Cowbirds, with their sleek brown heads and shiny, black-feathered bodies, even look like they might be up to no good on occasion. New research suggests that indeed, they’ll go to even greater lengths than previously imagined to ensure their continued success, sometimes at great cost to other bird species.
Cowbirds are brood parasites, meaning they lay their eggs in the nests of other birds, including everyone from yellow warblers and song sparrows to eastern phoebes and red-winged blackbirds. They are the only brood parasite, as these lay-and-run birds (or insects, or fish) are known, found in North America.
Each brood parasite has its own modus operandi. Cowbirds parasitize the nests of hundreds of species of host birds. They’re not picky. The common cuckoo also parasitizes many other bird species; however, maternal lines of females specialize in one particular host. As a result, their eggs have evolved to mimic that host’s egg size and coloration. Cowbird eggs just look like, well, cowbird eggs (white, speckled with brown).
The cowbird does share one important trait with another brood parasite, Europe’s great spotted cuckoo. Both species stick around after egg-laying to ensure that the host mother raises the foster kid. If she doesn’t, it turns out, her genetic offspring are toast.
This tactic has been dubbed “mafia behavior” and was first proposed in 1995, when scientists were struggling to find an explanation for host birds’ apparent indifference to the presence of an obvious imposter in the nest. The scientists reasoned that caring for the demanding (and sometimes murderous) nestling must benefit the host mother’s progeny in some way.
To find out, the scientists observed the great spotted cuckoo and its usual target host, the magpie. When the scientists experimentally removed cuckoo eggs from a magpie nest, the cuckoos usually returned and damaged or destroyed that nest. In that light, the safer bet for the magpie is to care for the cuckoo egg. Then, at least some of her offspring might survive. This retaliatory behavior was only known to exist in the great spotted cuckoo, until recently.
In 2002, a team of scientists from the Illinois Natural History Survey began removing cowbird eggs from the nests of one cowbird host, the prothonotary warbler, in order to maximize warbler production. To their dismay, that effort resulted in more nest and egg failures than before egg removal. The only possible culprits, they believed, were cowbirds - the same cowbirds, in fact, who laid the impostor eggs in the first place. Were cowbirds capable of mafia behavior, too?
Dr. Jeffrey Hoover, who led the team, knew that cowbirds vigilantly monitor host nests before they lay their eggs, in order to synchronize their egg-laying effort with that of the host mother. What Hoover didn’t know - what nobody knew - was that the cowbird, like the great spotted cuckoo and its magpie, will then stick around just to make sure the warbler accepts it.
To test his theory, Hoover modified some of the nest boxes in which warblers and cowbirds had laid their eggs such that only the warblers, and not cowbirds, could fit through the opening. Then, he and his team removed the cowbird eggs from every nest. Over half the nests with openings large enough for cowbirds were damaged in some way.
Hoover suggests that this strategy optimizes a cowbird’s chance of nesting success. If a warbler nest is trashed, the warbler will usually start a second nesting attempt somewhere nearby. This allows the cowbird to have a second “nesting” opportunity, too. Indeed, second-chance nests were parasitized by cowbirds 85 percent of the time.
Hoover also suggests that this behavior may have shaped cowbird-warbler interactions through time: by destroying the eggs of egg-ejecting warblers, cowbirds are “selecting” against future egg-ejectors.
Mafia behavior may increase not only the current cowbird egg’s success but that of future eggs, too. “I would speculate,” says Hoover, “that [cowbirds] may even monitor the fate of nests that they have parasitized to provide themselves with information regarding who they should parasitize in the future (this could tell them which species of host are good at raising cowbird chicks).”
If a cowbird can wait around for her egg to hatch, why can’t she care for it properly herself? One theory involves cowbirds’ historical association with roaming bison herds. A constantly moving cowbird’s only choice was to lay its egg in the existing nest of another bird. Over time, they may have entirely lost the ability to build nests and raise young.
However, as in the case of other brood parasites, cowbirds’ parasitic breeding strategy may simply have evolved as a means for maximizing reproductive potential: a lone cowbird is capable of laying over 40 eggs in one season. Now that’s a recipe for success!