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

## TL;DR

This study examines how digestion in predatory birds affects isotopic signatures in their excreta and prey remains, providing correction values for more accurate diet and environmental reconstructions.

## Contribution

The paper provides the first evaluation of isotopic diet-excreta offsets and digestion effects in predatory birds, offering tentative correction values for isotopic analysis.

## Key findings

- Diet-excreta isotopic offsets vary slightly between individual birds and depend on tissue type and acidification of excreta.
- Digestion significantly affects bone δ13C and 87Sr/86Sr isotopes, which could impact diet and environmental reconstructions.
- Researchers should use caution when using digested bone to infer diet or environmental conditions due to significant isotopic shifts.

## Abstract

We investigated isotopic diet‐excreta offset (Δdiet‐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 Δdiet‐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 (
Bubo bubo
) and red‐tailed hawk (
Buteo jamaicensis
) 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 Δ13Cdiet‐excreta and +0.5 to 1‰ for Δ15Ndiet‐excreta when working with proteinaceous tissues and unacidified or acidified excreta, respectively. For bioapatite, we suggest +9 to 10‰ for Δ13Cdiet‐excreta and +0.001 for 87Sr/86Srdiet‐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, Δdiet‐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.

Isotope analysis of predator feces and consumed prey remains can be used to non‐invasively track diet and reconstruct past environment and climate conditions. However, one must account for the isotopic difference between diet and feces, as well as the possible influence of digestion on consumed prey tissues. We investigated the influence of digestion by a red‐tailed hawk (Rhett) and an eagle owl (Caspian) on murid rat tissues. We also investigated the isotopic offset between diet and feces for the two birds. We provide tentative correction values that can be used to account for diet‐feces offset for birds of prey and suggest researchers be cautious of using potentially digested bone to evaluate the diet of consumed prey or infer past climate or environmental conditions.