Wednesday, January 30, 2013

Broodfellas: Nest parasitism and Mafia involvement among cuckoos

Ah, mother birds: so caring, so dedicated to the art of raising healthy, successful young. Surely there are few better examples of committed mothers than a delicate bird returning to its nest with a beak full of worms for its eagerly waiting chicks.

And then there are mother birds that produce infanticide-committing, constantly-eating, greedy monsters and destroy the nests of those who don’t agree to raise them.

The common cuckoo Cuculus canorus, mother of the most evil baby birds you'll ever see. Photo by Jari Peltomaki.
Such is the wonderful life of a nest parasite. Nest, or brood, parasitism is a much more interesting phenomenon than we imagine it to be: it takes much more than just a sneaky mama bird and an unsuspecting host. In fact, neither of those needs to be true for a brood parasite to be successful. Though brood parasitism has been recorded in several species, including ducks, whydahs, and honeyguides, there is no better example of the complexity and interesting adaptations of parasitism than the cuckoo.

Babies and Gentes
While only about 40% of cuckoo species are brood parasites, those that are display a variety of ingenious traits allowing them to ensure the safety and growth of their offspring, even when they are absent as parents. Since before the egg is even laid, its host is predetermined. Female cuckoos belong to several different genetic varieties, known as “gentes.” These different lines of females produce eggs which match the eggs of their chosen hosts.

Random males can mate with a female of any gente. However, females will always produce eggs of their own gente, suggesting that the genetics for egg coloration are inherited maternally. The mystery of how gentes are maintained is puzzling, as cuckoos are known to parasitize a wide variety of hosts, ranging from warblers to robins. Gentes may reflect the hosts which raised the mother cuckoo in her own infancy, though the mechanisms controlling them are still mysterious.

A newly-hatched cuckoo making short work of reed warbler Acrocephalus arundinaceus eggs. Photo by Mike Richards.
As soon as the egg hatches, the young cuckoo gets to work disposing of its nestmates. In some species of cuckoo, the egg spends up to 24 hours longer in the mother’s body, allowing it to hatch faster than its nestmates. Using a shallow depression in its back, the newly-hatched, naked cuckoo dumps the other eggs over the side of the nest, ensuring that it gets all the attention it needs. Which is a lot.

The video below shows just how burdensome the task of raising a young cuckoo can be for the host. In this clip, a young common cuckoo receives an insect from its host parent, a reed warbler. The size comparison between the two species is enormous: even a young cuckoo can weigh five times as much as a warbler, and takes even longer to fledge.

And so, the life of the cuckoo goes on: the chick eventually fledges and leaves the nest and, one day it will produce its own brood of home-wrecking parasites. But what causes birds to simply accept a parasite in their nest to begin with? Why care for the cuckoo’s chick at all?

Don Cuckooleone: An offer you can’t refuse
Amotz Zahavi, a behavioral ecologist at Tel Aviv University, suggested a theory which sounded rather outlandish at first. He related the cuckoo’s powers of persuasion to that of the Mafia: the cuckoos, he suggested, get other birds to comply not with brains, but with brawn. It’s hard to imagine birds, some of earth’s most graceful and fragile of animals, engaging in gang-like violence against one another.

A male and female great spotted cuckoo Clamator glandularius in the process of creating even more little parasites.
The idea, appropriately labeled the “Mafia hypothesis,” was largely unsupported until Manuel Soler of the University of Granada put it to the test. On the plateau of Hoya de Guadix in Spain, Soler documented the interaction between great spotted cuckoos Clamator glandularius and magpies Pica pica over three breeding seasons (1990-92). Soler recorded the number of instances in which magpies removed cuckoo eggs from their nests, and the resulting number of attacks.

The results of Soler’s observations revealed some chilling statistics: 86% of magpie nests were attacked by cuckoos when their eggs were removed. When magpies accepted the eggs, only 12% of nests were attacked. About 22% of unparasitized nests were attacked. Arm- or, rather wing-twisting on the part of the cuckoos apparently worked successfully: when the magpies attempted to build nests and breed again, they readily accepted the cuckoo’s eggs.

A Burchell's glossy-starling Lamprotornis australis feeding a young great spotted cuckoo in Africa.
Fortunately, the breeding success of the magpies was not drastically effected. For the cuckoo Mafia to destroy all the magpie nests in the area would be impractical and biologically impossible. Researchers continue to search for similar behavior among other species of cuckoo throughout the rest of the world, but nothing to the extent of Soler’s experiment has been observed.

Nature can often be cruel, but as Michael Corleone says, “it’s not personal… it’s strictly business.”

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