In the avian world, cuckoos are the villains you root for. These diabolical birds can trick others into raising the cuckoos' young instead of their own. From a thick playbook of deceptions, one trick cuckoos use is to impersonate local bullies. This apparently convinces their victims to let cuckoos walk right into their nests.
Cuckoos live all over the world, and most species are model citizens, building their own nests and raising their own offspring. But many species are so-called brood parasites, which sneak their eggs into other birds' nests. Some species match the egg's color to their host's eggs to disguise it, while others don't bother, depending how clever their preferred targets are. In certain species, male cuckoos goad the host parents into chasing them off while females creep into the nest and lay their eggs.
The parasitic cuckoo hatches earlier than the other eggs in its nest and gets a head start in begging the host parent for food. It may mimic the appearance of its nestmates. Often, it kicks them out of the nest altogether. The clueless host parent feeds and raises the young cuckoo until it can fly off on its own.
Obviously, it's in the best interest of host birds to keep cuckoos out of their nests in the first place. Parasitic cuckoos and their host species engage in a constant evolutionary arms race, with the parasite's tricks and the host's defenses always improving. Thanh-Lan Gluckman, a Ph.D. student at the University of Cambridge, and her advisor, Nicholas Mundy, studied one of these tricks: plumage that disguises cuckoos as birds of prey.
It's no secret that certain cuckoos resemble certain raptors, and vice versa. The hawk in the photo above (left) is named the African cuckoo-hawk because of the likeness. Gluckman and Mundy wanted to measure that likeness: How similar is the plumage of raptors and cuckoos? And is that similarity stronger in species that live in the same area, suggesting the cuckoos have evolved to mimic specific birds?
The researchers focused on the chest feathers, where many cuckoo species have a "barred" pattern that's similar to many raptors'. It makes sense that the cuckoo's front side would be disguised, rather than its back, because that's what a host bird sees as a cuckoo swoops toward its nest. (And the last thing it sees before its young are replaced with aliens.)
Using museum samples, the authors photographed the plumage of representative birds. Then they transformed the images digitally to represent how they'd look through a bird's eyes. Characteristics of the barred pattern—how big the markings are, how consistent or variable the pattern is, and so on—were compared between five Old World cuckoo species (each representing a different genus) and raptors that share their territory.
All five cuckoos had patterns that matched a local raptor, such as a hawk or buzzard that overlapped with their territory. But when the scientists compared pairs that didn't live in the same area, there was no match. This suggests that cuckoos don't just imitate raptors in general; instead, they've evolved to match specific birds that live around them.
If the raptor it's imitating is local, that means the cuckoo's intended victim—the bird whose nest it's about to invade—will recognize it too. Gluckman says the purpose might be frightening host birds so they don't attack, "or making them misjudge what the cuckoo is for long enough to access the nests." A cuckoo can toss its host's eggs from the nest and lay its own in just ten seconds, she says. Alternately, males that are imitating a dangerous raptor might convince host birds to chase after them, also buying the female time to sneak into the nest.
It will take more research to show exactly how host birds react to an approaching cuckoo disguised as a bird of prey. Probably how they should react is to just move underground, because nothing else seems to be working.
Images: Left, African cuckoo-hawk by Ken Clifton; right, Oriental cuckoo by Tom Tarrant (both via Flickr). Thanh-Lan Gluckman, & Nicholas I. Mundy (2013). Cuckoos in raptors' clothing: barred plumage illuminates a fundamental principle of Batesian mimicry. Animal Behaviour DOI: 10.1016/j.anbehav.2013.09.020