Is What Comes out the Same as What Goes in? A Preliminary Investigation of the Isotopic Impacts of Digestion by Red‐Tailed Hawks (<scp><i>Buteo jamaicensis</i></scp>) and Eurasian Eagle Owls (<scp><i>Bubo bubo</i></scp>)

We investigated isotopic diet-excreta offset (Δ_diet-excreta) for predatory birds, and the isotopic influence of bird digestion on consumed prey tissues. Foraging ecology of predatory birds can be non-invasively monitored using excreta or regurgitated prey. However, one must account for Δ_diet-excreta and any influence of digestion on prey tissues. Neither of these has been previously evaluated for predatory birds. We worked with a captive Eurasian eagle owl (<i>Bubo bubo</i>) and red-tailed hawk (<i>Buteo jamaicensis</i>) fed frozen murid rats. We collected rat feet, as well as regurgitated pellets and excreta from each bird's enclosure. We analyzed carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes in undigested rat muscle, undigested and digested fur, and bone collagen (extracted from pellets), δ¹³C, oxygen (δ¹⁸O), and strontium (⁸⁷Sr/⁸⁶Sr) isotopes in rat bone bioapatite, and δ¹³C, δ¹⁵N, and ⁸⁷Sr/⁸⁶Sr in bird excreta. Diet-excreta offset differed slightly between individuals and depended on how we estimated diet (muscle alone or muscle + collagen) and if excreta were acidified. We tentatively suggest using +1 to 1.5‰ for Δ¹³C_diet-excreta and +0.5 to 1‰ for Δ¹⁵N_diet-excreta when working with proteinaceous tissues and unacidified or acidified excreta, respectively. For bioapatite, we suggest +9 to 10‰ for Δ¹³C_diet-excreta and +0.001 for ⁸⁷Sr/⁸⁶Sr_diet-excreta. Fur isotopes, collagen δ¹⁵N, and bioapatite δ¹⁸O were unaffected by digestion, but ⁸⁷Sr/⁸⁶Sr decreased by 0.0005 to 0.001, and collagen and bioapatite δ¹³C shifted 0.5 to 1‰ (decreasing collagen-apatite spacing by 1.5 to 2‰). For both birds, Δ_diet-excreta for carbon and strontium were similar to previous studies, but nitrogen was notably different, possibly because excreta contained some urine or urates, or because raptors have distinct digestive physiologies. The influence of digestion on bone δ¹³C and ⁸⁷Sr/⁸⁶Sr is large enough to affect interpretations of diet and bioavailable strontium. Researchers should use caution if relying on potentially digested bone to evaluate the diet of consumed prey, establish strontium baselines, or infer past climate or environmental conditions.