Comparing Apples to Oranges: What is the Relative Nutritive Value of Avian and Rodent Prey to an Apex Arctic Predator?

Effective conservation and management of raptors necessitates an understanding of their diet and feeding habits, especially in environments highly susceptible to anthropogenic change. The Gyrfalcon (Falco rusticolus) is an apex avian predator of the Arctic tundra ecosystem that specializes on Willow Ptarmigan (Lagopus lagopus), Rock Ptarmigan (L. muta), and Arctic ground squirrels (Urocitellus parryii). Despite being approximately equal in overall biomass, the relative nutritive value of ptarmigan and squirrels to breeding Gyrfalcons is unknown. Because Gyrfalcon habitat and dietary specialization make them particularly susceptible to changing prey distributions and abundances resulting from climate-induced landscape changes, we compared the nutritive value of ptarmigan and ground squirrels collected from the Seward Peninsula, Alaska. We isolated the edible biomass of twenty ptarmigan and ten ground squirrels through lab dissections, and prepared tissue samples for protein, fat, and energy assays. We found that ptarmigan contain significantly more edible biomass than squirrels (243.2 g and 114.9 g for ptarmigan and squirrels, respectively). Further, 45% of a ptarmigan’s total body mass is edible compared to only 25% for a squirrel, suggesting that ptarmigan yield more nutritive benefit than squirrels per prey delivery. Results from tissue assays were not available at the time of this presentation. Our preliminary results suggest that Gyrfalcon populations may be disproportionately affected by changes in ptarmigan abundances or distributions than those of ground squirrels