Obligate Brood Parasitism: Birds That Cheat


Brown-headed Cowbird

Brown-headed Cowbird

The Brown-headed Cowbird cheats. We’re not talking about philandering males (or females) here. We’re talking about obligate brood parasistism.

What that means is that female cowbirds don’t brood their eggs. Male and female cowbirds don’t feed their kids. Instead, the female lays her eggs in other birds’ nests and lets those birds brood and raise her kids. In many cases, the cowbird hatchlings are much larger than their nest mates, and command all the food the involuntary foster parents bring. The result is that the victims’ hatchlings starve as the vitims spend all their time feeding the alien.

The evolutionary leverage is pretty fierce. A victim who can recognize a cowbird egg and throw it out of the nest, or thrown the alien hatchling out of the next. So some species have developed those skills.  Rejection skills occur in at least three forms: nest desertion, like the Blue-gray Gnatcatcher, where the victims simply abandon the nest and try elsewhere, hopefully in a place free of cowbirds; burying of the egg under nest material so it does not get brooded and does not hatch, like the Yellow Warbler, and pitching the egg or the nestling out of the nest, like the Brown Thrasher. The Gray Catbird may be the best at evicting cowbird eggs; it evicts cowbird eggs over 95% of the time.

But evolutionary forces work both way. Brown-headed Cowbirds can be testy about being outsmarted. There’s a study by the Florida Museum of Natural History published in 1983, documenting that the cowbirds returned to ransack the nests of a range of host species 56% of the time when their egg was removed. In some cases, the cowbird also destroyed nests in a type of “farming behavior” to force the hosts to build new ones. The cowbirds then laid their eggs in the new nests 85% of the time.

Among Common Cuckoos, another brood parasite, the Cuckoo has evolved the ability to make its eggs match the eggs of the targeted victim species. The victim species cannot recognize the intruder egg.

At least five different families of birds have evolved brood parasitism. Of course, it’s not confined to birds. It’s seen in insects, arachnids and fish as well. The child-rearing strategy may be offensive to humans. But it is highly successful among birds. There’s no shortage of Brown-headed Cowbirds.

At least among brood parasites, cheaters prosper.

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2 thoughts on “Obligate Brood Parasitism: Birds That Cheat

  1. Fascinating article – we are currently watching a pair of starlings feeding their cowbird “babies” (who are already much larger than the parents).

  2. Handsome bird. I have been using the cuckoo bird nesting habits to describe the political scene for a few months. My version was simple and not as scientifically interesting as your information, WC; but the same principle applies. Watch out for the cowbird and the common cuckoo. They won’t bother building their own party, oops, nests. Interesting phenomenon.

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