Cuckoos are nest parasites. That means they lay eggs in the nests of other birds, which then put the effort into raising the chicks. So you'd think they'd be quiet about it. Yet female cuckoos have a tendency to make a bubbly, chuckling call while they’re laying their eggs. That’s a strange thing for them to do, because host birds aren’t too fond of cuckoos.

That chuckle doesn’t sound at all like the male cuckoo call. But it does have similarities to the call of the sparrowhawk, which led researchers Jenny York and Nicholas Davies to wonder whether the call might be a purposeful deceit—a ruse to distract the hapless host birds while their nest is being violated. York and Davies’ paper in Nature Ecology & Evolution this week provides evidence that the cuckoo chuckle does indeed seem to distract host birds by making them fear a sparrowhawk attack.

Faking it

Plenty of species have evolved to mimic predators, often for their own protection. For instance, the wasp beetle mimics a stinging wasp, causing predators to steer away. Cuckoos seem to be a fan of that tactic: some cuckoo species look quite a bit like hawks, and this seems to protect them from mobbing by other birds.

Has the female common cuckoo evolved a cry that imitates a hawk? Both the cuckoo chuckle and the sparrowhawk cry consist of a rapid series of notes with a similar pitch and a similar rhythm. They sound different to a human ear. But, write York and Davies, “manipulation by imperfect mimicry is frequent in the natural world, and resemblance to hawk calls in some key features might be sufficient to trick hosts.” The way to find out is to determine whether the call seems to be effective in freaking out the host birds.

Host birds like reed warblers are wise to the possibility of cuckoo parasitism. They mob adult cuckoos and sometimes reject host eggs in their nests by kicking them out or abandoning the nests altogether. They also “monitor cuckoo activity in the vicinity of their nest and vary these defenses in relation to local parasitism risk,” write York and Davies. Because of this, female cuckoos are quick and stealthy when they lay their eggs—aside from the chuckle call.

Distracted parenting

York and Davies conducted three studies to compare the effects of the chuckle call and the sparrowhawk call. They filmed reed warblers in their nests and used speakers placed near the nests to play a series of different bird calls: the male cuckoo’s characteristic cuckoo-clock-like call, the female’s chuckle call, the sparrowhawk cry, and (as a control) the call of the extremely unthreatening collared dove. The birds didn’t react to male cuckoo calls or dove calls, but they adopted a vigilant response to hawk calls and the female cuckoo’s chuckle: they began looking around their nests for threats.

But warblers are a target species for cuckoos, so maybe they respond to both as a threat even if they can tell the difference between them. To test this, York and Davies conducted a similar experiment with birds that are prey for the sparrowhawk but not targeted by cuckoos: the great tit and blue tit. The birds reacted vigilantly to both the chuckle call and sparrowhawk call, but not to the dove or male cuckoo call.

It’s possible to explain the cuckoo’s mimicry in evolutionary terms if it buys them more reproductive success—if it increases the likelihood that eggs laid along with a fake hawk call go unnoticed by host birds, who are too busy worrying about being eaten to pay close attention to their nests. So, York and Davies tested whether reed warblers distracted by hawk or chuckle calls might lower their nest defenses.

The researchers faked a foreign, parasitic egg in 75 different reed warbler nests by taking one egg out, painting it brown, and putting it back. Then they played one of the four calls near each nest. The majority of the foreign eggs—66 percent—were rejected in nests that heard male cuckoo or dove calls, but only 26 percent of the eggs in the hawk and chuckle call nests were rejected. That suggests that the chuckle call helps to distract the reed warblers long enough for a foreign egg to go unnoticed.

The results are a nifty example of an evolutionary arms race: cuckoos that have their eggs accepted by the hosts have more babies and fare better in evolutionary terms; meanwhile, host birds that reject the cuckoo egg save their own young. So while host birds, over time, adapt to the cuckoo’s tricks, the cuckoo just adapts to become even trickier.

Nature Ecology & Evolution, 2017. DOI: 10.1038/s41559-017-0279-3 (About DOIs).