Why Do Mockingbirds Accept Invaders' Eggs?

By Veronique Greenwood | December 22, 2011 11:35 am

In the form of brood parasites, the bird world has enough irresponsible moms to start a reality TV show: cowbirds, for instance, lay their eggs in other species’ nests, stab most of the hosts’ eggs to death, and then leave their offspring to be raised by the host parents. The standing explanation for this involves most host birds being not that sharp on the uptake (watch a tiny warbler fussing over a cuckoo chick ten times its size (above) and you’d think that too). But maybe, a new study suggests, it’s sometimes to the host’s benefit to let imposter eggs stay in their nests.

The researchers chose mockingbirds as their hosts and cowbirds as their parasites, because mockingbirds usually fight like crazy to keep cowbirds of their nests but get strangely quiescent once the invaders have laid their eggs, a behavior that piqued the researchers’ interest. Once all the birds in the sample population had laid, the researchers went around adding and removing eggs from nests to see whether having a certain number of cowbird eggs affected mockingbird survival. They found that mockingbird eggs that shared their digs with cowbird eggs and suffered repeated cowbird invasions were more likely to survive, apparently because when each cowbird arrived, it would stab a certain proportion of the eggs in the nest regardless of whether they were host eggs or the eggs of the previous cowbird. Letting the parasite’s eggs stay, then, means that more of the host eggs avoid getting stabbed. The researchers conclude that when there are a lot of cowbirds around and hence a high probability of multiple nest hijackings, it makes sense for mockingbird parents not to shove out the invaders’ eggs.

Nice. And with all this dubious parenting and wanton violence, it’s straight from an episode of Teen Mom meets Cops, no?


CATEGORIZED UNDER: Environment, Living World
  • MikeG

    This is very narrow minded short term strategy. If the mockingbirds shoved the parasite eggs out of their nest, it would in the short term drastically reduce the cowbird population size. In the long term, it may result in a behavioral change in cowbirds, forcing them to rear their own young.

  • http://www.squatlo-rant.blogspot.com Squatlo

    While the egg-in-the-nest theory might hold water, how does one explain the fact that other species of birds will raise and feed the cowbird hatchlings? I’ve seen robins in our yard being followed by their own young, as well as immature cowbirds, and the robins work diligently to feed them all equally well. One would expect the survival of their own to surpass the parental urge to shelter interlopers, but that’s not what we observe in nature.
    This might be a trait more akin to higher primates who will “adopt” the offspring of others.

  • Kara

    Actually, it’s been found that some brood parasites will watch to see if the eggs they’ve laid are removed by the host. If the eggs are removed by the host, then the brood parasite will exact retribution by returning and destroying all of the host bird’s eggs. So the hosts are forced to decide between raising the brood parasite and less of their own offspring or losing all of their own offspring. So there is certainly no altruistic “adoption” involved nor is it a narrow-minded short term strategy. Raising the brood parasite’s chicks is simply the lesser of two evils.


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