Cowbirds
laying eggs in the nests of other species exemplify avian
brood parasitism. Some solitary breeding birds (especially
waterfowl) occasionally parasitize their own species,
producing abnormally large clutches, and cooperative
breeders often have strategies for getting other members of
the group to incubate their eggs. Such parasitization of
conspecifics had been thought to occur widely in colonial
birds as well, although it was only recently demonstrated.
Cliff Swallows have now been shown to have an unusually high
degree of intraspecific brood parasitism, a phenomenon
enhanced by the synchronized breeding within their
colonies. In Southwestern Nebraska,
Cliff Swallow colonies contain up to 3,000 nests, making
them among the densest known aggregations of vertebrates.
They cluster their gourd-shaped mud nests under bridges, in
culverts, beneath the eaves of buildings, and on the faces
of cliffs. Attention was drawn to brood
parasitism in this species in the course of a study that
recorded egg-laying intervals in over 700 swallow nests.
More than two eggs often appeared in a nest within a 24-hour
period. As no bird is known to lay more than one egg daily,
this indicated that more than one female was laying in the
same nest. Following that discovery, a colony of 190 nests
was observed for the entire period of egg laying, and 30
nests were selected for intensive scrutiny. About
three-quarters of the swallows using those 30 nests were
color banded for individual recognition. On five occasions,
banded birds were seen to enter the nests of others and lay
a single egg when the owners of the nest were absent; one
bird was responsible for two of the parasitic incidents. All
of the parasitic birds also had clutches in their own nests.
It was estimated that nearly a quarter of the swallow nests
in large colonies were parasitized. Typical brood parasites such
as cowbirds are notoriously quick egg layers. The parasitic
swallows were also fast, each spending less than a minute in
the host nest during laying. Indeed, one parasitic bird
managed to lay an egg in a mere 15-second visit, while the
host swallow was distracted by a battle with another
intruder. Eggs of parasites sometimes
appeared in nests several days after the hosts had started
to incubate their own eggs. In spite of this, the parasite
eggs hatched synchronously with the host eggs. This means
that they required less incubation time than the host eggs,
an adaptation frequently found in brood parasites that
attack members of other species. Swallows were also
frequently observed entering neighbors' nests and tossing
out eggs. Presumably many of the vandals were parasites that
later replaced a tossed egg with one of their own. If this
is the case, then the frequency of parasitism in the study
colony may be even higher than estimated from the number of
appearances of "extra" eggs within a day -- since many
parasitic eggs would go undetected by the daily egg census.
It is therefore possible that well over a quarter of the
nests in some colonies harbored parasites. Parasitized swallows fledged
fewer of their own young, on average, than did swallows not
burdened with "adopted" offspring. Parasitic swallows, on
the other hand, increased their fitness. Not only were they
successful in fledging all of their young, but all of their
eggs identified as having been laid in host nests were
fledged by the foster parents, as well. Instead of averaging
about three young, they managed to produce four or
five. So far, it has not been
possible to determine what distinguishes parasitic from host
swallows. Are they genetically different, or is their
parasitic behavior related to their environment? It is not
clear whether the presence of a certain percentage of
parasites is a stable situation, or whether high levels of
parasitism might, in some manner, lead to the decline of
entire colonies. Theoretically, if all birds in a colony
were to take up parasitism, the fitness of all should be
reduced simply from the mutual destruction of
eggs. Just as the Handbook was
about to be printed Charles and Mary Brown reported (Nature
331:66-68, 7 January 1988) that Cliff Swallows actually
carry their eggs in their beaks to the nests of other
individuals. This is a previously unknown mode of brood
parasitism, and the first systematic study of egg transport
in North American birds. SEE:
Brood
Parasitism;
Cowbirds;
Cooperative
Breeding;
Coloniality;
Variation
in Clutch Sizes. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.