Cuckoos evolve to be useful freeloaders

Scientists have found out why crows are happy to raise cuckoos along with their own young. It's a protection racket, Yi-Di Ng discovers. 

A baby crow with its mouth open shares the nest with a cuckoo which the adult crows tolerate. – Vittorio Baglione

The cuckoo’s reputation as the animal kingdom’s most notorious freeloader may not be entirely deserved. Cuckoos are known for laying their eggs in the nests of other birds, leaving the hapless foster parents with an extra chick to rear. But new research has found that a certain species of cuckoo actually helps its foster family by protecting the nest from predators.

“This is an exceptionally exciting discovery. This study provides the first solid evidence that brood parasites can benefit their hosts,” says Naomi Langmore, who has studied cuckoos at the Australian National University.

The discovery came by accident. Daniela Canestrari, an evolutionary biologist from the University of Oviedo in Spain, was studying crow breeding habits when she noticed something odd. Great spotted cuckoos laid their eggs both in crow and magpie nests. Parent magpies fought back by chasing them away and tossed cuckoo eggs out of their nest – although they didn’t notice them all. By stark contrast the crows did nothing to avoid raising a baby cuckoo.

Intrigued, Canestrari and her colleagues set out to discover why. Were the crows simply witless, or were they welcoming the freeloaders because they saw some benefit? To answer the question, the researchers compared how crows’ nests fared with and without cuckoo chicks. They deliberately placed a cuckoo chick in each of 14 crows' nests and removed one from each of 16 nests. Their results, published in Science this month, confirmed that crows do indeed benefit from fostering cuckoo chicks. While 76% of the nests carrying cuckoo chicks raised baby crows to maturity, only 54% of the nests with no cuckoos were successful. But the team still had no clue why.

Cuckoo chicks help their foster nests by repelling predators.

The answer came unexpectedly. As they were handling the cuckoo chicks, the researchers noticed that the chicks would release a dark oily slime from their rear ends. “It was really disgusting,” recalls Canestrari. “It smelled rotten, acidic, and very pungent.” Immediately she guessed that the smelly substance – a mix of acids and sulphurous compounds – could drive predators away. Experiments confirmed her suspicions: predators refused to eat meat slathered with the secretion, confirming that cuckoo chicks help their foster nests by repelling predators.

Cuckoo chicks release a foul-smelling slime, which could drive predators away. – Daniela Canestrari

Looking back through data collected over 16 years, Canestrari noted that the benefit of raising a cuckoo was greatest in years when predator numbers were highest. In northern Spain where the study took place, predation by cats and birds is a big concern for crows. In bad years up to 78% of nesting crows fail to raise any chicks at all due to predation.

If cuckoos are such helpful foster chicks for crows then why don’t the magpies do it too? They simply can’t afford to, says Canestrari. Magpies are smaller than crows and the oversized cuckoo chicks would end up leaving the magpie chicks hungry and the parents drained as they try to provide enough food. By contrast it actually costs the crows less to raise a cuckoo chick than one of their own: a cuckoo chick is only a third the size of a crow chick and develops much faster. The cuckoo fledges at 16-18 days after hatching while crows take 30-32 days. According to the data, crow chicks raised alongside cuckoo chicks were just as healthy as chicks in cuckoo-free nests.

The discovery adds a “whole new dimension to our understanding of cuckoo-host interactions”, Langmore says. “Until now the interactions between brood parasites and their hosts have been held up as one of the classic examples of antagonistic co-evolution. This study shows that the interactions can become even more complex than that, with the benefits of parasitism actually outweighing the costs in some years.”

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