Cuckoos are notorious freeloaders, conning other species into rearing their young, often at the expense of the hosts' chicks. But a new study suggests that the parasitic birds are not mere opportunists.

Like thieves who yell "fire" to clear a store before robbing it, cuckoos appear to have evolved hawklike plumage patterns and physical traits that temporarily scare potential hosts from their nests so they can lay eggs in them and get other birds to rear them.

Bird experts have long noted that Old World cuckoos resemble certain small hawks. Even the ancients observed the likeness. Roman naturalist Pliny, for one, believed that cuckoos converted to hawks for the winter, a notion that Aristotle rejected on the ground that cuckoos have neither talons nor curved beaks.

One explanation for the resemblance is "protective mimicry." Cuckoos taste good to hawks, so anything that makes them look more hawklike and less like lunch helps them survive. Another possibility: rather than using mimicry for protection, cuckoos use it to improve their chances with their "prey".

By resembling hawks in key respects—elongated body, long wings and tail—they frighten smaller species. Only in this case, they are less interested in eating their victims than in driving them away from their nests for a while.

The new study offers the first evidence for the latter explanation. Zoologists Justin Welbergen and Nick Davies of the University of Cambridge in England set up several peanut feeders in and around their bucolic  campus for great and blue tits, two common European species not known to be targets of cuckoos. They then recorded the birds' reaction to the presence of stuffed sparrow hawks (Accipiter nisus)—a close relative of the sharp-shinned hawk—and to common cuckoos.

Tits seemed equally agitated by either interloper, as long as the cuckoos had barred bellies like the raptors. But the little birds showed no such fear of teal or doves, even when the doves were given barring.

"Cuckoos with barred underparts were treated like hawks, while unbarred specimens were treated like innocent doves," says Welbergen, who published the findings in the Proceedings of the Royal Society earlier this year. "This indicates that naive small birds, with no history of cuckoo parasitism, can mistake cuckoos for hawks, suggesting that discrimination by hosts with a history of cuckoo parasitism is an evolved antiparasite response."

Scott Robinson, of the Florida Museum of Natural History in Gainesville, and an expert in brood parasites, says experimental demonstrations of mimicry are tricky. The British researchers "solved this problem very nicely," he says. "It makes perfect sense that cuckoos would want to scare host species."

Stephen Rothstein, a zoologist at the University of California, Santa Barbara, offers another theory: He believes that mimicry may be a way of avoiding the pestering attention of species the birds don't target, who might alert potential hosts to their presence.