Albion Monitor /Features
 Alien Invaders: Native Bird Acts Like a Transplant

Following the Crowd

by Simone Wilson

Cowbirds always lay their eggs in the nests of other birds

Spongers get a bad rap, and sponging is the cowbird's stock in trade. Instead of building their own nests, cowbirds always lay their eggs in the nests of other birds, hoping mother vireos, thrushes and flycatchers won't notice they're incubating one ungainly egg that's different than the rest.

Granted, a cowbird lays only one egg per nest. But since cowbirds aren't territorial, several in the immediate neighborhood may visit the same nest, like boors crashing a party when the host goes out for more beer. Cowbirds aren't especially choosy -- they've been known to lay eggs in the nests of over 220 species of fellow birds. Not all species make good foster parents, though. Some are savvy enough to spot the bogus egg and shove it out of the nest or peck it to pieces.

Whatever the response of the host bird, at least one of its own chicks is lost, because the cowbird typically destroys one egg for each one it leaves. If the mother bird is nervous about the intruder egg, she sometimes abandons the nest altogether, and then the whole clutch of eggs is lost. If it does hatch, the cowbird chick quickly becomes the bully of the nest: it hatches earlier, grows faster and grabs more than its share of the food.

Thrives on habitat destruction

Essentially the cowbird is a native that behaves like an introduced species: It moves in where habitat is disturbed, takes advantage of man's changes and thrives at the expense of other birds.

For many species in North America, the coming of man and his farms meant loss of habitat as well as decreased food supplies. But for the cowbird, man's encroachment was a big windfall.

About the size of a blackbird, the cowbird was largely confined to grasslands west of the Mississippi before colonists started coming ashore and knocking down trees to plant their crops. Cowbirds like open country and the edge of woodlands, and the extensive forests that prevailed in the east didn't leave the cowbird much elbow room for feeding and social displays.

Once the colonists made open patches, cowbirds started to move in. It's this "forest fragmentation" that leaves an opening for opportunistic species that prefer the forest's edge, because a forest broken up into small patches has more edge than one big woodland expanse. Birds that nest at the edge of the forest bear the brunt of the cowbird's nest mooching: vireos, warbles, tanagers, thrushes and flycatchers. The cowbird is a major culprit in the decline of eastern songbirds.

But cowbirds need more than just elbow room to increase their range. Like all species, they need extra food to expand in number, and it wasn't until the 20th century that leftover grain, especially in the rice fields of the south, sparked a cowbird explosion.

Technically the cowbird is native to California, since it existed near the Arizona border. "But they were abundant only along the Colorado River," says Ron Jurek, wildlife biologist for California Fish and Game. "Historically they didn't get across the desert."

The farming practices of European settlers changed all that, again by fragmenting forest lands and leaving plenty of waste grain lying around in the wake of harvests. Cowbirds made themselves at home throughout California, and now, says Jurek, "they're extremely abundant."

Their expansion comes at the expense of birds they parasitize. "The mortality of birds is pretty high anyway, but that's alright as long as the production of young keeps up with the losses," Jurek explains. If a species is in trouble, however, cowbird depredations "can be the breaking point."

For more information, visit the Smithstonian, which offers an excellent web page on cowbirds.

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Albion Monitor March 10, 1996 (

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