There are times when the sky goes dark with cowbirds, flying into new territory like a tiny version of the winged monkeys in The Wizard of Oz. The females swoop in at dawn and lay their eggs in OBNs, then move on. (If you want to take over the world, it seems, don’t get attached to it. Any of it.)
A female cowbird has to be able to detect the characteristics of a good host nestone where the diet includes insects, for examplethen lay her egg there at exactly the right time. “If they lay their egg even a day later than when the host starts incubating it, their young’s going to be hatched a day after everybody else, and they’re going to be at a complete competitive disadvantage,” notes White, who has slipped a Cadbury candy into a nest as a decoy. “Because the mothers typically don’t feed the ones that need the food; they feed the ones that beg the most.” Cowbirds “seem to grow faster, beg more, and actually hatch a few hours earlier than typically your host species does,” he adds. “And that’s why they get all the food.”
That, in turn, appears to affect the birds’ development. “There were more survivors from the early-hatch than the late-hatch batch,” White notes. “They gained weight faster. There are a whole bunch of early markers that suggested the early guys were healthier than the late guys. They eventually all caught up in weight, but their behaviors are somewhat different, so I don’t know what to make of it yet. After I analyze the data, maybe I will.”
So far, he hasn’t seen much difference between, say, cowbirds raised by red-winged blackbirds and those raised by juncos. But he hasn’t yet attempted a study of the subject. “Cowbirds parasitize over 200 different species, and we’ve never seen any other systematic differences from babies,” he says. “In some parasite species there is a big influence on the young, but I don’t think there is much of that with cowbirds. There’s not much for them to learn early on and they just don’t learn anything.
“I think what happens to a cowbird around Day 30 is that they get hungry, and just about every single cowbird from any kind of nest gets hungry and goes and finds food, and they end up getting together with a bunch of other cowbirds who are also hungry and looking for food. And I think that’s where a lot of the learning begins for themsocial learning about how to be a cowbird.”
Three young malescall them Manny, Moe, and Jackare sitting on a branch, talking trash and getting nowhere. Every now and then they face off with each other; give a big song and a big displayfanning out their wings, then bowing, like sumo wrestlers, and wiping their beaks. Eventually they go off and sing to their femalesso far, with little success.
“They’re all kind of doofuses when it comes to courting females,” says White. “It’s getting late in the breeding season and they’ve learned a bit, but”
Just then, Manny flies over and sings to a female. She sidles away a little, but doesn’t take off. White looks pleased. “That’s a pretty good response for a female,” he says. “So he should take that positively and sing another song to her.” Manny obliges: Glup, glup whistle. “Now another male’s coming inthat’s just gonna mess up his whole technique.”
Contrary to my expectations, the feathers don’t fly. What goes down is more like a very mild, PG-rated version of street rappers battling.
“The facing off between males is usually with song,” says White. “I’ve seen maybe 10 fights in the entire breeding season, so they’re not frequentbut they can be significant to the male. One fight can really change a male’s status if he gets his eye pecked or something like that.” But the juveniles, he adds, “never really get in any kind of serious fights.”
The cowbird’s song provides a tangible way to measure behavior, “because you can print out sound spectographs of songs and compare them directly,” says White. “You can really characterize these songs, quantify them, and it becomes a printout of behavior, if you will.
“What a male is doing at any particular moment in time really is an image of what he’s learned and what social experiences he’s had,” he adds. “If he has a super-fantastic song, it’s probably because he had a social environment where he wasn’t tested, or he’s copied somebody, or he’s been in a good relationship with a female and has been able to hone his song into something good. You can say the same thing for his whole behavior pattern.”
When a male cowbird sings, he takes a big breath, puffs up his chest like Pavarotti, and emits a variation on the theme of Glup glup, whistle. The glup part has an alluring, liquid quality to it, and sounds like a noise your computer might make to indicate an incoming email. The song starts off lowvery low, with notes in the barely audible 100-hertz rangebut can soar up to a piercing 12 kilohertz when they give their whistle. (Female cowbirds do not sing, though they occasionally give a chirping “rattle” that drives the males wild.)
To get the right sound, male cowbirds switch back and forth between two airways in their throats. This doesn’t happen overnight, White points out. “They spend their whole first year just singing a pile of garbage until that sort of comes in.” Their repertoire is limited to about six variations on a themeunlike, say starlings, which will incorporate everything from a car alarm to a dog’s barking into their song.
A few seconds after Moe sings, Jack counter-sings: Glup glup glup, whistle. Though White has found counter-singing to be a key predictor of reproductive success, the exact evolutionary motivation for doing it is still open to debate.
The standard view is that it’s “all about male-male competition,” says White. “Males compete for females, so if you can establish yourself that high in the social order, you’ll have priority access to females, and you won’t be disrupted when you’re courting a female. But the more I watch these guys, the less I believe it. In fact, the more I’m starting to see male-male singing as performance for the female. I think it provides the females with more information.”
After all, a female can’t really judge a male’s genetic quality by his song, since he might just be copying another male’s song. In fact, a male with a great song will sometimes be attacked by other males until he learns to tone it down. But if a male sings to another male and can defend himself while doing sothat’s information a girl can use. Females, says White, “are really, really sensitive to that kind of thing.”
On the whole, “we’ve overemphasized male-male competition and underestimated female-female competition,” he adds. “The females are much more subtle and very polite, not big and in your face like the males. But there’s a whole lot of female-female competition. One female shows her interest in a male, the other females can’t let that go.”
The more he studies cowbirds, the more he’s coming to a view that some human males will undoubtedly find sobering.
“I’m starting to think it’s by the act of the female picking the male that the male becomes a good male,” he says. “That the female is really making the quality of the male for him.”
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©2005 The Pennsylvania Gazette
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COVER STORY: The Cowbird Variations