First Documented Cases of Polygyny in the Grasshopper Sparrow

—We studied the breeding biology of Grasshopper Sparrows (Ammodramus savannarum) on restored grasslands in Maryland from 1999 to present. We report the first documented cases of social polygyny in this species. Polygyny increased reproductive success for males in two of four cases, but its rarity suggests it is only a facultative behavior for this usually socially monogamous species Received 31 January 2009. Accepted 3 June 2009. Social polygyny is a mating system in which males pair bond with, frequently control, or gain access to two or more females simultaneously (Gill 2007). Seventy-one species of passerine birds (26%) within the United States and Canada have been reported to exhibit some form of social polygyny (Ford 1996, Hanley et al. 2007), although within most of these species polygyny seems to be the exception rather than the norm. Ford (1983) lists 1 Chester River Field Research Center, P. O. Box 421, Chestertown, MD 21620, USA. 2 Eastern Shore Land Conservancy, 601 Locust Street, Suite 302, Cambridge, MD 21613, USA. 3 14621 Nebraska Avenue, Tampa, FL 33613, USA. 4 Department of Biology, University of Maryland, College Park, MD 20742, USA. 5 Corresponding author; e-mail: [email protected] 822 THE WILSON JOURNAL OF ORNITHOLOGY N Vol. 121, No. 4, December 2009