Swallows along the Platte, part two


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Part one of this essay discussed the benefits and costs of social life for cliff swallows and their foraging and nesting behavior. The essay continues with discussion of the cliff swallow’s opportunistic breeding behavior and colony size.

By Charles R. Brown

Do unto others before they do unto you

The cost of being parasitized by swallow bugs. The nestling cliff swallows on the right is from a nest where the bugs were removed by fumigating with an insecticide; the nestling on the left is from a nest in the same colony with natural numbers of bugs. Both nestlings were the same age (10 days old). (Mary Bomberger Brown)The cliff swallow’s highly social nature also illustrates a fundamental consequence of living together for most animals: there are plenty of opportunities to exploit others around you. By color-marking cliff swallows and intensively observing them at their nests, we discovered that these birds are constantly trying to use their neighbors to their own selfish advantage. A bird will intrude into the nest next door, steal its neighbor’s nesting material (grass stems) or the wet mud on its nest, attempt to copulate with its neighbor’s mate, throw out one of its neighbor’s eggs or in some cases even lay an egg in its neighbor’s nest. These are not cases of mistaken nest identity, as cliff swallows clearly know whose nest is whose. Sometimes these trespass attempts are almost continuous, as birds repeatedly try to enter several of their neighbors’ nests in rapid succession. Typically, cliff swallows guard their nests almost constantly, probably to try to prevent their neighbors from doing these things to them, but there are enough lapses in guarding that bad things do happen. Often one bird will steal grass from its neighbor, only to have that neighbor return the favor at a later time; I once watched what looked like the same grass stem change “hands” several times as it was repeatedly stolen from a nest and then stolen back.

Probably the most unique way that cliff swallows exploit others is by foisting some of their eggs off onto their neighbors. Mary and I discovered relatively early in our research that these birds are like other species in that they sometimes lay eggs in the nests of other colony members. Typically, a female finds a neighboring nest unattended, slips in and lays an egg, and the unsuspecting neighbor is none the wiser. This enables the parasitic bird to then lay an additional egg in her own nest, and in that way increase her reproductive output. It also “spreads the risk” by not putting all her eggs in the same nest, should her own nest be depredated or be washed away by a storm. Up to 20 percent of swallow nests have parasitic eggs laid in them, and the odds of both finding a nest to parasitize and having your own nest victimized increase in larger colonies.

But cliff swallows have evolved an even neater trick to parasitize their neighbors. If you lay a parasitic egg, you have to time the laying to coincide with when your neighbor lays her eggs. Lay too early and the neighbor won’t be at the appropriate stage to care for (incubate) the eggs. Lay too late and the neighbor’s own eggs will have a head start in development and hatch before yours. This means parasitic laying is only possible during a fairly narrow window of time (three to four days). Yet, if you carry an egg already laid in your own nest into a neighbor’s, you have a longer window of time (the entire 16-day incubation period) to find your neighbor’s nest left unguarded. Cliff swallows do just that. They physically transfer eggs in their beaks from their own nests into ones nearby. They even seem to be able to predict (how we don’t know) which nests in the colony are less likely to have lots of swallow bugs, and eggs are transferred into those nests at a higher frequency than into the more infested nests. In this way, females seem to insure that at least some of their offspring are reared in nests with fewer bugs and are thus more likely to survive.

Why colonies vary in size

One big reason that the cliff swallows of the Platte Valley have proven to be such marvelous research subjects is that they live in colonies that vary so much in size. A few birds are found each year that live by themselves, and others form colonies containing up to 6,000 nests on a single bridge. Colonies spanning this entire range are found every year, with most birds living in groups of several hundred nests. This variation, which I was unaware of when I selected Keith County as a research site, has allowed us to study not only how the costs and benefits of social life vary with group size but also what ecological conditions might cause colonies to differ in size in the first place. This is a fundamental question about animal behavior but, surprisingly, one that has not been widely addressed.

One factor that determines, in part, the number of cliff swallows at a given colony site (e.g., on a particular bridge) is the nature of the habitat surrounding it. Feeding on small, swarming insects, the swallows prefer areas that support large numbers of swarms. Insect abundance and distribution is influenced by the type of vegetation, land use and amount of water within the birds’ foraging range. Sites that have a moderate amount of water (ponds, rivers, canals) relatively close and also have diversity in other kinds of habitat are the ones that support the largest swallow colonies. Sites surrounded by all one kind of habitat (e.g., a cornfield) tend not to regularly attract as many birds, probably because there isn’t enough insect food in the vicinity. The overall land use in the Platte Valley, with moderate amounts of standing or flowing water plus a mixture of cropland, prairie, riparian woodland and town-like areas, probably accounts for the high abundance of cliff swallows in this area, simply because conditions are so good for insects.

Perhaps the most surprising single discovery about cliff swallows in the last 28 years is the realization that colony size is, in part, based on genetics. It is well known that some animals produce increased levels of stress hormones in social situations, and some 15 years ago I guessed that cliff swallows might “perform” better (i.e., be more successful in raising young) when in groups of particular sizes. Few colleagues, however, thought that an individual’s choice of what size group it would live in could be determined by genes.

To determine if there were innate preferences for groups of particular sizes, we performed an experiment in which some nestlings born in a colony of one size were raised in a colony of a very different size. We had monitored when nestling cliff swallows hatched, and as soon as they were old enough (three to four days), we banded them for permanent identification. We then switched some babies born in large colonies to nests in small colonies to be raised by foster parents; we did the reverse with additional hatchlings.

If the babies’ subsequent choice of where to live was based largely on genetic tendencies, the youngsters should later choose colonies that matched in size where they were born (where their parents chose to live), not where they were reared. On the other hand, if early experience as a fledgling in a particular social environment dictated later choices, the exchanged birds should choose colonies that matched the size of the sites where they were reared. The next summer we caught breeding birds, looking to see where our experimental birds returned and what size colonies they chose as first-time breeders. Yearling cliff swallows chose breeding colonies that matched in size with where they were born and rarely occupied sites similar to where they were reared. Thus, cliff swallows seem genetically programmed to use colonies of a particular size, and this accounts in part for the diversity of colony sizes seen in the population along the Platte River.

Although unappreciated by most people and certainly ignored by the many tourists cruising across Interstate 80 each summer, the cliff swallows of western Nebraska have provided insights that have reshaped the way scientists think about group living in animals. They are a fascinating treasure that everyone who lives in the state should protect and take the opportunity to enjoy.


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