How Bald Eagles on the Copper River respond to temperature and salmon abundance

Salmon are a critically important food source for eagles.

In this paper, we used a 15-year dataset on the nesting population of Bald Eagles along the the Copper River in Wrangell-St Elias National Park and Preserve to investigate the role of weather conditions and salmon abundance on nest occupancy and success over time. Salmon are an important food resource for Bald Eagles in this system, and we found that nest success was positively related to early season salmon run strength as one might expect. However, warmer springs appeared to moderate the impact of salmon abundance, possibly through increased runoff due to increased glacial melt. These findings help us to understand the potential drivers of Bald Eagle population dynamics, and demonstrate more broadly how complex interactions between weather conditions and food resources can affect natural systems.

Direct and indirect effects of temperature and prey abundance on bald eagle reproductive dynamics

Abstract

Understanding the mechanisms by which populations are regulated is critical for predicting the effects of large-scale perturbations. While discrete mortality events provide clear evidence of direct impacts, indirect pathways are more difficult to assess but may play important roles in population and ecosystem dynamics. Here, we use multi-state occupancy models to analyze a long-term dataset on nesting bald eagles in south-central Alaska with the goal of identifying both direct and indirect mechanisms influencing reproductive output in this apex predator. We found that the probabilities of both nest occupancy and success were higher in the portion of the study area where water turbidity was low, supporting the hypothesis that access to aquatic prey is a critical factor limiting the reproductive output of eagles in this system. As expected, nest success was also positively related to salmon abundance; however, the negative effect of spring warmth suggested that access to salmon resources is indirectly diminished in warm springs as a consequence of increased glacial melt. Together, these findings reveal complex interrelationships between a critical prey resource and large-scale weather and climate processes which likely alter the accessibility of resources rather than directly affecting resource abundance. While important for understanding bald eagle reproductive dynamics in this system specifically, our results have broader implications that suggest complex interrelationships among system components.

Schmidt, J. H., J. Putera, and T. L. Wilson. 2019. Direct and indirect effects of temperature and prey abundance on bald eagle reproductive dynamics. Oecologia https://doi.org/10.1007/s00442-019-04578-8