Probable causes of nesting failures in Oregon's bald eagle population

During the last 4 decades, numerous studies on a variety of raptor species have described breeding success and productivity, but few studies have reported the causes of nesting failures. Newton (1979:358) described causes of nesting failures for 6 species of raptors, but only 2 of these species (bald eagle [Haliaeetus leucocephalus], prairie falcon [Falco mexicanus]) inhabit North America. Assessing causes of nesting failures is difficult because some causes are easier to determine than others and some failures inevitably remain unexplained. The difference between the number of young produced/successful nest and occupied nest from 1979-1992 for Oregon's bald eagle population (Fig. 1) averaged 0.60 young/site and reflected the influence of nesting failures on productivity (young/occupied nest). Overall, the annual failure rate (% of occupied sites failing to produce young) averaged 40% (range = 34-49%) for this population from 1979-1992 and was higher than the 35% specified for recovery in the Pacific states (U.S. Fish and Wildl. Serv. 1986).