Ecological and environmental correlates of territory occupancy and breeding performance of migratory Golden EaglesAquila chrysaetosin interior Alaska

Abstract

Understanding relationships between environmental conditions and reproductive parameters is important when interpreting variation in animal population size. The northwestern North American population of Golden Eagles Aquila chrysaetos canadensis initiates courtship and nesting in early spring when prey diversity is low and weather conditions are severe. Snowshoe Hare Lepus americanus and Willow Ptarmigan Lagopus lagopus, the primary prey of Golden Eagles early in their nesting season in interior Alaska, both exhibit cyclical fluctuations in abundance, providing the opportunity to investigate such relationships. We used Bayesian hierarchical models to explore variation in territory occupancy, nesting rates, nesting success and productivity of Golden Eagles from 1988 to 2010 in Denali National Park and Preserve, Alaska, in relation to annual and site‐specific parameters including prey abundance, weather conditions, elevation and human activity. We also investigated the long‐term fluctuations of breeding performance over the course of the study. The abundance of Hares influenced both the number of Eagles that laid eggs and the number of Eagles that produced fledglings. The conditions on the breeding ground did not explain observed declines in nesting rates and fledgling production, suggesting that other factors such as change in the age structure of the population, increased intraspecific competition or deterioration of migration and wintering habitat are driving the long‐term trends of these parameters.

Government-sponsored Falconry Practices, Rodenticides, and Land Development Jeopardize Golden Eagles (Aquila chrysaetos) in Western China

“…comm.). Declines in breeding success over 23 yr in Alaska could not be tied to conditions on breeding grounds, suggesting that conditions on wintering grounds, which include the southwestern U.S., may have negatively affected breeding condition of returning migrants (McIntyre and Schmidt 2012). Additionally, our ability to interpret trends in occupancy may have been limited by adding territories to our sample over the course of the study.…”

“…We do not, however, expect sampling bias affected our conclusions on differences among study areas because we used consistent methods to locate nesting territories in all areas. We were not able to collect any data on environmental factors (e.g., prey abundance, weather, vegetation), but acknowledge other researchers have found Golden Eagle reproductive success was correlated with environmental factors, particularly prey abundance (Steenhof et al 1997, McIntyre andSchmidt 2012). Limited funding and personnel restricted us to relatively few survey occasions per year to document occupancy ( 2 visits), and production of early-season (one visit) and fledging-age nestlings (one visit).…”

“…Our terminology followed the definitions of Steenhof and Newton (2007) and McIntyre and Schmidt (2012), and recent recommendations of Steenhof et al (2017). We defined a Golden Eagle nesting territory as a 3.2-km-radius circular buffer around a nest or nest cluster located during the nest survey, where only one pair of eagles nested in a year.…”

“…recommends management to maintain stability of Golden Eagle populations at the level of Bird Conservation Regions (BCRs). Relevant to this study, the 471,600-km 2 Southern Rockies Colorado Plateau BCR (of which our 67,873-km 2 Navajo Nation study area makes up 14.4%) was the only BCR with a negative population trend for Golden Eagles during -2012. Apparent negative trends of populations in the southwestern U.S. create a pressing need to understand potential factors affecting survival and reproduction of Golden Eagles in this region.…”

Long-Term Study of Reproductive Performance in Golden Eagles in Relation to Food Supply in Boreal Sweden

“…It is appropriate for species that lay eggs every year. Brood size at fledging (a synonym for young per successful pair or nest) is usually less variable than other measures of reproduction (Steenhof et al 1997, McIntyre andSchmidt 2012), but it is less meaningful for assessing reproduction of a population. Estimates of reproduction based solely on brood size at fledging are misleading because they do not include information on nonlaying pairs and unsuccessful pairs.…”