Birds
Cowbird
Molothrus · also called cowbirds
A cowbird is a bird that has outsourced parenthood entirely. It builds no nest, incubates no eggs, and feeds no chicks. Instead the female slips her eggs into the nests of other birds — more than 200 species in the case of the brown-headed cowbird — and lets them do the work, often at the cost of their own young.
Overview
Cowbirds are New World blackbirds in the genus Molothrus, and all are obligate brood parasites. They feed mainly on seeds and grain, and on insects stirred up by grazing herds — the habit that gives them their name and, historically, tied the brown-headed cowbird to the bison herds of the plains. Following the herds meant the birds could never stay put to raise a family, and parasitism is thought to be the result.
How it works
A female cowbird watches other birds build, waits until the nest is unguarded, often removes one host egg, lays her own, and leaves. Freed of all parental duty, she can lay a remarkable ~40 eggs a season. The cowbird chick usually hatches first and grows fast, taking most of the food, so some or all of the host’s own chicks starve — a warbler may fledge a cowbird three times its weight and lose its whole brood.
Defences and the “mafia”
Some hosts reject cowbird eggs — puncturing them, burying them, or abandoning the nest — but many small songbirds that met cowbirds only recently, as forests fragmented, have no defence. There is evidence that cowbirds retaliate against nests where their egg was removed, raiding them, which may be why so many hosts accept the egg.
Status
Least Concern and abundant — the brown-headed cowbird expanded massively with land clearance. The conservation problem runs the other way: cowbirds are a serious threat to rare hosts like Kirtland’s warbler and the black-capped vireo, which are protected by cowbird-removal programmes.
Cowbird: common questions
Why do cowbirds lay their eggs in other birds' nests?
Because their way of life makes tending a nest of their own impractical, and evolution found a workaround. The brown-headed cowbird developed alongside the roaming bison herds of North America, feeding on insects the herds kicked up, which meant it had to keep moving and could not settle in one spot for the weeks a nest requires. Laying eggs in the nests of resident birds let the cowbird keep following the food while other species did the incubating and feeding. Once the behaviour was established, natural selection refined it, and the cowbird lost the ability and the instinct to build nests entirely.
What happens to the host bird's own chicks?
Often they die. The cowbird egg usually hatches a day or two before the host's eggs, giving the cowbird chick a head start, and it tends to be larger, beg more loudly, and grow faster, so it takes most of the food the parents bring. Smaller host chicks are frequently outcompeted and starve, or are crowded out of the nest. Some hosts fledge a cowbird and none of their own young. Unlike a young common cuckoo, the cowbird chick does not usually push the other eggs out, but the outcome for the host brood is often just as bad.
Can host birds fight back against cowbirds?
Some can. A number of species recognise and reject cowbird eggs, either puncturing and removing them, burying them under a new nest floor, or abandoning the nest and starting again. The yellow warbler often builds a fresh nest layer over a parasitised clutch, sometimes several storeys high. Larger, sharp-eyed hosts like robins usually eject cowbird eggs outright. But many small songbirds that only recently came into contact with cowbirds, as forests were fragmented, have not evolved defences and accept the egg, which is why cowbirds hit those species hardest.
Is it true cowbirds punish birds that reject their eggs?
This is the mafia hypothesis, and there is some evidence for it. Studies of brown-headed cowbirds and, more strongly, of the shiny cowbird found that when hosts ejected a cowbird egg, their nests were more likely to be raided and their remaining eggs destroyed, as if the cowbird were retaliating and forcing the host to accept parasitism as the lesser cost. The idea is debated and the effect is not universal, but it offers a possible reason why so many hosts tolerate cowbird eggs even when they seem able to recognise them.