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Henderson, Michael

Michael Henderson in Alaska - drop-off at remote nest location (by helicopter)
Drop-off at remote nest location

Effects of Protective Nesting Site Properties on Gyrfalcon Breeding Success and Parental Investment in Western Alaska

Thesis Abstract

Habitat suitability for wildlife is defined at scales ranging from the landscape to an individual breeding site. Areas that fulfill habitat requirements for birds disproportionally maintain populations, and the identification of variables that distinguish optimal breeding sites helps to prioritize conservation. Nesting site characteristics that protect breeding raptors from harsh weather can promote a more favorable microclimate and increase breeding success, although previous attempts to understand this effect in breeding Gyrfalcons have yielded ambiguous results. Additionally, breeding adults incur substantial costs from the physical shielding of eggs and nestlings, particularly in the Arctic, and it is possible that protective properties can decrease nest attendance rates, thus lowering costs of breeding. My objective was to quantify Gyrfalcon nesting site characteristics and assess how breeding success and nest attendance varies by protective qualities of nesting sites. I studied Gyrfalcons on Alaska’s Seward Peninsula from 2016 – 2018 by installing motion-activated cameras to monitor breeding attempts and quantify nest attendance rates. I found that the degree of physical exposure in the horizontal plane was negatively correlated with the probability of hatching and fledging (providing hatch occurred), as well as overall productivity. The negative effect of horizontal exposure on fledging probability and overall productivity was greatest at sites that were also more exposed in the vertical plane, although this interaction did not affect hatching probability.

Camera install
Camera install
Gyrfalcon chicks
Gyrfalcon chicks

Productivity more than doubled in nests that provided a refuge in which nestlings could seek shelter, such as a crevice or an overhang. Additionally, nest attendance rates were highest in nests that were maximally exposed in the horizontal plane, particularly when nestlings were two to three weeks old. The increased parental investment and concurrent decreased productivity associated with horizontal nest exposure demonstrated that nesting site characteristics can have both direct and indirect effects on breeding Gyrfalcons. The compounding effects of poor nesting site suitability suggests that breeding sites are a relevant scale for effective conservation of Arctic breeding raptors. As the Arctic continues to see rapid increases in temperature and precipitation, physical protection of nesting sites will likely become increasingly important for the conservation of the world’s largest falcon. Understanding factors that dictate habitat suitability, at all spatial scales, will help prioritize the conservation of valuable habitat as the tundra landscape continues to see accelerated climatic changes.