Extreme drought increased home range sizes and space use of Aquila chrysaetos (Golden Eagles) in coastal southern California

Aquila chrysaetos (Golden Eagles) are long-lived top predators, vulnerable to a variety of threats. There is increasing concern about the declining population in coastal southern California, which has largely coincided with habitat loss due to urbanization of the region. This Mediterranean-type ecosystem is also experiencing more prolonged and intense droughts, which can reduce the abundance of key prey species, such as jackrabbits (Lepus californicus) and ground squirrels (Otospermophilus beecheyi). However, the impact of drought on Golden Eagles is poorly understood. Here, we used high-resolution GPS tracking data to calculate eagle home ranges and hypothesized that home range size would be influenced by drought such that home ranges would be larger to meet their resource needs with worsening drought severity. Fifty individuals were captured over seven breeding seasons, spanning the time both during and after a historic drought. We also compared space use for different stages of the breeding season throughout each year. Golden Eagle home ranges increased by 77% during extreme drought compared to wetter conditions. Drought-breaking rainfall corresponded with much smaller home ranges compared to home range sizes during drought years. As drought is projected to be more prolonged and frequent in this region with climate change, this may result in increasingly large home range sizes at the same time as eagles are facing ongoing habitat loss and fragmentation from various land use changes. Our results suggest that the frequent and severe drought that is projected for this region could lead to lowered nesting density and increase the risk of further population decline.