COWBIRDS DO MATH?
August 7, 2011
Q. Do cowbirds
deposit their eggs in the nests of other cowbirds?
A. No. Cowbirds,
like European cuckoos, are "obligate brood parasites," which
means they lay eggs in nests of birds outside their own species. In fact,
European cuckoos and cowbirds no longer even construct their own nests.
(Nonobligate brood parasites lay eggs in the nests of other birds in their
species, as well in their own nests.) At least 100 species of birds are
known victims of the cowbird's sneaky behavior.
are native to areas of the United States where the buffalo once roamed.
They made a living by eating insects stirred up by the herds as they moved
across the landscape. The living was easy, but building a nest and feeding
babies was an impossible task. Following the food source created by the
buffalo herd might place you a hundred miles away from your nest and hungry
babies within a few days. So, over evolutionary time cowbirds came up
with another solution--they laid their eggs in the nests of more sedentary
birds while they themselves tagged along after the buffalo. Most buffalo
herds have disappeared, and some cowbirds have moved east, yet they have
maintained their brood parasite strategy.
to the phony foster chick scam, some birds engage in another practice
that, from the perspective of humans, is particularly unsavory: siblicide.
Siblicide, in which nestmates kill a brother or sister and throw its carcass
out of the nest, has been reported for many types of birds, including
eagles and egrets.
do not generally kill their nestmates. But the cowbird's behavior is not
as altruistic as it appears; it is, in fact, self-serving in an unexpected
way. Rebecca M. Kilner of Cambridge University and colleagues studied
brown-headed cowbirds to learn why a baby cowbird typically does not kill
the other baby birds, which would lead to its getting all the food its
foster parents bring to the nest. As the researchers put it, nestling
parasites like cowbirds "should be ruthlessly self-interested and
should kill host offspring soon after hatching." The scientific challenge
was to determine why a species performs in a manner seemingly contradictory
to its best interest.
set up tests using eastern phoebes, a type of flycatcher, as the host
species and arranged for a single cowbird egg to be in each of 20 nests.
In 10 nests, they removed all eggs of the phoebe on the day the cowbird
egg hatched so that the parent birds had a single baby they assumed was
their own. In the other 10 nests, they removed all the hosts' eggs on
the day the cowbird hatched but added two newly hatched phoebes. Adult
phoebes were tending 20 nests; 10 had a single cowbird and 10 had a cowbird
with two phoebe nestmates.
were surprising. Cowbirds with two nestmates gained weight more rapidly
than those in a nest alone. The investigators filmed the nests and found
that parent birds brought food to three birds about 2½ times more
often than to a single cowbird. However, a cowbird in the nest with multiple
birds took more than half the food brought to the nest, so it fared better
than a lone cowbird getting all the food. The mystery of the cowbird's
seeming altruism toward other baby birds was revealed: not engaging in
siblicide turns out to be a strategy for getting more food.
Do the cowbirds always win? Nope. Nature seldom works that way. Some birds
are aware that the cowbird egg does not belong in the nest. Recognizing
it as a parasite, the would-be foster parents let it languish unattended
or actually remove it from the nest. Yellow warblers will place new nest
material over the uninvited egg and lay their own eggs on the new layer.
The record is six layers of nesting material covering six cowbird eggs.
The name of the game, after all, is survival of the fittest and there
are many ways to play it.
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