Seasonal variation in resource selection by subadult golden eagles in the Great Basin Desert

Golden eagles Aquila chrysaetos are a long-lived and wide-ranging species believed to be stable or in slight decline across North America. Golden eagles have an extended subadult stage (4-5 years) that is critical to maintaining recruitment into the breeding population and population viability. Compared to adult golden eagles, the ecology of subadult golden eagles (hereafter, subadults) has received little attention. We investigated patterns of resource selection for subadults in the Great Basin Desert of the western United States during summer and winter, 2013-2019. We monitored 46 subadults with GPS transmitters and related locations (n =99 037) with predictors hypothesized to influence seasonal patterns of space use with mixed-effects logistic regression. Subadults selected for ridges and upper slopes in summer and winter, but higher elevations in summer. Subadults showed weak selection for lower ridge density in summer, which was likely facilitated by selection for areas with greater thermal wind current potential. In contrast, subadults showed strong selection for higher ridge density in winter. Subadults selected areas further from roads in summer and closer to roads and electrical transmission lines in winter, which may be related to winter scavenging of road-killed ungulates. Resource selection functions suggested subadults selected for shrublands and woodlands in both seasons, but odds ratios revealed that during winter subadults avoided shrublands and increased selection of woodlands relative to summer. Subadults selected for areas with infrequent fires in both seasons; areas with frequent fires were avoided in summer but selected for in winter. Seasonal changes in resource selection suggested that subadults used woodlands more than expected, potentially reflecting spatial partitioning by subadults to lower-quality habitats to minimize competition with breeding adults during winter when energetic demands for thermoregulation were presumably higher and prey more limited.

Automated summary

Golden eagles are a stable or slightly declining species in North America. The resource selection patterns of subadult golden eagles in the Great Basin Desert were investigated during the summer and winter seasons. The study found that subadults preferred ridges and upper slopes, with higher elevations favored in the summer. Their selection of lower ridge density in summer was likely influenced by thermal wind currents, while higher ridge density was preferred in winter. Subadults also showed a preference for areas further from roads in summer and closer to roads and electrical transmission lines in winter, possibly due to scavenging opportunities. They selected shrublands and woodlands in both seasons, but their preference for woodlands increased during winter. Areas with infrequent fires were preferred in both seasons, while areas with frequent fires were avoided in summer but selected for in winter. These seasonal changes in resource selection suggest that subadult golden eagles adjust their habitat use to minimize competition with breeding adults during winter when resources are limited.