The great spotted cuckoo sneaks its own egg into the nest of carrion crows. The chick grows up as a parasite, tricking crows into taking care of it, while it competes with the host chicks for food. But before you jump to judgment, a new 16-year study shows how cuckoos also help their host crows out by producing a gross-smelling fluid that repels feral cats and other predators.
A team led by Daniela Canestrari from the University of Oviedo in Spain studied 741 nests of carrion crows (Corvus corone corone) in northern Spain. Some of them were already parasitized by great spotted cuckoos (Clamator glandarius) while others weren’t. To be fair, unlike other cuckoo species who kick out the host babies, these cuckoo chicks are happy to share a nest with a couple other crow chicks.
The team confirmed that the parasitic birds do limit the crows’ reproductive success by competing for food. Parasitized nests only had 2.07 crow fledglings on average while nests with no cuckoos had 2.56. But they also found that the parasitized nests were actually more successful overall than their cuckoo-free counterparts. Nests were more than 40 percent more likely to raise at least one crow chick successfully if they contained a cuckoo chick.
Turns out, cuckoo chicks protect the crows from predators. With some chemistry work, the team found that the cuckoos secrete a noxious substance out of their cloacas (used for mating and excreting). The mix of caustic and repulsive compounds are dominated by acids, alcohols, indoles (often found in coal tar and feces), and several sulfur-containing compounds (the stuff of rotting eggs). The stinky mix is an effective repellent against quasi-feral cats and birds of prey who scour nests for food.
"It is really disgusting," Canestrari tells New Scientist. "It's pungent, produces a burning sensation in the throat, and looks like rotting matter." But it also wards off enemies, and when pressure from those predators is high, the brood parasites actually help boost the crows’ population.
They smeared the foul stuff on to meat and offered it to cats, other crows, and rapturous birds. Only one of eight cats took a bite of the “treated” meat, even though eight of nine cats happily ate all 10 pieces of uncontaminated “control” meat. Crows and raptors avoided the stinky meat too.
The team also conducted experiments with the nests. They deliberately added or removed cuckoo chicks from nests, comparing them to natural "control" nests free of human intervention. The nests with a deliberately added cuckoo chick did best; they had a 71 percent success rate when it comes to rearing at least one crow chick. The control nests where cuckoos had naturally laid eggs also did well, with a 61 percent success rate. However, the cuckoo-free control nests managed to rear a crow chick with 38 percent success rate. The worst performers were the nests where the researchers actually removed a cuckoo chick; their probability for success was 31 percent.
"Our work is the first confirming that a brood parasite can benefit its host," Canestrari add. Findings like these blur the lines between parasitism, commensalism, and mutualism, showing us how the way organisms interact with each other totally depend on environmental factors. Perhaps the crows knew this, and that’s why they tolerated the cuckoos? That last part is still a mystery.
The work was published in Science this week.
Images: Vittorio Baglione (top, below) & Daniela Canestrari (middle)