Of the many forms of social behavior cliff swallows exhibit, one of the most interesting is their tendency to parasitize the parental care of other birds in a colony. A female will monitor her close neighbors, and if a nearby nest is left unattended, she will lay one of her eggs in that nest. Only resident females who have nests of their own do this, and sometimes two females parasitize each other! An even neater trick is these birds' ability to physically carry eggs from their own nest into a neighbor's. Duping neighbors into caring for one's chicks probably increases the parasitic females' overall reproductive success, in part because they often select nests to parasitize that have fewer blood-sucking insects than in their own nests.

Trying to determine what behavioral and ecological factors are both advantageous and disadvantageous to cliff swallows, depending on colony size, was our initial objective, and that question remains a focus to date. We have measured, for example, how the detection and avoidance of predators, the ability to find food, the extent of infestation by parasites, food depletion, competition​ among neighbors for resources, and the possibility of misdirecting parental care to other birds' offspring vary with colony size under natural conditions. These costs and benefits generally tend to increase as colony size gets larger.

A primary focus has been to use banding and recapture to measure how annual survival varies for cliff swallows living in colonies of different sizes. For this, we have used one of the largest mark-recapture data sets on any bird: we have banded over 230,000 cliff swallows since 1982 and captured birds in mist nets over 407,000 times. Current work is aimed at measuring reproductive success of birds in different sized colonies, and eventually we will use the data on survival and reproductive success to examine how fitness changes with colony size, year, and other characteristics of the cliff swallow's environment.

One of our main interests is in learning what rules cliff swallows use to choose their colony site and size each year. Do certain birds always use small colonies and others always large colonies? How important is an individual's familiarity with the colony site itself or the surrounding landscape from an earlier year in its choice of where to settle? Cross-fostering experiments have shown that birds born in colonies of a particular size tend to settle their first year in colonies of a similar size, regardless of where they were reared. This demonstrates a genetic tendency to occupy particular colony sizes.

​Little is known in general about why animal groups vary in size​. We have charted the histories of use for over 220 cliff swallow colony sizes, some since 1982. We are examining these patterns to determine if colony sites vary in size and use in response to parasite infestations, local habitat features, previous colony size, or the individual composition of the colony that previously used the site.

​A theme running through much of our research is the effect that swallow bugs—bedbug-related nest parasites that live in cliff swallow colonies—have on these birds’ social behavior and ecology. Bugs represent the most serious cost of living together for cliff swallows, and we have studied their effects on the birds’ survival, colony choice, dispersal, nesting behavior, and physiology. We have used fumigation at some sites to remove bugs, enabling us to study how cliff swallows respond to their absence. Bugs have been a major player in the transmission of Buggy Creek virus (below) to cliff swallows. However, we still do not understand many aspects of swallow bug life history.

We have examined how hormone profiles of cliff swallows in different sized colonies differ. For example, testosterone levels of both males and females are higher in larger colonies, probably an adaptation for the more frequent fighting and higher aggression seen in large groups. The stress hormone, corticosterone, increases when birds are exposed to more blood-sucking parasites in larger colonies, but in the absence of parasites, birds seem to be more stressed in smaller colonies. Some evidence indicates that stress hormone levels predict what colony size a bird chooses and are correlated with annual survival.

A major objective during the period from about 2006-2010 was to study a viral pathogen, Buggy Creek virus, and how it affected cliff swallows in colonies of different sizes. The virus is transmitted to birds by blood-feeding swallow bugs that live in the birds’ nests year-round. Buggy Creek virus has little effect on cliff swallows, but it severely affects invasive house sparrows that occupy cliff swallow nests at some sites. The invasive sparrows appear not adapted to this virus as a result of their relatively recent exposure. We have used the cliff swallow/swallow bug/Buggy Creek virus system to explore general questions about the ecology of bird-associated virus transmission. Although not a human pathogen, Buggy Creek virus is similar in some ways to viruses such as West Nile and western equine encephalitis viruses, which do affect human health.

One advantage of a long-term study is the ability to document rapid evolutionary changes in response to variability in the environment, including climate change. We have found that body size of cliff swallows has undergone a change since the early 1980’s, with birds now skeletally larger but with shorter wings and tails than when our study began. These changes reflect natural selection brought about in part by severe weather events (late spring cold snaps that reduce the availability of the birds’ food), causing the population to shift from smaller to larger birds almost overnight. Vehicles also exert a selection pressure on cliff swallows, because the birds often nest around roads. Swallows with shorter wings are more likely to escape an oncoming car, and consequently selection has favored birds with shorter wings over time and has resulted in fewer birds being killed on roads now than in the 1980’s. Finally, we are finding evidence that cliff swallows’ ability to tolerate the parasites that live in their nests has improved over time, a response to the high infestations of swallow bugs these birds have been exposed to over the last 35-40 years in our study area.

We recently initiated studies of personality in cliff swallows, building on research with other species illustrating that the behavioral composition of animal groups can often vary. Some groups consist of a higher percentage of bold or risk-taking individuals, while other groups contain more shy animals. We are investigating whether cliff swallows sort among colony sizes based on their personalities and whether the reproductive performance of an individual depends on its personality and/or those of others in the colony.