‘THAT’S A BAD BIRD,’ we may say in judgment of the brown-headed cowbird, who uses the nests (and nanny services) of other species to raise its young. But is such negative anthropomorphism valid—or are cowbirds actually brilliant survivalists? I asked my friends at the BirdNote public-radio program, my partner in an ongoing series. (Browse all past installments.)
Even the genus of the cowbird’s Latin name, Molothrus ater, probably deriving from the Greek molobros for “greedy fellow,” implies a bad reputation. Older sources say it also means vagabond, tramp or even parasite, and cowbirds are brood parasites—that is, laying their eggs in another species’ nest and not providing parental care. (Those are heavily speckled cowbird eggs in a house finch nest up top.)
That we put our human values on cowbirds for leaving their children behind reminded me of something from a class on bird behavior:
‘THINKING of animals like people is misleading and unhelpful, and offers no assistance in understanding animal behavior,” said the Cornell Lab of Ornithology instructor, Kevin McGowan. “Thinking of people as animals with the same survival goals can provide profound insights into what we do.”
With that in mind, I confessed to BirdNote consulting scientist Dennis Paulson (also an expert on dragonflies, remember?) that I actually like cowbirds. I enjoyed the first male I’d seen in months, strutting his stuff here last week, like the male in the photo below. But am I wrong, I asked Dennis—is the cowbird’s bad rap deserved?
“Cowbirds didn’t evolve a brood parasitic habit because they are too lazy to raise their own kids, or they enjoy the thought of reducing the populations of other perching birds,” says Dennis. “They did so because natural selection favored this unique form of reproduction.”
A very quick view of cowbird evolution (also reported in this recent BirdNote segment taped in Big Bend National Park in Texas) puts forth one popular theory: that cowbirds originated on the Great Plains, following herds of bison that kicked up seed that the birds ate. But the herds kept moving, and if female cowbird (above) stopped to nest—no more food. So they try using other species’ nests, and it works.
But as good a story as it is, Dennis says it’s not the real one.
“Cowbirds did not evolve brood parasitism because they followed herds of bison,” he says. “They spring from a group of blackbirds that almost surely originated in South America and spread northward through the Americas. No bison anywhere down there, and some cowbird species live in rain forest, where there are no herds of anything to follow.”
Plus, Dennis points out, the females are much more sedentary than the bison scenario would expect–which is why though they do follow large animals, it’s only on a very local scale. “They stay in a relatively limited home range, and are excellent nest finders only because they know what’s going on in that home range,” he says. “They have to follow other females around and find where they are nesting, then they have to check those nests, presumably daily,” to see if an egg has been laid yet.
“I call cowbirds ‘superb birders’ because of that skill,” says Dennis.
What’s important to figure into the cowbird evolution story is another factor, says Dennis, one caused by humans: We changed the literal landscape.
“We have favored cowbirds by opening up the landscape, providing them with cows and horses in lieu of the bison they often associated with, and having feed lots and crops where they can feed to their heart’s content in winter,” he explains. “They almost surely didn’t have such pronounced effects on their hosts until we came along.” Opening up the landscape also reduced habitat, and populations, of some other birds while giving cowbirds an edge.
Now cowbirds are almost chicken-like in their prolific egg production, laying 25 or more eggs in maybe a half-dozen clutches through the season. Impressive—but not without other impacts.
Cowbirds really can reduce populations of small songbirds, Dennis explains, especially if they are already uncommon, such as the Kirtland’s warbler, the black-capped vireo, or Southwestern willow flycatcher.
His attitude: “It’s just fine that cowbirds are a part of our environment until it turns out that some endangered species or population is being hammered by them, and then I think it’s perfectly appropriate that we start reducing cowbird populations in those areas.”
SOME HOST bird species have evolved counter-measures of their own. The yellow warbler, for instance, “evolved the counter-strategy of roofing over the eggs,” says Dennis, “which makes the yellow warbler something special in the ‘arms race’ between parasite and host. But plenty of their eggs get parasitized anyway.” Listen in to how this species piles on a new nest at the sight of unwelcome eggs.
Certain passerines—the perching birds—can easily remove the cowbird eggs because they have a large bill, such as robins, shrikes, kingbirds, thrashers and jays. They’re called “egg ejectors,” says Dennis. (At the BirdNote site, read Dennis’s article on cowbirds, parasitism and more.)
“But blackbirds, which might be able to do so, don’t,” he adds, with this exception: Many Bullock’s orioles pierce the cowbird eggs first to remove them from their nest; they couldn’t hold onto the egg in any other way. And marsh wrens do likewise.
Does that make the marsh wren or Bullock’s oriole a bad bird, a good bird, or what’s your take on the bigger scheme of things?
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(Cowbird eggs in house finch nest copyright James Marvin Phelps Photography; male display photos by Gregg Thompson; female cowbird by Tom Grey.)