Brown-headed Cowbird: Being a Parasite Is Not Easy

One of the annual March arrivals in Eliza Howell Park is the Brown-headed Cowbird. I usually see the first one in the second or third week of the month; this year it was March 8.

The cowbird is not usually a sought-after species among bird watchers. It is common, not particularly striking in appearance, and few record its song. Thr first picture is of a female; the second a male.

Both photos courtesy of Margaret Weber

The cowbird is not a favorite, but it is well-known. It is a brood parasite, the most common brood parasite in North America. It lays its eggs in the nests of other species for them to hatch and feed.

Cowbirds got their name from their foraging practice of accompanying grazing mammals (originally bison on the Plains), seeking the insects that scatter as the animals move through grassland.

Not a forest bird, their range expanded to include a much larger section of the country in the 19th century as forests were fragmented. Their current range is indicated in this map fron the Cornell Lab of Ornithology.

A female Brown-headed Cowbird lays perhaps 30 eggs a year, usually one per nest of 30 different “host” birds. This means locating all these nests and knowing when the time is right to fly in and lay an egg quickly. They often remove one of the eggs already present. They pick a time when the clutch is conplete or nearly so and incubation is about to begin. (I pride myself in finding nesting birds in Eliza Howell, but a cowbird makes me recognize that I am just a beginner.)

They often choose the nests of species smaller than themselves with the result that their fast-growing chick has an advantage over its nest mates. Occasionally, they will parasitize larger birds; the smaller egg in this N. Cardinal nest is a cowbird egg.

Some host species recognize and remove cowbird eggs and some make new nests, but many cowbird eggs are successfully hatched. And fledged young cowbirds somehow find a way to join other cowbirds and not identify with / mate with their host species. The whole cowbird repoduction process is much more demanding than suggested by a common human judgment that they are taking an easy way out by letting other birds raise their young.

Occasionally, the cowbird might constitute a threat to an endangered species, as in the case of Kirkland’s Warbler here in Michigan. The warbler’s population plummeted following the transformation of its habitat and that same environmental change allowed cowbirds to enter the area. Kirkland’s Warbler had no inherited way of managing cowbird parasitism.

The Kirkland’s Warbler is a Michigan favorite.

Photo courtesy of Margaret Weber.

The Kirkland’s Warbler recovery plan included trapping and killing cowbirds.This might have been a useful short-term strategy, but the need says much more about the effects of human habitat change than it does about the behavior of cowbirds.

I have come to see the Brown-headed Cowbird as a bird with a fascinating history and unusual “nesting” practices. The more I learn about birds, the more impressed I am by the diversity of their behaviors.

I stop for a while when I see a female cowbird, She is worth watchung carefully.

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