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Caldwell Hahn

USGS Patuxent Wildlife Research Center
12110 Beech Forest Road
Laurel, MD 20708-4015

Photo of Caldwell Hahn

Telephone: 301-497-5653

Fax: 301-497-5624


Research Wildlife Biologist

Primary Responsibilities: 
We are working on studies at the interface of wildlife disease ecology and human health.  In particular, we are working on several aspects of West Nile Virus as well as theoretical questions of the evolution of immunity.          

The rapid geographic spread of West Nile Virus across the United States has stimulated a wide range of work on transmission and spread of this disease, which is highly pathogenic to both wildlife and people.  We have conducted comparative studies, identifying which avian species are competent reservoirs, maintaining the virus and facilitating its spread by mosquito vectors.  In owls, we also study disease resistance mechanisms by documenting maternal transfer of WNV antibodies in Eastern Screech Owls. Currently we are looking at within-clutch patterns of maternal allocation of WNV antibodies based on egg lay order, offspring gender, and plumage pattern. 

In songbirds, we use comparative studies to illuminate the principles of evolution of immunity and the immune system. We utilize the life history strategy of the Brown-headed Cowbird, the principal North American brood parasite, to compare life history parameters of cowbirds vs related non-parasitic species.  We predicted differences in disease resistance among the blackbird species based on differences in life history, because they differ in geographic range and life history traits of that include mating and breeding systems. In particular, based on our extensive earlier work on cowbirds, we predicted that the cowbirds would be significantly more disease-resistant than related non-parasitic species.  Higher exposure to parasites is the basis for parasite-mediated selection for increased diseased resistance.

Wellesley College, B.A.
Tufts University, M.S.
Rutgers University, Ph.D. 
Smithsonian Tropical Research Center, Post-doctoral Fellow
Harvard University, Museum of Comparative Zoology, Post-doctoral Fellow

Areas of Expertise/Interest:
Behavioral ecology
Ecological immunology
Life history


Active Projects: 

Transmission and spread of infectious diseases: role of migratory songbirds

Immunological studies in the Screech Owl (Megascops asio): West Nile Virus Susceptibility

Evolution of immune systems: comparative studies of brood parasites and their relatives

Selected Publications/Products: 

Hahn, D. C., S. G. Summers, K. J. Genovese, H. He, and M. H. Kogut. 2013. Obligate brood parasites show more functionally effective innate immune responses: An eco-immunological hypothesis. Evolutionary Biology online early April 16 abstract

Hahn, D. C., S. G. Summers, L. H. He, K. J. Genovese, and M. H. Kogut. 2013. Enhanced innate immune responses in a brood parasitic cowbird species: degranulation and oxidative burst. Avian Diseases 57(2):285-289. abstract

Hahn, D. C. and W. K. Reisen. 2011. Heightened exposure to parasites favors the evolution of immunity in brood parasitic cowbirds. Evolutionary Biology 38(2):214-224. abstract

Hahn, D. C. 2011. Patterns of maternal yolk hormones in eastern screech owl eggs (Megascops asio). General and Comparative Endocrinology 172(3):423-429. abstract

Kozlowski, C. P. and D.C. Hahn. 2010. Developmental changes in circulating androgens of nestling screech owls from hatching till fledging. Wilson Journal of Ornithology.122(4):755-761. abstract

Reisen, W. K. and D. C. Hahn. 2007. Comparison of immune responses of brown-headed cowbird and related blackbirds to West Nile and other mosquito-borne encephalitis viruses. Journal of Wildlife Diseases 43(3):439-449. (PDF)

Hahn, D.C, N.M. Nemeth, E. Edwards, P.R. Bright, and N. Komar. 2006. Passive West Nile virus antibody transfer from maternal Eastern Screech-Owls (Megascops asio) to progeny. Avian Diseases 50(3):454-455. (PDF)

Hahn, D.C., J.S. Hatfield, M. Abdelnabi, J. Wu, L.D. Igl, and M.A. Ottinger. 2005.  Inter-species variation in yolk steroid levels and a cowbird-host comparison.  Journal of Avian Biology 36(1):1-7. (PDF)

Hahn, D. C. and J. R. O'Connor. 2002. Contrasting determinants of abundance in ancestral and colonized ranges of an invasive brood parasite. Pages 219-228 in J. Michael Scott, Patricia J. Heglund, Michael L. Morrison, Jonathan B. Haufler, and William A. Wall, editors. Predicting species occurrences : issues of accuracy and scale. Island Press, Washington, DC. xvii, 868 pp. (PDF)

Hahn, D. C. and J. S. Hatfield. 2000. Host selection in the forest interior: cowbirds target ground-nesting species. Pages 120-127 in J. N. M. Smith, T. Cook, S. Rothstein, S. Robinson, and S. Sealy. Ecology and management of cowbirds and their hosts: studies in the conservation of North American passerine birds. University of Texas Press, Austin, TX. ix, 388 pp.

Hahn, D.C, R.D. Price, and P.C. Osenton. 2000.  Use of lice to identify cowbird hosts.  Auk 117(4):943- 951. (PDF)

Hahn, D. C., J. A. Sedgwick, I. Painter, and N. Casna. 1999. A spatial and genetic analysis of Cowbird host selection. Pages 204-217 in M. L. Morrison, L. S. Hall, S. K. Robinson, S. I. Rothstein, D. C. Hahn, and T. D. Rich, editors. Research and management of the Brown-headed Cowbird in western landscapes. Studies in Avian Biology 18. 312 pp.  (PDF) 

Burger, J., D. C. Hahn, and J. Chase.  1979.  Aggressive interactions in mixed-species flocks of migrating shorebirds.  Animal Behaviour 27(2): 459-470. abstract 

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