Two-year-old nesting behavior and extra-pair copulation in a reintroduced Osprey population

Abstract

A reintroduced population of Ospreys (Pandion haliaetus) in eight counties surrounding Minneapolis/ St. Paul, MN was monitored from 1984-2005. We recorded the behavior of marked birds at 306 occupied nests. Two-yr-old Ospreys were involved in nest attempts more often than previously reported. Twelve were observed; eight of these (five males and three females) occupied nests, and the remaining four were floaters. Readily available nest sites and abundant food resources may have been factors in breeding attempts by these young Ospreys. Age at first known occupied nest averaged 4.13 yr for males (N = 38) and 3.36 yr for females (N = 11). Age at first known successful nest averaged 4.39 yr for males (N = 38) and 3.64 yr for females (N = 11). In 2003, two 2-yr-old males fledged chicks. We also observed Ospreys copulating with more than one partner in the same breeding season on 14 occasions (10.52% of nests where eggs were laid). All but one extra-pair visit was observed between 2003 and 2005, when the nesting population of the study area exceeded 33 pairs. Fertility insurance, genetic quality, sperm competition, and parental care may have been factors favoring extra-pair-copulation behavior in this population. Because multiple individuals were observed copulating outside the pair bond, the paternity of chicks in these clutches of Ospreys cannot positively be attributed to the social mate. © 2008 The Raptor Research Foundation, Inc.