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Dec. 13, 2011

Egg Counting

by Sam Flatow

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I’m starting to believe that birds in general are jerks. I can understand why. Think about it, your ancestors are dinosaurs, some of the biggest, meanest monsters ever, and you’re stuck as a pelican. No Mr. Pelican, I won’t make fun of you because you always look sad or confused.

Take the ultimate bird-jerk, the cuckoo. Refusing to care for their own young, a cuckoo will sneak into other avian nests, lay its egg, and trick the nest owner into raising a baby serial killer. And when I say “tricks the nest owner into raising a baby serial killer,” I mean that when the baby cuckoo hatches, it blindly murders its nest mates so it can monopolize all the resources and space his adopted parents provide. And when I say “blindly murders its nest mates,” I mean the deed is done before the little jerk develops sight.

This strange adoptive relationship is called brood parasitism, and it’s not unique to the cuckoo. A recent study published in Proceedings of the Royal Society B looked at a similar scenario between the chalk-browed mockingbird and the shiny cowbird.

But the cowbird isn’t as cunning as the cuckoo. The cowbird will assault a mockingbird nest and, while under attack from the protective homeowners, poke a hole in a mockingbird egg, and lay one of its own. There’s no hiding, no espionage, no tiny bird cameras and photos of secret entrances, they just shove their way in, punch some holes, dump some eggs, and wing it. It’s not quite a smash ‘n’ grab, more of a smash ‘n’ drop.

You’d think that when the attack was over, the mockingbird would just dump out any unwelcome eggs, but that’s not what happens. The researchers found that chalk-browed mockingbirds who have had their eggs destroyed will keep any replacements–because those new eggs increase the chance that their own will survive. Now if another creature attacks the nest, the cowbird eggs act as decoys, hopefully absorbing some of the carnage, and saving the mockingbird’s own genes from going to waste.

This statistical advantage is so useful that mockingbirds will rear the outsider egg, spending as many resources on alien genes as they would their own. This isn’t altruistic behavior–the mockingbird isn’t thinking “well, I can’t blame the children because the parents murdered my future babies.” No, this is a cold numbers game.

About Sam Flatow

Sam is an assistant producer at Science Friday where he prepares the tasty SciFri snacks and blogs about smart cephalopods and zombie ants.

The views expressed are those of the author and are not necessarily those of Science Friday.

Science Friday® is produced by the Science Friday Initiative, a 501(c)(3) nonprofit organization.

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