How cuckoos became con artists of the bird world
They've been revered in Native American traditions, immortalized in the plays of Shakespeare and carved on Bavarian clocks.
Some cuckoos raise their own young, but others are brood parasitesbirds that leave their eggs in other birds' nests, duping the hosts into rearing their offspring and freeing themselves in the process to lay more eggs instead of investing time and energy in foraging and feeding babies.
By studying evolutionary relationships across the spectrum of cuckoo behavior, "we hoped to learn how brood parasitism evolved," says Robert Payne, professor of ecology and evolutionary biology. "We think that dinosaurs were good parents for their eggs, and birds inherited parental care from their dinosaur ancestors. That's why it's of interest to see how the parasitic cuckoos lost this parental behavior."
In his office at the Museum of Zoology, where he is a curator, Payne shows off specimens, some collected more than a century ago. He and collaborator Michael Sorenson of Boston University used these, along with field observations, song recordings and specimens from other museums, to work out relationships among the various species. But to Payne, the feathered skins are not just research materials; they're things of beauty.
"This one's called a Blue Coua," he says, pointing to a specimen with peacock-blue feathers and a purplish tail. "I like the color. Next year I'd like to go to Madagascar and see some of these alive."
Though he's never seen Blue Couas in the wild, Payne has observed plenty of other cuckoos, including the Black-billed Cuckoo and the Yellow-billed Cuckoo, which live in Michigan. But it was in Africa that he first became fascinated with the birds' habits. On a research trip in the 1960s, he began investigating the idea that brood parasites can afford to lay more eggs than can birds that expend a lot of energy raising their own young.
"The idea is, the parasites don't have the limitations that normal birds have, so the prediction is that they would lay more eggs," he says. When Payne compared the ovaries of parasitic Black Cuckoo females with those of nonparasitic species to see how many eggs they had laid in their lifetimes, he found that the prediction held true.
A book written by Payne, "The Cuckoos" contains descriptions of all 141 species, from the Chattering Yellowbill to the Roadrunner, along with 20 color plates. Also included is a family tree analysis that offers insights into the evolution of the birds' social behavior and patterns of parental care.
In more recent work, Payne and Sorenson have tried to understand how cuckoos became parasitic in the first place. The question puzzled Charles Darwin, who thought the behavior might have arisen from species that occasionally lay eggs in other birds' nests, but also raise their own broods, as Yellow-billed and Black-billed Cuckoos.
Another parenting pattern that might lead to brood parasitism is cooperative breeding, seen in cuckoos such as Anis and the Guira Cuckoo. In these birds, several mated pairs share a nest where they all lay eggs and care for the young. But it's not all peace and love in the avian communes. Sometimes a female pitches out nestmates' eggs when she lays her own.
Payne and Sorenson's analysis, which combined traditional morphological methods with Payne's song recordings and Sorenson's molecular genetic analysis, show that brood parasitism originated more than once in the cuckoos; not all parasitic species are from a single lineage. And their analysis confirms what Darwin hypothesized: the trait seems to have come from the habit of occasionally laying eggs in another bird's nest, not from cooperative breeding.
Another question that piqued Payne's curiosity was this one: Are brood parasites wily or just too witless to raise their own offspring? To get at the answer, one of his research associates poured BBs into empty cuckoo skulls to estimate the birds' brain sizes. Parasitic cuckoos, he found, have smaller brains, suggesting that they may have lost abilities they once had.
Even after decades of research, Payne says there's plenty more work to be done.
"I still want to do field work on some of the little-known cuckoos in Madagascar and Southeast Asia . . . collecting songs, watching behavior, working with local people who know where the birds are nesting," he says. "We're also looking at other families of birds with extreme behaviors from the point of view of learning how those behaviors might have evolved."
So for now, at least, Payne isn't about to close the book on cuckoos.