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28 September 2022 Spatiotemporal Responses of Wintering Bald Eagles to Changes in Salmon Carcass Availability in the Pacific Northwest
Ethan S. Duvall
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Abstract

Each winter, thousands of bald eagles (Haliaeetus leucocephalus) from across western North America migrate to Pacific Northwest rivers to feed on the carcasses of post-spawning chum salmon (Oncorhynchus keta). However, declining salmon populations and impacts of climate change are reducing the availability of salmon carcasses as a wintering food source for eagles. The ability of eagles to adjust to these impacts is crucial to their survival, and their responses are currently unknown. I hypothesized that eagles are responding by redistributing to non-river habitat in search of alternative food sources. Specifically, I examined the redistribution of over-wintering eagles from river habitat to nearby agricultural areas in response to seasonal declines in carcass availability. Over two consecutive winters, I conducted weekly eagle surveys on a 30-km stretch of the Nooksack River and a 22.5-km stretch of farmland northwest of the river. I examined the association between salmon carcass distribution and eagle abundance on the Nooksack River, and evaluated the temporal relationship between eagle abundance on the Nooksack River versus neighboring farmland. I found a strong negative association between eagle abundance on the river versus adjacent farmland, and observed eagles primarily concentrated near dairy farms and waterfowl rafts. My results suggest that eagles are responding to declining carcass availability by redistributing to non-river habitat in search of alternative food sources, and that a substantial fraction may migrate to agricultural areas.

© 2022 by the Northwest Scientific Association. All rights reserved.
Ethan S. Duvall "Spatiotemporal Responses of Wintering Bald Eagles to Changes in Salmon Carcass Availability in the Pacific Northwest," Northwest Science 95(3-4), 307-316, (28 September 2022). https://doi.org/10.3955/046.095.0306
Received: 27 October 2020; Accepted: 19 May 2021; Published: 28 September 2022
KEYWORDS
agriculture
Bald Eagle
chum salmon
distribution
Pacific Northwest
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