, you probably are hearing a single cowbird youngster.
I heard this call coming from the cottonwood tree bearing the south savanna Baltimore oriole nest . As I like to do, I found the cowbird and waited to see what would come to feed it. The foster parents were the orioles. Here the female feeds the cowbird.
I watched for a few minutes, during which the female fed the cowbird three times, the male oriole fed the cowbird once, and he fed the baby orioles (still in the nest) once. A period of rain and work duties kept me from checking for a couple days, by which time there was no sign of any of these birds near the nest.
Over the years I have seen cowbird fledglings being fed by yellow warblers, song sparrows, scarlet tanagers, blue-gray gnatcatchers and cardinals. Cardinals are common at Willowbrook Forest Preserve, but for several years the only time I saw cardinal fledglings was late in the season, after the cowbirds were done. All early products of cardinal nests were cowbirds. This kind of selective pressure is what has led many birds to evolve the capacity to recognize and reject cowbird eggs. Obviously such an ability remains to emerge in many others. Incidentally, once the cowbird becomes independent, it instinctively seeks out other young cowbirds and behaves as a cowbird from then on.