In Plutarch's Life of Aratus, the cuckoo asks a group of birds why they flee from him, as he is not ferocious. The birds answer that they're afraid of him, because one day he will be a hawk. Glance at a cuckoo and a sparrowhawk side by side, and it's easy to see how the ancient Greeks often confused the two species. Now, researchers have shown why Plutarch's frightened birds made the same mistake.
Cuckoo and sparrowhawks aren't closely related, yet they share similar size, shape, and plumage--particularly their pale, barred legs, bellies, and breasts. Ostensibly, cuckoos have evolved this guise to scare away smaller birds so that they can swoop in on their nests and quickly lay an egg. When the birds return, they treat the cuckoo's egg as one of their own; once the cuckoo chick hatches, it heaves the other eggs overboard.
Reed warblers, which have evolved side by side with cuckoos for thousands of years, no longer fall for the ruse. In an as-yet-unpublished experiment by behavioral ecologists Nick Davies and Justin Welbergen, both of the University of Cambridge in the U.K., warblers viciously defended their nests when stuffed cuckoo models were placed nearby, whereas they fled in alarm from stuffed hawks. How would other birds react?
Davies and Welbergen placed an assortment of taxidermy models, including harmless controls (collared doves and teals) and sparrowhawks and cuckoos near peanut feeders. Blue and great tits, which are not normally harassed by the cuckoo, ignored the controls but fled the hawks and cuckoos, making agitated alarm calls (seet!) even after the models were removed. When plain white silk panels were pinned over the cuckoos' barred underparts, the tits fed calmly. But dressing the cuckoos in hand-barred silks--to emphasize these markings--sent the birds into a tizzy again. "They treated barred cuckoos like hawks and unbarred [camouflaged] ones like doves," says Welbergen, noting the importance of the barred patterns. The team reports its findings today in the Proceedings of the Royal Society B.
Mark Hauber, a behavioral ecologist at the University of Auckland in New Zealand, calls the experiments "powerful" tests of the long-held belief that cuckoos are indeed mimicking the plumage of sparrowhawks. The findings are "the first evidence for a 2000-year-old belief that has never been rigorously tested," adds behavioral ecologist Oliver Kruger of the University of Cambridge. A still-open question is how warblers have learned to tell the difference between the two birds. Davies and Welbergen say they'll try to answer that with their next set of experiments.