Cuckoo chicks make terrible nestmates. First, they show up uninvited in the nests of other species. Then they hog the attention of their adopted parents. And finally, adding insult to injury, they unceremoniously dump their nestmates over the side of the nest. Or at least that is what usually happens. However, great spotted cuckoos in Northern Spain that lay their eggs in the nests of carrion crows have taken a different path; they actually help their nest mates – at least sometimes. How do they accomplish this? Daniela Canestrari and an international team of colleagues have recently shown in Science that the answer is as biologically clever as it is amusing: cuckoo chicks stink.

When they are harassed, great spotted cuckoo nestlings void a sulphurous and phenolic anal secretion that is both noxious and repulsive. Just how bad is this stuff? When the researchers coated chunks of meat in nestling secretions, not even feral cats would eat it! Carnivorous raptors, too, turned up their noses in disgust. But such repellence is precisely the secret of their success.

It turns out that cats and other predators cause major problems for carrion crows. They attack nests, and in some years cause most to fail. This is where the cuckoos step in. By monitoring the success of crows' nests over 16 breeding seasons, the researchers discovered that in years with particularly high rates of predation, nests with cuckoos were more successful than those lacking them. In contrast, in years when predation was limited, the nests that were invaded by cuckoos did worse.

What explains these shifting costs and benefits? Using manipulative experiments that moved cuckoos between nests, the team provided direct evidence that adding cuckoos to crows nests increased their success by almost 2-fold, where success is defined as the survival of at least one crow chick. By virtue of their malodourous stench, cuckoo chicks apparently served as a deterrent to would-be predators. But it isn't all good, as the team found that the presence of cuckoos resulted in fewer crows fledging from successful nests. Taken together, these costs and benefits seem to just about balance out; in an average year with an average amount of predation, crows with cuckoos do no better or worse than those without them.

Simply put, great spotted cuckoos are not parasites of this population of crows, at least on average. In the good years they seem to do more harm than good, while in years with widespread predation the cuckoos are instead mutualists. Because crows are unable to predict the future, they play their odds each year, tolerating their smelly houseguests and getting it right often enough. At the same time, with their simple trick of emitting a putrid stench, these cuckoos have hit upon the ideal evolutionary strategy. They've made themselves indispensable, and in doing so they ensure their persistence for generations to come.

T. C. J.
J. M.
From parasitism to mutualism: unexpected interactions between a cuckoo and its host