As spring slowly gives way to summer along Nebraska’s Platte River, and the vast flocks of sandhill cranes and snow geese become only distant memories as they wing their way north to breeding grounds in Canada, Alaska or Siberia, another avian spectacular unfolds near the braided, winding channels of this storied river. Small, sparrow-sized cliff swallows (Petrochelidon pyrrhonota) begin returning in the thousands to form enormous nesting colonies underneath bridges over the Platte, in concrete culverts beneath the highways and railroads that crisscross the river valley, or on riverfront cliff faces and natural outcroppings that line the upper reaches of the North Platte River in far western Nebraska. With their gourd-shaped mud nests clustered closely together, neighboring pairs raise their young as a gigantic synchronous horde, which in some ways more resembles a colony of ants or social bees rather than birds. Cliff swallows rival even the more famous sandhill cranes in their propensity to live in large groups and do everything together.
Sometimes confused with the related barn swallow, cliff swallows are known by their orange rump, square tail and white forehead patch. The species breeds across most of North America but is more common in the western half of the continent. Migrating to northern Argentina, Uruguay and southwestern Brazil for the winter, cliff swallows have among the longest migratory journeys of any North American land bird. They are also famous as the swallows that mythically return to San Juan Capistrano on March 19, St. Joseph’s Day, each year. Cliff swallows historically built their nests underneath vertical ledges on the sides of steep cliffs and canyons in mountainous areas. But relatively recently the birds have discovered that human structures, like bridges, road culverts and underneath the eaves of people’s houses, are better places to live, probably because nests there are less likely to be destroyed by storms and might be harder for predators to reach.
The center of the cliff swallow’s universe in Nebraska (and probably the entire world) is the area around Keystone, just east of Kingsley Dam in Keith County. Here, the University of Nebraska’s Cedar Point Biological Station is situated, surrounded by dozens of swallow colonies and thousands of nesting birds. In the summer of 2009, I estimated over 41,000 active cliff swallow nests between Maxwell, in Lincoln County, on the east and Broadwater, in Morrill County, to the west. Even before people built roads and bridges in Nebraska, these birds were reported to be nesting in the Platte Valley: naturalist-surgeons with the U. S. Army found them on cliffs in the Ash Hollow area as early as 1845. To paraphrase the California ornithologist and swallow enthusiast William Dawson from 1923, the species’ overwhelming abundance in the area around Keystone and Lake McConaughy makes it difficult to notice any birds there except cliff swallows! Providing a natural laboratory for the study of animal social life, this concentration of swallows drew me to the biological station almost 30 years ago, where I have spent each summer since trying to understand this bird’s complex and fascinating social behavior.
Benefits and costs of social life
Many kinds of animals live in groups of different sizes during at least part of the year. Natural historians dating back to Aristotle have speculated on the advantages of living in close proximity to others of your ilk, but the modern study of animal sociality began only about 40 years ago when biologists John Crook, David Lack, Hans Kruuk, Henry Horn, Richard Alexander, John Hoogland and others adopted an evolutionary perspective on the formation of animal groups. They pointed out that there are both advantages and disadvantages to grouping. To understand why group life evolves in some species and not in others, one must observe how the positive and negative factors interact to affect the animals’ survival and reproduction.
Working on colonially nesting bank swallows (Riparia riparia) in Michigan, Hoogland and collaborator Paul Sherman were among the first scientists to measure how the costs and benefits of social living differed among birds living in colonies of different sizes. They found, for instance, that bank swallows in bigger colonies were parasitized by more blood-sucking fleas than those in small groups, yet the birds in the large colonies were more successful at thwarting attacks of predatory birds. The net gain seemed to favor bank swallows that formed colonies.
In 1982 when I first came to western Nebraska to study cliff swallows, we still had only a partial idea of what the different costs and benefits of living in colonies for birds might be. Some ecologists, like Hoogland and Sherman, felt that group living was most important in helping animals avoid being preyed upon: the more members in a group, the more “eyes” looking out for predators and thus a lower likelihood of getting eaten. Other scientists, like Amotz Zahavi, Stephen Emlen and John Krebs, believed that animals formed groups mainly to aid in the search for food, particularly when the animals feed on patchy or ephemeral resources. Simply put, if it takes a long time to find a food source (such as a school of fish in the sea or a swarm of insects in the air), individuals can avoid the costs in time and energy of searching for food themselves by instead watching where others in that group feed and then following suit.
Yet despite these advantages to being together, a substantial disadvantage to social life is the increased likelihood of contracting parasites or disease from other group members through intentional or unintentional physical proximity and contact. The rampant spread of flu and other infectious diseases among college kids cooped up in large dormitories in the winter attests to this reality. It’s now generally thought that all animals must pay this cost to some degree. Another universal drawback to social life is greater competition for resources. When many individuals of the same species all live together, they will likely eat the same foods or nest in the same kinds of places. This may force them to invest more time and energy in finding and defending food or nest sites from each other, or it may require them to travel farther in search of these commodities.
Swallow bugs and social foraging
My collaborator and fellow behavioral ecologist Mary Bomberger Brown, now at the University of Nebraska-Lincoln, and I quickly discovered in our research that the cliff swallows of the Platte Valley experience one classic cost of group living, an increased incidence of ectoparasites. The swallow bug (Oeciacus vicarius), a blood-sucking relative of the human bedbug, is adapted specifically to cliff swallows, and populations of these bugs can be quite large in swallow colonies (up to 2,600 bugs in a single nest). The bugs live in the swallows’ nests or in cracks and crevices of the nesting substrate near the nests and feed on both the adult birds and the nestlings inside the nest.
We found that cliff swallow nests in larger colonies contained more bugs per nest than did those in smaller colonies. Furthermore, by fumigating nests with insecticide and removing the bugs in some nests, we found that bugs exert a severe cost to cliff swallows in larger colonies: nestlings exposed to the many swallow bugs there often die before fledging, likely because of blood loss. Those that do survive to leave the nest are in poorer condition and less likely to live to return to Nebraska from South America the next year. Sometimes, bugs are so numerous in a colony that the cliff swallows there abandon it entirely, leaving their eggs or small nestlings to starve. (If swallows live on your property, have no fear: the bugs feed only on birds!)
With such a terrible price to pay for living together, swallows must gain some advantage of this lifestyle. The primary one, we believe, is the opportunity to use other colony members to help with finding food. In the early 1970s, it was suggested that bird colonies might serve as “information centers,” where birds could always reliably identify other members of the group who were knowledgeable at that moment as to the whereabouts of food. This would be particularly important if the species fed on food types that were unpredictable in where and when they could be found. Cliff swallows feed on swarms of insects concentrated in thermals of warm air that vary considerably in location over the course of even a single morning, and thus we knew early on that they would be good candidates for their colonies serving as information centers. I remember the very first morning in 1982 that I spent watching cliff swallows forage. I could tell immediately that group foraging was important for these birds, as solitary foragers were almost nonexistent and flocks stayed together cohesively as they looked for insects.
Mary and I subsequently verified that the swallows frequently observed others at a colony and learned who had been recently successful the same way we did: by seeing who came back with a beakful of insects to feed their babies. Birds who came back to their nests without food, the presumably unsuccessful ones, watched their neighbors, and when a nearby bird came back with food, the unsuccessful bird followed the successful one when it next left the colony to return to the food source. In this way, birds save the time and energy of hunting for the swarms, and the more birds present, the more quickly individuals can locate somebody who happens to know where the food is at any given time. This increased foraging efficiency compensates somewhat for the costs of bug parasitism in the larger colonies.
We discovered other advantages and disadvantages of social life. For example, birds in larger colonies are more likely to detect incoming hawks or snakes that prey on them. With all the closely spaced neighboring nests in larger colonies, birds’ nests can share common walls, and this reduces the time and effort of building a nest. Cliff swallows have to travel farther to feed as colonies grow, however, because the many birds present tend to deplete the insects near a colony. Birds in larger colonies also fight more as they compete for the best places to build their nests (typically toward the center of the colony). The net effect of all the different costs and benefits of social life seems to be that in some years, cliff swallows nesting in large colonies have higher survival and raise more young, whereas in other years birds in medium-sized or small colonies have the advantage.
Part two of this essay discusses the cliff swallow’s opportunistic breeding behavior and colony size.