About the Cliff Swallow Project

The Cliff Swallow Project has its roots in a study conducted in the early 1970's on bank swallows in Michigan. Biologists John Hoogland and Paul Sherman measured the costs and benefits of living in colonies, using natural variation in the bank swallows' colony sizes. They studied, for example, how bigger colonies might offer advantages to bank swallows by helping the birds detect and avoid predators more successfully. However, they also found that there were disadvantages of large colonies because more fleas infested the birds' nests there. Charles Brown decided to ask similar questions for cliff swallows, which was the initial impetus for the project. When he became Hoogland's graduate student in 1981, the project was conceived as Brown's Ph.D. research. Charles made the decision to study cliff swallows in about five minutes.

Brown was fortunate to locate the Cedar Point Biological Station in western Nebraska as his study site, where field work commenced in 1982. The area around Cedar Point has probably the highest density of cliff swallow colonies of any region in North America, and colony sizes there vary widely. Some colonies can reach 6000 nests in size, while others can be as small as 2 nests, and a few birds nest as single pairs. This natural variation in colony size has contributed heavily to the longevity of the project, because it has allowed us to address many fundamental questions about the evolution of social behavior in animals.

Charles Brown- The Cliff Swallow Project

Our research has sought generally to understand the selective pressures that bring about social living in cliff swallows, how the birds' fitness changes in different sized groups, and what ecological conditions lead to variation in group size.

The main areas of investigation have been:

Laying Eggs in a Neighbor's Nest

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.
The Costs and Benefits of Coloniality

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.
Fitness Consequences of Coloniality

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.
Choice of Colony Size

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.
Why Do Colonies Vary in Size?

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.
Parasite Ecology

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.
Hormones and Colony Size

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.
Buggy Creek Virus and Disease Transmission

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.
Rapid Evolution

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.
Personality and Group Size

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.

Researchers

Charles R. Brown, Ph.D
Charles began studying cliff swallows in 1980, and has conducted research on them full-time since 1982. His cliff swallow research in western Nebraska is one of the longest-running field studies of birds done in North America. He received a B.A. in biology from Austin College in 1981, and a Ph.D. in biology from Princeton University in 1985. He is currently a professor of biological science at the University of Tulsa, where he has been since 1994. Charles’ research on cliff swallows was recognized by the Elliot Coues Award from the American Ornithologists’ Union in 2009 for extraordinary contributions to ornithological research, and by the Exemplar Award from the Animal Behavior Society in 2011 for major long-term contributions to the study of animal behavior. His book, Swallow Summer (University of Nebraska Press, 1998), was described by Booklist as one of the best popularizations about field research ever written. For more, see Charles' Career Inspiration Story from the Perot Museum of Nature and Science.
Laci Cartmell
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Brooke Kester
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Valerie O’Brien
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Olivia Pletcher
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Hannah Reeb
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Amy West
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Mary Bomberger Brown
Mary began studying Cliff Swallows in 1982 and worked on the project full-time through 2006. Her other research interests included habitat selection in Wilson’s phalaropes and conservation and management of interior least terns, piping plovers, and greater prairie-chickens. She received a Ph.D. from the University of Nebraska in 2011 and was an associate professor at the University of Nebraska-Lincoln at the time of her death in 2019. For her work on cliff swallows, she was co-recipient of the Elliot Coues Award from the American Ornithologists’ Union in 2009 for extraordinary contributions to ornithological research.
Gigi Wagnon
Amy Moore
Catherine Page
Stephanie Strickler
Christine Sas
Bruce Rannala
Erin Roche
Stacey Hannebaum
Abinsah Padhi
Jerome Foster
Samrrah Raouf
Linda Smith

Undergrad Field Research Assistants

Laura Molles	(Yale University)
Laura Monti	(Yale University)
Shyam Narotam	(University of Tulsa)
Cecily Natunewicz	(Yale University)
Cheryl Ormston	(University of Tulsa)
Charlene Patenaude	(University of Dallas)
Sunita Quick	(Colorado College)
Barbara Raulston	(University of Texas at
Gabriela Redwine	(Yale University)
Craig Richman	(University of Pennsylvania)