Column: Cowbirds are tricksters of the bird world

Published 6:44 am Thursday, April 3, 2008

By Staff
Correction to last week's column on bluebird houses: Entrance hole size should be one and one half inch, not one inch.
Some of the more recent migrant birds now drifting in are cowbirds. Last week all I saw were males but now the girls are showing up as well. I don't know if they migrate separately or if the split was just coincidence. Probably the latter. Anyhow, I'm not sure any bird can be called down right evil, but if you're willing to go that far the cowbird is certainly it. They just don't seem to have any redeeming qualities. Cowbirds were originally denizens of the great prairies, hanging out with buffalo which stirred up insects, making easy pickings for the cowbirds. If cowbirds took time out for nesting they couldn't stay with the wandering bison so they developed the trait of laying their eggs in other bird's nests, counting on the surrogate parents to raise their chicks. Cowbirds are the only North American bird that does this nest parasitism.
They are masters at this deadly trickery. The female cowbird often takes watch from a shrub or tree looking for other birds building nests. She keeps track of these nests and once the other bird begins egg laying the cowbird sneaks in during an unattended time and lays one of her eggs. Often she removes one or two of the other bird's eggs so the egg pile remains roughly the same size (birds don't count so good). She even goes so far as to take the removed egg(s) some distance away so it is not noticed. She repeats this going from nest to nest as many as 20 times. It has even been documented where cowbirds not yet ready to lay eggs "farm" a nest. They destroy the hosts eggs so the host will build another nest which the cowbird will then parasitize to get their eggs in synch with the host.
Cowbirds are known to parasitize the nests of over 200 species of birds ranging from warblers, phoebes and the various sparrows to wood thrushes and meadowlarks. They prefer open cup nests, but occasionally parasitize cavity nesters like blue birds and woodpeckers as well. Interestingly, most individual cowbirds specialize in a particular host species. The timing of the parasitism is precise and the cowbird chick usually hatches one or two days prior to the others. Cowbird chicks grow fast and soon outsize the rightful occupants by three times or more. Surprisingly, they don't simply oust the smaller chicks for a relatively quick demise, but rather hog all the food brought by the surrogate parents. One by one the smaller chicks slowly die from starvation. As one ornithologist put it, "For every cowbird that comes into maturity a brood of some other species must perish."
Sometimes the host bird recognizes the scam and either removes the cowbird egg or builds another nest over the top and starts over. Revenge is supposed to be a strictly human trait but cowbirds are an exception. A University of Florida study of Prothonotary Warblers found that when the cowbird egg was removed from the warbler nest over half the time the cowbird ransacked the nest, destroying the eggs.
It wasn't until the late 1800s when logging and agriculture opened up the forests that cowbirds became an issue here in Michigan. One of their more infamous acts was contributing greatly to the demise of the Kirtland's Warbler. It was found cowbirds were parasitizing 70 percent of the Kirtland's nests. In the 1970s an aggressive cowbird trapping campaign was instigated in Kirtland's Warbler nesting habitat and to this day continues to be an instrumental tool in the recovery of the Kirtland's Warbler.
Unfortunately, cowbird parasitism remains a major factor in the declining migratory songbirds throughout much of the U.S. Since they are native they are protected under the Migratory Bird Treaty Act so it is illegal to harm or harass a cowbird or its eggs without a permit. Personally, I think is one case where the Act could stand some revision. Carpe diem.