1. a Laboratory of Ornithology , Cornell University , Ithaca , New York , 14850 , USA
2. b Microbiology & Immunology, College of Veterinary Medicine , Cornell University , Ithaca , New York , USA
3. c Department of Biology , Virginia Tech , Blacksburg , VA , 24061 , USA
4. d Department of Natural Resources , Cornell University , Ithaca , New York , USA