While many birds are excellent parents, others lack any maternal or paternal instincts altogether. The common cuckoo, a nesting bird in Europe and Asia, is a well-known brood parasite that would rather slip its eggs into the nest of other bird than raise its own young. In scientific terms, “brood parasite” refers to creatures that rely on others to raise their young. In addition to some birds, this tactic is also employed by some species of insects and fish.
The strategy is effective, if, in the human way of thinking, rather heartless. In biological terms, however, this “foster parenting” allows brood parasites to ensure a new generation without expending much energy on the part of the actual parents. Some recent contacts with readers have reminded me that not all of our feathered friends would qualify for “parent of the year.”
Mike Dickenson of Bristol, Tennessee, contacted me on Facebook about a discovery he made in a nest built under the steps of his house.
“I noticed two blue eggs,” he said. “I checked a few days later and noticed two gray eggs also. Did another bird sneak her eggs into the nest?” Mike also informed me that some of the eggs hatched shortly after he discovered them.
James Rowland of Erwin, Tennessee, sent me a message on Facebook asking me to identify a bird in a photograph he had taken. “What is this bird?” James asked. “It’s larger than a sparrow.”
He added that he observed and photographed the bird near the Covered Bridge in Elizabethton, Tennessee. A study of the bird in the photo revealed a very nondescript bird in largely gray plumage. Few of our birds are this plain and gray with almost no standout characteristics.
In both cases, one of North America’s most successful brood parasites was involved. I responded to Mike and told him that is was entirely possible that a female brown-headed cowbird slipped some eggs into the nest beneath his steps. I likewise informed James that the bird in his photo looked like a brown-headed cowbird. I added that the bird was either a female or a young bird, since a male would have the brown head that gives the species its common name.
In North America, one of the best-known feathered brood parasites is the brown-headed cowbird. While many brood parasites are specialists, with females slipping their eggs into the nest of a specific species of host bird, the brown-headed cowbird approaches brood parasitism in a less discriminating manner. Female cowbirds have been known to lay their eggs in the nests of at least 221 different species of birds. No baby brown-headed cowbird ever knows its biological parents.
How did the brown-headed cowbird turn to a life of foisting eggs onto unsuspecting foster parents? The answer is connected with the American bison, also known as buffalo. When the bison roamed the Great Plains of the United States by the millions, flocks of brown-headed cowbirds followed the great herds, feeding on the insects flushed by the hooves of millions of bison. As the herds stayed on the move constantly, the cowbirds also developed a nomadic lifestyle. After the bison herd diminished, the cowbirds survived a potential crisis by simply transferring their bovine affinity from bison to domesticated cattle.
At times, this random and undiscriminating approach to reproduction fails. Some finches feed their young a diet that consists of a great deal of vegetable matter. Young cowbirds fed this protein deficient diet fail to thrive and ultimately perish.
Other birds blissfully bring a rich assortment of protein snacks — insects, spiders and other small invertebrates — that permits the young foster bird to thrive, at times at the expense of the host bird’s own young. About 20 years ago I observed a willow flycatcher bringing food to a young brown-headed cowbird at least twice the size of the “parent” trying to feed it. I’ve also seen song sparrows, dwarfed by a cowbird changeling, trying to keep their enormous baby bird well fed.
Cowbirds are members of the blackbird family, which includes such relations as orioles, meadowlarks and grackles. All cowbirds are confined to the New World and include species such as the screaming cowbird of Argentina, Bolivia and Brazil, as well as the bronzed cowbird of Central America and the southern United States, especially the states of California, Arizona, New Mexico, Texas, and Louisiana. Other cowbird family members include giant cowbird and the shiny cowbird.