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Dryad

Details and raw isotope data for samples related to a semi-controlled feeding study involving rats fed to a red-tailed hawk (Buteo jamaicensis) and Eurasian eagle owl (Bubo bubo).

Data files

Apr 16, 2025 version files 44.55 KB

Abstract

Rationale: We investigated isotopic diet-excreta offset (Ddiet-excreta) for predatory birds, and the isotopic influence of bird digestion on consumed prey tissues. The foraging ecology of predatory birds can be non-invasively monitored using excreta or regurgitated prey. However, one must account for Ddiet-excreta and any influence of digestion on prey tissues. Neither of these has been previously evaluated for predatory birds.

Methods: We worked with a captive Eurasian eagle owl (Bubo bubo) and red-tailed hawk (Buteo jamaicensis) fed frozen rats. We collected rat feet, as well as regurgitated pellets and excreta from each bird’s enclosure. We analyzed carbon (d13C) and nitrogen (d15N) isotopes in undigested rat muscle, undigested and digested fur and bone collagen (extracted from pellets); d13C, oxygen (d18O), and strontium (87Sr/86Sr) isotopes in rat bone bioapatite; and d13C, d15N and 87Sr/86Sr in bird excreta.

Results: Diet-excreta offset differed slightly between individuals, and depended on how we estimated diet (muscle or muscle + collagen), and if excreta were acidified. We tentatively suggest using +1-1.5‰ for Δ13Cdiet-excreta and +0.5-1‰ for Δ15Ndiet-excreta when working with proteinaceous tissues and unacidified or acidified excreta, respectively. For bioapatite, we suggest 9-10‰ for Δ13Cdiet-excreta and 0.001 for 87Sr/86Srdiet-excreta. Fur isotopes, collagen d15N, and bioapatite d18O were unaffected by digestion, but 87Sr/86Sr decreased by 0.0005-0.001, and collagen and bioapatite d13C shifted 0.5-1‰ (decreasing collagen-apatite spacing by 1.5-2‰).

Conclusions: For both birds, Ddiet-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 d13C and 87Sr/86Sr is large enough to affect interpretations of diet and bioavailable strontium. Researchers should use caution when using potentially digested bone to evaluate the diet of consumed prey, establish strontium baselines, or infer past climate or environmental conditions.