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.”