Birds see colours better than we can, so why don’t they shove the intruder’s eggs out of nest? Perhaps it is not in their psyche.
The pied cuckoo chick was a giant amongst its foster family of yellow-billed babblers. Every morning for a few days, we watched the flock of dowdy brown birds frantically stuff leftover dog food, insects, and other tidbits from the garden into the gaping maw of the ever-hungry monster chick. The family must have been blind to think the fledgling with a pointed crest, prominent black and white plumage, and a long tail was its own.
Preventing a nest parasite such as the pied cuckoo from laying eggs in its nest should be the first line of babbler defence. But babblers don’t chase anything, let alone cuckoos.
Once a pied cuckoo lays its egg in a yellow-billed babbler’s nest, the latter has no way of telling the difference. The cuckoo eggs are turquoise blue like the poor babbler’s.
However, pied cuckoos brazenly lay large white eggs in the nests of red-vented bulbuls, whose eggs are speckled brown. The host birds are too small to dislodge the enormous cuckoos’ eggs. Neither can they peck through the strong shells, among the strongest in the cuckoo family, and destroy them.
When a female pied cuckoo sneakily airdrops her egg in a bulbul’s nest, the large egg crash-lands on the fragile bulbul eggs, and often damages them. She might also deliberately push a bulbul egg over the edge so the tiny nest can accommodate her own.
Would bulbuls eject strangers’ eggs if they were able to? Oliver Krüger, University of Bath, the U.K., conducted experiments with Cape bulbuls in South Africa, and found the birds don’t eject any eggs, even small bulbul eggs painted white. Birds see colours better than we can, so why don’t they shove the intruder’s eggs out of nest? Perhaps it is not in their psyche. The bulbul can cut its losses by abandoning the infiltrated nest, and making a fresh start elsewhere. But most don’t.
Krüger found abandoning a nest comes at great price. Predators are more likely to take chicks later in the breeding season, and starting afresh runs the risk of losing the second brood entirely. Besides, there is no guarantee a nest will remain cuckoo-free.
Babblers may be cuckoo-blind, but why do bulbuls choose to feed these giant chicks? Early in the breeding season, the small birds chase away any adult cuckoos lurking near their nests. Yet they don’t seem to recognise the changelings that resemble their biological parents.
Pied cuckoos seem to have it made. But there’s one trick babblers and bulbuls can learn to get ahead in this game of one-upmanship.
Nestlings of the superb fairywren of Australia look identical to those of its nest parasite, the Horsfield’s bronze cuckoo.
About four to five days before the eggs hatch, the mother calls to the developing embryos an average of 16 times an hour. Each female embeds a specific signature code in her call that is unique to her and no other. When the chicks hatch, they include their mother’s code in their begging calls, and the mother recognises her offspring. No code, no food.
The incubation time of the bronze cuckoo is 12 days, three days less than the fairywrens. Cuckoo chicks have less time to learn their foster mothers’ secret code, and therefore, can’t imitate it well. The mother fairywren doesn’t recognise them as her chicks. So the cuckoo relies on other credulous hosts to rear its progeny.
Here in India, despite the gullibility of their foster parent species, pied cuckoos are not common. In all these years at the farm, we’ve seen babblers rear a cuckoo chick only once.
That’s because the species of irresponsible parents haven’t perfected one thing: timing. The cuckoos frequently lay their eggs late, and their chicks don’t hatch by the time their nest mates do. The foster parents don’t incubate the remaining eggs much longer and the cuckoo eggs rot. If cuckoos ever learn to time their egg-laying right, there’s little to save their naive hosts.