What’s wrong with this picture? The chicks should all look the same, but one nestling is big for its age, and has a much bigger and brighter mouth than its nestmates. That’s because it’s not really a baby Eastern Phoebe – it’s a chick from one of North America’s most widespread parasitic birds, the Brown-headed Cowbird (Molothrus ater).
Cowbirds are brood parasites: they don’t sneak under other birds’ feathers to grab a meal (they actually eat insects and seeds), but they leave their eggs in other birds’ nests. Each spring, when other birds are finding mates and building nests, female cowbirds are hiding in the shadows: spying on nesting birds and until they see a chance to rush in and lay an egg in their brood. Cowbirds are veritable egg factories: each female can lay more than 3 dozen eggs in a summer, spread one at a time into nests from about 140 different host species. And although a few hosts can recognize cowbird eggs and either eject them, cover them over, or abandon the nest completely to start over, most host species wind up raising a cowbird changeling along with their own chicks.
Interestingly, although a cowbird chick hatches first and grows faster than its nestmates, it doesn’t typically take the European cuckoo route of tossing its foster siblings over the side. Instead, it helps its nestmates make a fuss when the adults come by the nest – and the extra noise convinces the parents to go out and forage more than if the cowbird chick was home alone. But when they bring back the goodies, the cowbird’s extra big mouth and aggressive begging makes sure that the parents feed it the most – over half the feedings instead of a more equitable one-third, according to a 2004 study by Mark Hauber (now at Hunter College) and his colleagues at the University of Cambridge. This means that a cowbird chick with nestmates actually gets more food (and grows faster) than when it’s alone in a nest. And once it’s grown and fledged, it’ll fly off to take its own turn making changelings.
Photo by Mark Hauber.
Kilner, R. (2004). Brood Parasitic Cowbird Nestlings Use Host Young to Procure Resources Science, 305 (5685), 877-879 DOI: 10.1126/science.1098487