Robinson, Bryce – Gyrfalcon Diet During the Brood

Rearing Period on the Seward Peninsula, Alaska, in the Context of a Changing World. 2016.

Alumni Research

Climate change is predicted to affect system dynamics in Arctic ecosystems. Many Arctic organisms depend on spring snowmelt for the resources required for reproduction.

Due to this, any change in the timing of spring events has the potential to affect organisms at different trophic levels in different ways, and carries with it conservation implications. The Gyrfalcon, Falco rusticolus, depends on ptarmigan as its key prey source for most of the year. Fluctuations in ptarmigan numbers require the presence of alternative prey to supplement the Gyrfalcon’s diet. On the Seward Peninsula, the Gyrfalcon shows a 50-day nest initiation window. This window acts as an important aspect of the breeding biology of Seward Peninsula Gyrfalcon because it provides an opportunity to assess the potential impacts of change on Gyrfalcon population stability. Differences in productivity and success between early and late nesters could highlight the implications of change on the reproductive effort of the Gyrfalcon.

My project aimed to assess the importance of Gyrfalcon nest initiation date related to prey use during the nestling period on the Seward Peninsula in western Alaska. To assess diet I installed motion sensor camera traps and collected pellets and prey remains to catalogue the diet during the nestling period. This information was then compared to nest initiation date, success, and output in hopes of better understanding the role of diet in population stability of a key Arctic predator, the Gyrfalcon.

The January-February 2016 issue of Audubon Magazine features a story on Bryce Robinson and his research work with the Peregrine Fund on Alaska’s Seward Peninsula.  Read the full Audubon Magazine article online

Listen to and/or read the interview on Boise State Public Radio: Idaho Researcher Looks At What Gyrfalcons Can Tell Us About Climate Change

Thesis title:

Gyrfalcon Diet During the Brood Rearing Period on the Seward Peninsula, Alaska, in the Context of a Changing World

Abstract:

As climate change impacts increase so does our need to understand their effects on ecosystem dynamics. I studied Gyrfalcon (Falco rusticolus) diet during the brood rearing period to improve our knowledge on dietary habits during nesting, and provide necessary information for understanding climate change impacts to Arctic ecosystems. I studied diet over two breeding seasons on the Seward Peninsula, Alaska, using two methods: motion-activated cameras and the collection of prey remains. I observed three important dietary shifts: the proportion of ptarmigan in the diet declined significantly throughout the season, the proportion of large prey items declined significantly throughout the season, and there was a between-season shift in predominant prey type from ptarmigan (Lagopus lagopus and L. muta) in 2014 to squirrel in 2015. The decrease in the proportion of ptarmigan coincided with an increase in Arctic ground squirrel (Urocitellus parryii) in the diet, indicating a within-season switch from ptarmigan to squirrel as the main prey item.

Despite the shift in prey composition, dietary breadth did not change. These results suggest that the Gyrfalcon is a facultative specialist, an important consideration regarding the predicted impacts of climate change on species interactions in the Arctic, as facultative shifts between prey types may help offset negative impacts to population dynamics caused by changes in prey populations. I also compared dietary characterization by method to assess whether biases existed between camera and prey remains data. I then related my results to Roseneau (1972) who used prey remains collection to quantify diet of the same population 46 years previous as a case study to assess the use of past diet studies to monitor changes in Gyrfalcon prey use.

The number of total prey items and the number of squirrels was significantly greater from camera data than prey remains analysis. This result suggests that prey remains analysis under-represented the contribution of squirrel to the diet. The underrepresentation of squirrel to the diet as provided by prey remains analysis, and the differences between the contribution of squirrel in my study and Roseneau (1972), reveals the limitations of prey remains analysis for a complete characterization of diet, and also suggests the limitations of using Roseneau as a baseline study to assess dietary change in western Alaska.

I further summarized 19 prior Gyrfalcon diet studies to evaluate their value as baselines by which to compare and confidently assess the impacts of climate change on Gyrfalcon diet.

I conclude that historical studies of Gyrfalcon diet are of limited value for measuring the impacts of climate change due to biased methods, short duration, small samples sizes, and a lack of standardization of methods. My case study involving two diet studies on the same population illustrates the benefits of camera data to quantify diet and illustrates the clear need to develop recurring, standardized diet studies across the circumpolar Gyrfalcon range to assess dietary shifts as a measure of climate change impacts on Arctic communities.
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