when perfect isn't enough

Egg signatures and forgeries in the cuckoo-drongo arms race

- JESS LUND

In the miombo woodland of southern Zambia, a pair of Fork-tailed Drongos tend to their nest. Within a sparsely woven cup slung between two thin branches lie three pale cream eggs, each one marked with tan speckles. Suddenly a flash of grey appears as a female African Cuckoo darts towards the nest. With raucous alarm calls, the drongos attack the cuckoo as she hunches over the nest for a few seconds, surrounded by a flurry of feathers and wings. Then, as quickly as she came, the cuckoo shoots off. The drongos inspect their clutch – it still contains three identical eggs.

Little do they know that in the chaos the cuckoo removed one of their eggs and laid one of her own in its place. In about two weeks’ time the cuckoo’s egg will hatch and the chick, blind and naked, will push the remaining two drongo eggs out of the nest. Now the only chick in the nest, the imposter will be the sole recipient of the food brought by the unsuspecting drongo parents.

The African Cuckoo’s tactic of offloading the costs of parenthood onto another species, termed ‘brood parasitism’, is rare; only one per cent of bird species rely on this strategy. But sub-Saharan Africa is particularly rich in brood-parasitic bird species. Cuckoos, honeyguides, whydahs, indigobirds and the Cuckoo Finch all avoid parental care by tricking other species, the ‘hosts’, into incubating their eggs and raising their chicks.