photographs of Brown-headed Cowbirds (top) and a Barn Swallow (bottom) by Rohan Kamath
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
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.
Precocial and Altricial Young;
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.