Parasitic Swallows

photograph of a Cliff Swallow gathering mud
for a nest by Tom Grey
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 colony.

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.