The great spotted cuckoo is a nest parasite that sneaks its eggs into the nests of other birds, but new research shows that such parasitism isn't always a bad thing for the cuckoo's hosts.
When they are chicks, these parasitic birds secrete a stinky substance that can drive away predators and protect the appropriated nests. And, when the threat of predation is high, the presence of parasitic cuckoos can actually benefit the host population as a whole, researchers say.
The findings emphasize how dependent species' interactions are upon their environments, suggesting that terms like "parasitism," "commensalism," and "mutualism" may not be as black-and-white as researchers once thought.
Daniela Canestrari from the University of Oviedo in Spain and a team of Spanish and Swiss researchers studied the nests of carrion crows in northern Spain—some of which were parasitized by great spotted cuckoos and others that were not—for more than 16 years to make their discovery. Their report  is published in the 21 March issue of the journal Science.
"This secretion is really disgusting and difficult to describe. It's pungent, long-lasting, and it burns your throat slightly. If you get the substance on your hands, it's really difficult to get the smell away. Even if you wash your hands, it takes hours to get off."
"It's really difficult to realize that something like this is going on without a long-term data set," explained Canestrari. "Experimental approaches are often useful, but for some interactions—especially in the field of ecology—you really need to collect data in the field because it's difficult to replicate complex ecological situations."
By monitoring the carrion crows' nests year after year, the researchers found that, contrary to conventional wisdom, parasitized nests were more successful overall than their cuckoo-free counterparts (meaning that they were more likely to produce at least one crow fledgling).
Upon closer inspection, Canestrari and her colleagues observed that when grabbed cuckoo chicks secrete from their rear ends a strong-smelling substance -a mix of caustic and repulsive compounds, made mostly of acids, indoles, phenols and several sulfur-containing compounds-that effectively repels cats and birds of prey.
So, even though the cuckoos limit their hosts' reproductive success by competing with the crow chicks for food, the parasitic birds also save the crows from certain predators, the researchers say.
"This secretion is really disgusting and difficult to describe," said Canestrari. "It's pungent, long-lasting, and it burns your throat slightly. If you get the substance on your hands, it's really difficult to get the smell away. Even if you wash your hands, it takes hours to get off."
The researchers would like to understand out how the cuckoos produce this compound, and they are hoping to receive funding to study the physiological processes involved. "At the moment, we just know that it's different from feces," explained Canestrari. "Bacteria are probably involved in its production, but we don't know which."
For now, their research stands as a testament to long-term field studies, and their results serve as a reminder of how deceiving species' interactions can be to the untrained eye.
"Interactions that have been clearly classified as 'parasitic' or 'mutualistic' might be more complex," concluded Canestrari. "Perhaps we need to look more closely at these interactions before giving species such tags or labels."