An Avian Villain Does Good — Sometimes

A Great Spotted Cuckoo Chick Raised Alongside a Carrion Crow Chick

Great Spotted Cuckoo Chick Raised Alongside Carrion Crow Chick Credit: Vittorio Baglione

Great spotted cuckoos, found all along southern Europe during breeding season, are notoriously lazy parents. They lay eggs in the nests of unsuspecting carrion crows, which raise them alongside their own, and at their expense — or so we believed. Researchers in Spain report an unexpected finding in Science this week: cuckoo hatchlings, in fact, confer protection on the entire brood.

Each spring, on average, the crow lays five eggs and doesn’t seem to worry about an egg or two the cuckoo might’ve snuck in. When the brood hatches, the cuckoo nestlings exude a nasty-smelling secretion made of phenols, indoles, and sulfides whenever cats, raptors or other crows drop in to hassle the chicks. That seems to do the trick and keep the predators away. Crow-parents can focus on finding food for the young. All this adds up to greater reproductive success for crows in a season when predator pressure is high. In a good year, however, more mouths to feed means fewer crows will fledge from the nest.

Canestrari and her colleagues have drawn upon 16 years of nesting data from a bird population in Northern Spain. They also transferred cuckoo chicks into crow’s nests to see how it affects success rates and chemically analyzed the substance which proves beneficial to the brood. Depending on the environmental context, host-guest species can run the gamut of interactions: parasitism (one thrives at the expense of the other), commensalism (one thrives, no gain or loss for the other), or mutualism (both benefit), the study’s authors write.

“This is a wonderful study combining long-term data and experiments to show that avian brood parasitism is not only one of the most exciting systems to study coevolution (how two interacting species evolve reciprocally) but also that interactions between species are often more complex than initially thought,” says Oliver Krüger, Professor of Animal Behavior at Bielefeld University in Germany, who was not involved in the research.

So is the great spotted cuckoo’s villainous reputation undeserved? Not so fast. The female lays about twenty-six eggs per season and sneaks a few into European magpie nests as well. The magpie being a smaller host, its chicks rarely survive when they share a nest with the cuckoo, says Canestrari. Host hatchlings starve to death in competition for food. Consequently, early in the breeding season, adult magpies tend to chase away cuckoo interlopers and evict alien eggs from the nest.

But the carrion crow does nothing against the sneaky guest.  Now, we know why.