CUCKOOS are arriving from Africa. Pity the poor meadow pipits they will dupe; their clutches destroyed, they will work themselves to the bone raising the enemy’s chicks.
Being conned by a cuckoo must spell disaster for any small bird. However, new research from Spain suggests that, sometimes, it doesn’t. Having a cuckoo in your nest, it suggests, might even be a blessing. It depends on how big you are and the species of cuckoo which tries to take advantage of you.
Our European cuckoo lays a single egg in the nests of meadow pipits and other small birds, removing one of the victim’s eggs as she does.
On hatching, the cuckoo chick pushes the eggs or young of its hosts out to the nest. The foster parents then lavish all their attention on the squatter. At around 85g when fledging, the young cuckoo weighs as much as a brood of meadow pipits, or four times that of a foster parent. If the cuckoo hadn’t disposed of the hosts’ chicks, it would have starved.
The great spotted cuckoo, which spends the winter in Africa, breeds in Spain, Portugal, Italy and the south of France. There are a few Irish records. Resplendent in its black, white and yellow plumage, this cuckoo is bigger than the European one and behaves rather differently. The great spotted fools other birds into raising its young but is far less aggressive in doing so.
It doesn’t choose small birds as foster parents but targets magpies and carrion crows. Not only are these bigger than the cuckoo herself, they readily eat eggs and young not their own.
The great spotted female lays up to three eggs in a nest and the resulting chicks don’t expel those of the host. Instead, they pass themselves off as members of the foster parent’s family. Parent birds will feed the strongest, most demonstrative, chicks first; being smaller, the young cuckoos risk getting so little food that they starve. How they manage to survive in the face of such fearful odds has intrigued ornithologists.
Diana Bolopo, of the University of Valladollid, installed video cameras at seven carrion crow nests with cuckoo chicks in them. Six nests, which had not been parasitized, were filmed for comparison. She analysed the footage still-by-still to quantify begging and feeding behaviour. Her findings have just been published.
The foster parents, Bolopo found, tended to feed their own chicks in preference to the cuckoo’s. Human parents have to encourage their babies to eat but, with birds, the chicks call the shots. The more begging they do, the more the parents are driven to feed them. The young cuckoos indulged in exaggerated begging behaviour, inducing the foster parents to bring extra food to the nest. They then attracted sufficient attention to get themselves fed but never out-competed the crow nestlings in the scramble for food.
Paradoxically, having young cuckoos in the nest benefited the crow chicks; it saved them energy. Noisy begging, with vigorous movements of the head, is a significant drain on a youngster’s energy resources. By leaving the young cuckoos to do most of the begging, the crow chicks could afford to take it easy, conserving their energy. Being bigger than the young cuckoos, they still got the lion’s share of the food.
These results raise intriguing questions. Why, for instance, don’t great spotted cuckoos evict their host’s chicks, as European ones do? One possible explanation is that, with more than one young cuckoo in most nests, a chick bent on eviction would kill its own kin. Also, allowing the young to survive prevents the evolution of defensive behaviour, such as desertion of suspect nests, in the host species. Young cuckoos, raised in magpie nests, hatch before their host’s chicks do. Magpies feed these precocious youngsters first, so the cuckoos don’t need to evict the young magpies.
Bolopo, D et al: ‘High begging intensity of great spotted cuckoo nestlings favours larger-size crow mates’. Behavioural Ecology & Sociobiology. 2015.
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