Anyone with older brothers or sisters has probably been bullied around at home at least once in a while. But if your older brother or sister were a cowbird, you'd probably have a lot more to complain about.
How cowbirds connive to survive. I'm Bob Hirshon and this is Science Update.
Cowbirds and cuckoos are classic deadbeat parents. They lay eggs in other birds' nests, and leave the strangers to care for their young. But while a cuckoo chick will destroy the other eggs in its nest, cowbirds simply join the host's family. Mark Hauber, a research fellow at the University of California at Berkeley, wondered why.
Cowbirds seem to suffer from the presence of host nestmates, in terms of having to beg more intensively, and competing with the nestmates that are there, and do not belong to the same species, let alone they are not their siblings.
He and his colleagues, Becky Kilmer and Joah Madden, have figured out their strategy. They found that cowbirds actually eat more and grow faster if they share their nest than if they're raised alone. That's because mom and dad bring more food to a bigger brood. And because cowbirds are usually older and bigger than their nestmates, they can easily hog the extra resources.
And so the nestmates often starve, or get trampled by the larger, more aggressive parasitic chick.
He says understanding cowbird behavior is important because they're spreading further across America, and targeting new species as hosts. I'm Bob Hirshon for AAAS, the Science Society.
Making Sense of the Research
There are about ninety species of parasitic birds (birds that lay their eggs in other birds' nests), but the North American cowbird and the European cuckoo are probably the best known. Of the two, cuckoos are far more ruthless: as soon as the cuckoo chick hatches, it kicks the other eggs out of the nest and enjoys the life of a well-fed only child. It's brutal, but it works.
Given the cuckoo's strategy, it's surprising that the cowbird doesn't do the same thing. A casual observer would see cowbirds squawking and begging for food with a nest full of birds from another species. Why bother competing? Why not use a winner-take-all strategy like the cuckoo?
As you heard in the report, the cowbird actually does have a strategy, but it's best described as winner-take-most. The researchers studied the cowbirds' success by planting cowbird eggs in the nests of a songbird called the Eastern phoebe. The cowbird eggs either were placed alone in the phoebe nests or with two phoebe eggs.
Why two? Previous studies have shown that cowbirds seem to prefer two nestmates to more or fewer. The fact that the cowbirds in Hauber's study grew faster and ate more when they had nestmates confirms this evolutionary strategy. A cowbird with two nestmates can gobble up the lion's share of the food by outmaneuvering and out-begging its adopted siblings. More siblings makes the competition tougher; fewer siblings means there's less food to be had. (Of course, cowbirds with too many siblings sometimes level the playing field by starving out the weakest chicks, but this isn't as deliberate as the cuckoo's killing spree.)
Some species of North American songbirds have lived with cowbirds for centuries, and have reached a kind of equilibrium with them. But lately, cowbirds have been branching out into new areas, in large part because of deforestation (cowbirds prefer flat, open grassland). And as a result, they're parasitizing new species. Interestingly, birds that have lived with cowbirds for a long time tend to lay fewer eggs than the cowbirds' newer targets. Time will tell if these newly parasitized species start cutting their losses by laying fewer eggs themselves.
Now try and answer these questions:
- What's the difference between the parasitic strategies of cuckoos and cowbirds?
- How did the researchers show the advantages of the cowbird strategy?
- Why do you think these two different birds evolved such different approaches to parasitism?
- Describe an experiment that might explore the effects of cowbirds on the new species of songbirds that they are parasitizing.
Cornell University's page on Brood Parasites, from their Birds in Forested Lands website, has more information on cowbirds.
Cuckoos, Cowbirds, and Other Cheats, an award-winning book by N. B. Davies (ISBN 0-85661-135-2), provides an in-depth but accessible look at parasitic bird behavior.