Brood parasites birds

Pamushana | December 2020

Deceit, deception and death! That sounds rather dramatic as an opening for a story on birds, but it is shockingly true, and brood parasitism in birds is not common knowledge so let me tell you a little about it. Some birds don’t make a nest of their own and raise their chicks, instead they lay their eggs in the nest of other birds and trick them into incubating their eggs, feeding their young, and caring for the fledglings. It’s astonishing how they get away with it! Here’s how:

Breaking and entering

The female parasite bird susses out the neighbourhood and identifies the nest of a potential host bird. She “waits in the wings” for the host bird to leave the nest during a pause in egg-laying or incubation. Then she dashes in, very quickly lays her egg in the nest, and flies off never to be seen again. Cuckoo eggs have a strong shell making them more resistant to cracking, and the embryo starts developing while the egg is still inside the female, unlike most other birds where incubation only starts once the egg is laid.

Don’t count your chicks before they hatch

The host bird usually can’t distinguish the parasite’s egg from her own so she accepts the addition to her clutch and incubates the foreign egg with her own. A single egg is usually laid and often that egg matches the colour and pattern detail of the host’s eggs.

The lengths of deception

The incubation period is usually shorter too resulting in the parasite chick hatching before the host’s chicks and gaining an advantage in food and size. Often the female brood parasite or the parasite chick destroys the eggs or chicks of the host. Some of the chicks instinctively push the host’s eggs or chicks out the nest assisted by the scoop-like shape of their concave backs, and some have deadly bill hooks that they use to bite and kill the host chicks. In other species the parasite chicks are raised together with the hosts species’. Many parasite chicks have the same mouth and gape markings of the host’s chicks to convince the host adults that they must be fed. Some brood parasite chicks also show initial plumage mimicry.

The culprits

Cuckoos are the main culprits, but other deceivers include honeyguides, honeybirds, whydahs and indigobirds. The scarlet-chested sunbird (Chalcomitra senegalensis) described in Dharmesh’s story has been recorded as the host of the Klaas’s cuckoo (Chrysococcyx klaas). Some brood parasites target one specific host species while others can lay their eggs in several host species’ nests.

Not putting all your eggs in one basket

Instead of building nests and raising young, brood parasites are able to channel their energy into developing and laying more eggs in hosts’ nests thereby ensuring their numerous offspring have a good chance of survival. Some species have been recorded laying up to 26 eggs in one season!

The war wages on

Some host birds have caught on to the parasites’ tactics, and have come up with ways to outwit them, safeguard their nests and reject imposter eggs. But, of course the parasites have evolved counter strategies to overcome these developments, and so this evolutionary arms race continues.

Levaillant’s cuckoo (Clamator levaillantii)

These cuckoos target the nests of some of the babbler species. Both the male and female Levaillant’s cuckoo fly around acrobatically to distract the host birds. The male continues the distraction while the female lays the egg. Unlike many other species of cuckoo, the newly-hatched cuckoo chick does not push the other eggs and nestlings out of the nest. It leaves the nest after about ten days and becomes independent in four to six weeks

This is such a fascinating aspect of bird behaviour. It’s the stuff of soap operas and movies! If you’d like to know more I recommend a book called, Birds – The Inside Story, by Rael and Hélène Loon which was my source for most of the information in this article.