If
a bird of one species parasitizes a bird of another by
laying an egg in its nest, the act is relatively easily
detected by a human observer. The host and parasite eggs and
the host and parasite young usually differ -- but people's
sensory systems are better equipped to discern these
differences than are those of birds. A cowbird laying its
egg in the nest of a small warbler is pretty obvious, so
interspecific brood parasitism has received a great deal of
attention from ornithologists. Intraspecific brood
parasitism, an individual laying eggs in the nest of another
of the same species, is not nearly as easily detected by
birds or people-parasitic eggs and young are very similar to
those of the host. If both the host and parasite female are
unbanded, even an observed incident of one female laying in
another's nest may go unrecognized as such. Intraspecific parasitism is
common among ostriches and their relatives, game birds, and
a few passerines (such as Cliff Swallows). Many cases,
however, involve ducks. A female duck that is parasitized by
another of the same species may have her own reproductive
output reduced in several ways. Both hatching success of her
own eggs and survival of her hatchlings may be reduced, and
the larger brood may attract more predators. The female may
face the numerous risks and stresses of reproduction for
relatively little benefit, if a substantial portion of the
clutch is not her own. What can the host female do
in the face of parasitic attack? She can desert the clutch
and start over, thus not spending her efforts on a mix of
her own and "adopted" offspring, but then she wastes the
resources tied up in her own eggs. She can identify the
parasitic eggs and discard them. Or if she senses from the
presence of more eggs in the nest than she laid that she has
been parasitized, but cannot discriminate the other female's
eggs, she can adjust the number of eggs she lays
subsequently so as to maximize survival of her own
offspring. Female Common Goldeneyes are
apparently incapable of recognizing the eggs of other
females, perhaps because goldeneyes nest in deep, dark
cavities. A test was carried out in a Swedish population of
goldeneyes in which there was evidence of a regular decline
in fledging success with clutch size. Experimenters
simulated parasitism by adding one, four, or seven eggs to
clutches of goldeneye females in nest boxes. The goldeneyes'
reactions were recorded -- did they continue to lay, start
to incubate, or desert the nest? When one or four eggs were
added to the females' clutches, the rate of nest desertion
was no higher than it was in "control" nests, which had no
added eggs. If seven eggs were added, however, the female
never incubated. When females were
"parasitized" with only one additional egg, they laid
significantly more eggs than if four "parasitic" eggs were
added to their clutches. In addition, if four eggs were
added before a female had laid five eggs of her own, she
adjusted her final output downward, whereas if the four
interloper eggs were added to the nest after the female had
laid five to eight eggs of her own, she did not (natural
clutch sizes are mostly in the range of eight to
twelve). These experiments clearly
showed that goldeneye females can adjust their egg laying to
compensate for parasitic eggs added to their clutches. This
is not surprising, since goldeneyes are indeterminate layers
-- they do not, like many songbirds always lay exactly the
same number of eggs per clutch. Why should intraspecific
brood parasitism have evolved so commonly in ducks? One
suggestion is that many ducks, especially cavity nesters,
suffer from a shortage of suitable nest sites. This could
drive several females to lay in the same nest, but in the
goldeneye population under study, less than a third of the
nest boxes were used, so this is certainly not a universal
reason. In addition, ducks seem to be better able than
smaller birds are to find nests of other females of their
own species. Also duck young are precocial, which can lessen
the host's burden when caring for the young of parasites.
Altricial birds, in contrast, should be under heavy
selection to avoid being parasitized by their own kind. Loss
of young to starvation is commonly observed in such birds,
indicating that a high price would be paid for rearing an
"adopted" offspring. Another factor may be that a
female duck often returns to nest near the place of her
birth. Thus sisters or mothers and daughters would tend to
nest in the same area, and therefore in many cases
parasitize each other. This would reduce somewhat the
evolutionary costs of being parasitized, since the host
often would be rearing young carrying copies of the same
genes. That would also reduce selection pressures on the
hosts to evolve defenses against being
parasitized. Finally, ducks, unlike many
passerines, do not defend the immediate vicinity of their
nests during the laying period, easing the access of
parasitic females to the nest. This exposes ducks to
interspecific parasitization as well, generally by other
ducks. The Redhead appears to be our most persistent
parasitic duck. In one study on artificial islands in
reservoirs in Alberta, Redheads parasitized 19 percent of
685 duck nests, laying an average of 2.68 eggs per
parasitized nest. Mallard nests were most frequently
parasitized, but the percentage of parasitic eggs per nest
was highest when Lesser Scaups were the hosts. Why don't more passerines
exhibit intraspecific parasitism? Have they evolved powerful
defenses to prevent parasitism by members of their own
species? Studies were done in which the eggs of three
colonial passerines -- Pinyon jays, Barn Swallows, and
Great-tailed Grackles -- were exchanged between nests of the
same species. The experiments produced no evidence that
members of these species can discriminate their own eggs
from those of other individuals. Perhaps the birds have
other mechanisms for repelling brood parasites of the same
species, such as defense of the nest. Another possibility is
that the costs of being a parasite are too high for most
birds with altricial young. Nest failures are frequent, and
time taken sneaking around trying to parasitize another nest
must be subtracted from time that could be used in building
nests, laying eggs in the home nest, feeding, and other
activities that affect the success of the bird's own nesting
attempt. In short, the costs of being a parasite may often
outweigh the benefits in altricial birds, which may explain
why intraspecific parasitism does not seem to be widespread
in passerines. SEE: Brood
Parasitism;
Parasitic
Swallows;
Precocial
and Altricial Young;
Site
Tenacity. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.