Of Cuckoos, Cowbirds And Other Parasitic Birds

Earlier this month, a friend in Virginia attached a houselike bird feeder to one of the windows of her home. She was worried no bird would find it, but soon a finch moved in, made a nest and started laying eggs. My friend named her Molly.

But tragedy struck. While Molly was away one day, another bird visited the nest. That bird pushed out one of Molly’s eggs and laid one of her own. Molly returned, with little clue as to what had happened, and she now sits on a nest with four of her own eggs and one from the intruder:
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With the event caught on a motion-capture camera, and the eggy evidence left behind, the parasite was quickly identified as a brown-headed cowbird (Molothrus ater). Cowbirds are brood parasites. Cuckoos are the best known of these kinds of birds, but others include indigobirds in Africa and the black-headed duck. These are all birds that lay their eggs in other birds’ nests. And some, like cowbirds and cuckoos, destroy eggs already in those nests, giving these tales an especially gruesome twist.

It’s hard not to judge creatures like that — the consensus on Facebook is that Molly’s cowbird intruder is a murderer — but the natural world is rarely fairy tale perfect. Brood parasitism is just another life strategy that creatures use to pass on their genes to the next generation. By laying her egg in another bird’s nest, the parasite can pass off the cost of child-rearing to the other bird and instead invest her energy and resources in mating and producing more eggs. The brown-headed cowbird, for instance, will produce an average of 80 eggs in just two years. (Female cowbirds are a little like chickens in this way; they keep producing egg after egg for two months so they can take advantage of all the available nests during that time.)

The tradeoff for this strategy is that only about three percent of cowbird eggs (2.4 eggs) will survive to adulthood. That’s despite a few advantages that baby cowbirds get: They usually hatch a day earlier than the other eggs in their nests. They tend to get more food than the other fledglings, probably because they’re louder and more demanding. And mom already offed some of the competition.

What does in many cowbirds is that mama birds aren’t discriminating when they pick nests in which to lay their eggs. Scientists have observed brown-headed cowbirds placing eggs in the nests of 220 or so other species, but these events have only been successful in 144 of those species.

The house finch — Molly’s species — is probably not one of them. A 1996 study in The Condor documented 99 cases in Ontario, Canada in which brown-headed cowbirds parasitized house finch nests. Almost 85 percent of the eggs hatched, but none were successfully reared. The researchers suspected that it was due to a mismatch in diet. Passerine birds like cowbirds prefer a diet of arthropods, but in a house finch nest they’re likely to only get seeds. The baby cowbirds don’t get enough nutrition in finch nests.

Most other brood parasite species won’t have such problems because they’re specialists and only use the nests of one other species. Their eggs may be adapted to look like the eggs of that other species, and their offspring might mimic the other nestlings. (Cuckoos don’t pick one single species to mimic, but they can match their eggs to those of several different species.)

The story of Molly’s family is still unfolding, but there is reason to worry further. Cowbird nestlings have been known to push eggs and nestlings out of the nest and to smother their nest mates. With the cowbird egg facing its own uncertain future, this could get ugly. Nature can be cruel.

Keep up with Molly on the Molly.Finch channel on Ustream.

Image courtesy of Kris Trader