Cowbirds

It is time to write about cowbirds. No nice way exists to say this: they are parasites. They once multiplied at a prodigious rate in North America and may be endangering many other bird species. They are a menace. No one knows for sure how many reside in North America but the number exceeds 40,000,000 and may be as high as 80,000,000. Mostly they are Brown-headed Cowbirds although Bronze-headed Cowbirds live in the southwest United States.

Cowbirds don’t believe in nest building. It is so much easier just to use someone else’s nest. Cowbirds also don’t believe in child rearing. It is so much easier to let someone else do it. But they do believe in fornication. The females are egg machines and the males love to fertilize all those eggs. Because they waste no time building nests or feeding their young, they have time on their hands and they use it for lust. In low density populations they appear to be monogamous but when population densities climb, the birds are promiscuous, polygynous, or polyandrous as the mood hits them. There is some evidence that monogamous populations flock together and that promiscuous populations flock together, leading one to the unforgivable comment, that birds of a feather flock together and we ask your forgiveness for making that unforgivable comment.

Female cowbirds are devious little birds. In order to find a nest to lay her eggs, a female will sit quietly and watch for other birds building nests; or she will walk around on the ground searching for nests in use; or she may flap her wings excitedly, perhaps trying to flush birds from their nests. When she finds a nest she lays her egg in it as soon as the nest-owner is gone. (The spotted egg in the photo is the cowbird egg.)

When the nest owner returns there is one more egg to brood, unless the cowbird has eaten one eggs already there. Often the cowbird egg hatches a day before the legal residents’, giving the cowbird a head start on its nest mates. The baby cowbird is a little bigger and a little noisier and ends up with more food. (The red-mouthed nestling in the photo is the cowbird.)

Not all birds put up with this. Robins and Grey Catbirds simply toss the strange eggs out of the nest. Yellow Warblers bury the cowbird eggs under a new nest lining. Others simply leave. Western Kingbirds, Eastern Kingbirds, Blue Jays, Cedar Waxwings and Northern Orioles almost always reject cowbird eggs. Interestingly, these are species which probably co-evolved with cowbirds and are on to them. But most songbirds reject cowbird eggs at a far lower rate. In one study, cowbirds successfully parasitized 83% of Indigo Bunting nests in the study area. Some species, such as Kirtland’s Warblers, Least Bell’s Vireo and Black-capped Vireos, already endangered, are at significant risk from cowbirds. Extinction is close at hand for those species and it will not take much to tip them over the edge.

Cowbirds prefer cities. Songbirds living in cities are more likely to have unwanted cowbird babies to raise. No one knows for sure why cowbirds like densely populated human urban areas although they do enjoy bird feeders and lawns are good foraging sites.

Not much is being done to control them. A few trapping programs exist. There is one in Texas. In 2006 that program trapped and “dispatched” 24, 293 cowbirds. Ominously, the report did not define “dispatched.” But 24,000 “dispatched” cowbirds out of a population of at least 40,000,000 cannot make much of a difference. The rest still have all that free time.

Once cowbirds were not such a problem for other birds. They lived only on the Great Plains where they followed the Bison herds. Their primary food was seeds and insects stirred up by the great herds. But then we killed the buffalo herds so cowbirds moved east as humans cut down forests to make room for agriculture. Cowbirds can’t live in unlogged forests. The songbird species most susceptible to cowbird shenanigans now are the species that used to live deep in those forests and evolved without cowbirds around. Now their habitat is fragmented and shattered and they haven’t caught on to cowbirds which now have access to all those songbirds’ nests.

No wonder cowbirds like humans, we’ve made life easier for them. True, we killed the buffalo which deprived them of one way of life but gave them another, easier one. Now they have a far larger selection of nests to choose from which gives them even more free time.

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UPDATE – APRIL 29,2008

Natalie Angier writes about cowbirds in today’s Science section of the New York Times.  You can read it here.

Tags: Bison bison, Bronze headed Cowbird, Brown headed Cowbird, buffalo, forests, grassands, Parasite