Cuckoos are consummate freeloaders; they lay their eggs in other nests, tricking unrelated birds into caring for their young. But why do only some of these unwitting foster parents kick out the obviously foreign eggs? A new study shows that such eggs may sometimes resemble each other in the ultraviolet spectrum, which birds can see. Researchers say this study emphasizes that biologists must take a bird's-eye view when studying bird behavior.
In the evolutionary arms races between cuckoos and the birds whose nests they parasitize, cuckoo eggs that look like their hosts' eggs are thought to have an advantage. Meanwhile, hosts ought to improve their ability to identify and remove the imposters. But some hosts do not reject cuckoo eggs even when they're a bad match. Some hypotheses have been proposed to explain this behavior, but now there's a new possibility: Perhaps apparently badly matching eggs may in fact match pretty well--but just not in the wavelengths of light that humans can see.
Behavioral ecologists Michael Cherry of the University of Stellenbosch, South Africa, and Andrew Bennett of the University of Bristol, United Kingdom, studied Africa's Red-chested Cuckoo, which parasitizes the nests of the Cape Robin and 21 other species. They took eggs of the cuckoo and its hosts from museum collections of parasitized nests and asked 15 people to rate how similar the eggs were on a scale of 1 to 5. Then the researchers analyzed the eggs with a reflectance spectrophotometer, which measures attributes such as brightness, hue, and saturation in visible and ultraviolet light. Birds perceive this entire spectrum. The spectrophotometer identified resemblances the human observers missed, particularly those in the UV part of the spectrum, the researchers report in 22 March issue of the Proceedings of the Royal Society.
Other behavioral ecologists are impressed. "They've shown quite convincingly that what seem to be nonmatching cuckoo eggs to a human eye might match in the UV," says University of Cambridge's Nicholas Davies, who has developed much of the theory of cuckoo-host coevolution. "We've got to see eggs in the ways that birds see them," he says.