Rationale: We investigated isotopic diet-excreta offset (D_diet-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 D_diet-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 Δ¹³C_diet-excreta and +0.5-1‰ for Δ¹⁵N_diet-excreta when working with proteinaceous tissues and unacidified or acidified excreta, respectively. For bioapatite, we suggest 9-10‰ for Δ¹³C_diet-excreta and 0.001 for ⁸⁷Sr/⁸⁶Sr_diet-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, D_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 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.

This dataset summarizes raw isotope data for rat tissues before and after digestion by Rhett, the red-tailed hawk, or Caspian, the eagle owl, who were individually housed at the Cincinnati zoo. We have also included isotope data for excreta produced by the birds. The study was conducted for a total of 11 days. A foot from each consumed rat was collected before feeding the birds, and then a regurgitated casting and excreta sample were collected from each bird's enclosure the following day. 

Rat collagen was isolated by soaking samples in 0.5 N HCl at 4°C ultil they were demineralized, rinsing in ultrapure water five times. Lipids were removed from rat muscle, hair and bone by sonicating samples in trace metal grade petroleum ether 2 to 5 times, then sonicating in ultra pure water 2 to 5 times, and drying.

Rat bioapatite was processed by soaking powdered sample in 30% H2O2 for 72 hours, rinsing with ultrapure water five times, soaking in 1 M Acetic Acid buffered with calcium acetate, and then rinsing again with ultra pure water five times, and drying. 

Excreta were dried and homogenized. We also acidified a subset of samples to remove potential minerals. 

Stable isotope values were obtained at the Stable Isotope Biogeochemistry Lab at the University of Cincinnati and strontium isotope data were obtained at the Multicollector ICPMS Laboratory in the Department of Geology at the University of Illinois, Urbana-Champaign.