How the con artists of the bird world get other birds to raise their babies.
The brown-headed cowbird (Molothrus ater), a native of North America, doesn’t build a nest. It also doesn’t feed its chicks, stick around for their first flight, or comfort them during storms. How can a bird species survive when it doesn’t care for its young? The answer involves covert egg-laying, chick sabotage, and the unwitting generosity of other songbirds.
Cowbirds are constantly on the move, following herds of bison or cows to eat the insects cattle stir up from the ground. Since their food source is always roaming, the birds have no time to settle down and build a nest. Cowbirds lay their eggs in the nests of other songbirds, relying on them to provide free babysitting while they follow the herd. A single female cowbird typically lays about 80 eggs over the course of two years — no small burden for other birds stuck taking care of them!
This behavior, known as brood parasitism, allows the cowbirds to follow their food source without having to stay behind and tend their young. It’s the perfect solution, except for one problem: how do young cowbirds know how to follow in their parents’ footsteps instead of imitating the songbirds who raise them? Misimprinting, or mistakenly thinking the host species is their own species, can cause cowbirds to sing their hosts’ songs and even try to mate with members of the species they were raised by. This cross-species confusion may explain why only about 1% of all birds are brood parasites.
To get around the potential for misimprinting, cowbird fledglings fly away from their nests at night to meet up with other cowbirds. Likely motivated by an instinct to roost in a field rather than a forest, the fledglings strike out on their own under the cover of darkness, where the cowbird young run into a whole flock of birds just like them! By sneaking out at night to see their “real” family, the young cowbirds learn normal cowbird behaviors. They return before dawn, their host families none the wiser.
The cowbirds’ brood parasitism comes at a price to parasitized songbirds. Cowbird eggs tend to hatch faster than the other chicks in the nest, giving cowbird chicks a competitive advantage over their foster siblings. The cowbird chicks can fight for more food from the parents, sometimes smothering other chicks or throwing them to the ground. They grow faster, easily bullying the other chicks for more food. Meanwhile, the songbird parents have no idea the cowbird chick isn’t one of their own.
At least, that’s what we’ve always thought. But the songbirds may have another motivator for taking care of the greedy cowbird chicks besides ignorance: fear. In one experiment, researchers removed cowbird eggs from warbler nests. Later, the mother cowbirds returned to terrorize and destroy the nests in retaliation. This suggests songbirds are forced to take care of the cowbird chicks or face the retaliation of the angry mother cowbirds. Video surveillance shows the mother cowbirds returning to nests they have laid eggs in to check on them, monitoring whether the songbirds need to be taught a lesson. This helicopter-parent behavior likely motivates songbirds to raise cowbird chicks even if they do recognize them as interlopers.
While it may seem underhanded of cowbirds to
parasitize other birds, this is the way they’ve found to accommodate a mobile
food source. It is a smart way to get around a challenging problem, and it’s
not all roses for the cowbirds either — about 97% of cowbird eggs and nestlings don’t
reach adulthood. No one has it easy out there. We’re all just trying to make it
in a bird-parasitize-bird world.
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