THE COMMON COWBIRD
by
Walter L. Meagher
Photos by Wayne Colony
Not every bird is beautiful but all are interesting. Disappointed when colors are dull, when our aesthetic sensibilities are unresponsive, we may yet be stirred by the mobbing of grackles, the thieving of crows, even by the daring behavior of starlings. But the Common Cowbird (Peterson calls it the Brown-headed Cowbird as well) has two counts against it: it is dull (a small, dark blackbird, an Icterid) and it is naughty. Who does not know that the cowbird lays its eggs in nests of smaller birds, that the eggs are mothered by the parasitized female, and that when the eggs hatch the cowbird nestlings thrust the other nestling species out of the nest, just like the European Cuckoo? The cowbird will have provided for its young, but by a surrogate mom, and to the cost of that mother’s own reproductive potential.
Many birds can fail as parasitized parents, thus reducing the Common Cowbird’s success rate. For instance, these would be ill-chosen: the Common Tern, a Spotted Sandpiper, or a Ruby-throated Hummingbird. All are able to dislodge a cowbird’s egg. This is the Common Cowbird’s strategy: not put off by failures, it tries to lay eggs in any nests it can find. Of course, small birds are preferred, and most, like vireos and warblers, rarely succeed in dislodging the cowbird egg or nestling.
If the Common Cowbird’s eggs are dislodged, it has to lay more, and in this it has been compared to a chicken. The female cowbird has a long reproductive cycle and short time-off between clutches. For all its ravishments, the cowbird is a workaholic. One female may lay 80 eggs in a season, a reproductive rate of 2.4 adults per female and enough to double the cowbird population overall within eight years, but, of these, only 3% will result in mature offspring. Hence, the scourge of cowbirds across North America.
On the lookout for a nest to parasitize, the Common Cowbird visits nests in construction before host eggs are laid. If the host has not laid an egg, she will not lay her egg, for fear of detection; if two eggs have been laid, she will remove one, and lay hers; but if only one egg has been laid, she won’t remove it. If she has made a mistake, and laid an egg in an active rejector’s nest, the host bird may choose to attack the cowbird; certainly the host bird will discard the egg or eject the hatchling. But if the cowbird has chosen well, her egg will hatch a day before the host’s egg, and her offspring will grow faster and get much larger than the host’s offspring. The host’s reproductive season will have been wasted and the cowbird will prosper at her cost.
We started with the idea that the cowbird is dull in color; then she becomes the ugly witch, a parasite of warblers and vireos. But other birds are nest parasites as well, and one, the Ruddy Duck, is gorgeous. In fact, nest parasitism is common among ducks, made easy by the nests they construct. Of all the duck parasites, the beautiful Ruddy Duck is probably the most persistent and successful; little do we suspect this, spotting a ruddy, through a ‘scope’, far out on the presa!