Natal dispersal distance of bald and golden eagles originating in the coterminous United States as inferred from band encounters

Abstract

We reviewed band encounter data for Bald Eagles (Haliaeetus leucocephalus) and Golden Eagles (Aquila chrysaetos) to estimate natal dispersal distances for both species in the coterminous United States (U.S.). We filtered band recovery data to focus on individuals banded as nestlings, encountered at ages old enough to be breeding, and encountered at times of the year when they may be at or near breeding or prospective breeding sites. Our final data set included 96 Golden Eagles and 878 Bald Eagles. Distances between banding and subsequent encounter sites for both species were lognormally distributed. We employed both traditional and Bayesian methods of analysis, and obtained similar results from both approaches. Bayesian analysis of banding data suggest a median natal dispersal distance of 69.2 (95% highest density interval HDI = 63.5-73.1) km for Bald Eagles and 46.4 (HDI = 36.0-55.2) km for Golden Eagles. Median natal dispersal distance for female Bald Eagles 78.3 was (HDI = 35.4-128.6) km greater than for males; we lacked sufficient data to analyze natal dispersal distance by sex for Golden Eagles. Median Bald Eagle natal dispersal distance did not differ among eight regional populations, but there was evidence of a trend toward increasing natal dispersal distance from east to west across the coterminous U.S. Our findings are compatible with natal dispersal data in the literature for both species. The U.S. Fish and Wildlife Service uses estimated natal dispersal distance of Bald and Golden eagles to set one of the geographic scales at which the effects of permits that authorize the "take" (removal from the wild) of eagles is evaluated. Our analyses suggest that choice of a natal dispersal value in the range of the 50th-90th quantile of the distribution as an effect-area for modeling or effect-assessment for both species of eagle is reasonable. For Golden Eagles, this range is 46-175 km, and for Bald Eagles 69-346 km. © The Raptor Research Foundation, Inc.