An Obligate Parasite

by Rex Burress

The word parasite more readily suggests some plant or insect feeding off another species. Yet the term also is applied to a nondescript bird we commonly see throughout California as well as most of the United States.

When we consider the term biological parasite, we find that it means a plant or animal living on or within another organism, from which it derives sustenance or protection without making compensation. One might wonder what bird could be like that. Perhaps you know of the egg-laying habits of the cowbird family, and particularly Molothrus ater, the Brown-headed Cowbird, the common species of California.

If you watch the birds at Lake Merritt, a blackbird type with a brown head can be seen walking amid the sidewalk landbirds. It is the male Brown-headed Cowbird, more easily noticed than the drab female, and often they mingle with the more numerous Red-winged and Brewer's Blackbirds. Who could suspect that one of the dark-feathered crowd could be completely out of the nest-building and infant-rearing business, and dependent on other birds to perform that duty for them?!!

The issue of obligate parasitism came to my attention through an article called "The Wisdom of Nature" by Janet Lembke, in which she tells of this incredible oddity of a bird forsaking what are generally considered normal parental reactions of building a nest, laying eggs, and giving tender loving care to its young. Why has one small group of birds of the cowbird family developed the habit of laying eggs in other species' nests in expectation that they will complete the chore of infant rearing for them?

When John James Audubon discovered the cowbird's characteristic, he remarked, "This is a mystery to me, nevertheless, my belief in the wisdom of Nature is not staggered by it."

If I am not staggered, I am more than mystified. The cowbird evidently spends a lot of time watching other birds building their nest and, at an opportune moment while the attendant is away, it slips in and hastily lays an egg. Two hundred and fifteen bird species, most of them smaller than the cowbird, are known to be victimized, and most (acceptors) accept the foreign egg and dutifully incubate and attend its functions. Some species such as the oriole and robin (rejectors) notice the intrusion and remove the egg, or sometimes build a new nest layer on top for a restart. The cowbird lays up to 12 eggs, one for each nest it assaults, so a lot of detective work goes on as she watches and waits for the proper time. Grown baby cowbirds seemingly are able to reunite with their own kind.

Other bird species sometimes lay or "dump" some of their eggs in other bird's nests, usually birds of the same species - such as when the Pied-billed Grebe lays in other grebe nests, or various waterfowl unload a hot egg in a neighbor's nest. Wood Duck crews sometimes find an overloaded nest box where several hens have contributed. However, no bird other than the cowbirds has completely given up nesting responsibilities.

The name cowbird came about from the way they originally followed buffalo herds that stirred up insects, then adapted to cattle herds where there are commonly found feeding around the edges of fields.

Considering the extraordinary effort some bird species exhibit in fulfilling their parental duties - such as the penguins nurturing an egg into hatching by balancing it on their covered feet for nearly a month, then returning from a strenuous food quest in the ocean and finding their particular offspring in a crowd of thousands, or even the strenuous effort of the Cliff Swallow in making a mud jug nest for the babies - it leaves one in stupefied amazement that there could be the other extreme of a careless-appearing approach to raising a family.

The fact that the cowbirds have no desire to provide warmth for the eggs, or see their babies born and feel the comforting warmth beneath their wings, and experience the joy of fulfilling their mission, is unfathomable. There are a total of five cowbird species in the Americas. They are in the Troupial family, which includes blackbirds, orioles, and meadowlarks. The family is found only in the Western Hemisphere or New World, consisting of 91 species with 22 found in North America, including the widespread Brown-headed Cowbird and the Bronzed Cowbird of the southern United States.


Consider the thought of nesting birds in this excerpt from the poem "The Vision of Sir Launfal" by James Russell Lowell:

This poetic piece reflects a family condition observed in most birds, but not for the cowbird. Their song is more like, "Let me find someone's nest, I sing for someone else's nest!"

© 2001 Rex Burress
January 16, 2001