How birds avoid cuckoos

By John Upton

One of nature’s more ridiculous sights is that of a pair of adult birds rearing an oversized cuckoo chick.

Cuckoos are members of a large family of birds, some of which have done away with chick rearing, instead depositing their eggs in the nests of other species. This is called brood parasitism.

The parasitized birds rear the cuckoo chick as if it were their own, even as it grows to dwarf them in size — and as it pushes any other chicks from the nest to certain death.

Illustrated by Perry Shirley.
Illustrated by Perry Shirley.

(Ever noticed a dead chick beneath a tree and wondered how it fell out? Next time look up for a nest, and wonder whether a cuckoo is being raised therein.)

It seems that the the parental compulsion to raise young is so strong that the parasitized birds remain blind to the possibility that the brood contains none of their own DNA.

While many bird species remain oblivious to what would seem to be obvious signs that they are raising an unrelated chick, selection pressure has of course led to the evolution of some defenses against brood parasites.

Scientists compared defensive strategies developed by barn swallows living in China with those in Europe and found that they developed different defenses.

In Europe, martins and barn swallows, which seem to be better than some similar species at avoiding cuckoos, prefer to build their nests indoors. That’s where cuckoos are less likely to strike; they prefer open areas and avoid human habitation.

“Suitable cuckoo hosts breeding close to human habitation enjoy a selective advantage from breeding indoors,” the researchers wrote in a paper published in the journal Behavioral Ecology and Sociobiology. “These findings suggest that birds benefit from association with humans in terms of reduce risk of parasitism.”

The scientists say it’s harder for the barn swallows to build nests indoors in China. There, they have developed an alternative trick that’s largely lacking among their European counterparts: A Chinese barn swallow will often recognize a cuckoo egg. So when a cuckoo egg shows up in its nest, it will toss it over the side.

“These findings suggest that barn swallows in China have gained egg rejection behavior because they cannot avoid parasitism when breeding outdoors.”

Watch a Reed warbler feed a much larger cuckoo chick: