IS WHAT COMES OUT THE SAME AS WHAT GOES IN? FIRST STEPS TOWARDS UNDERSTANDING THE ISOTOPIC INFLUENCE OF DIGESTION BY PREDATORY BIRDS

Fecal isotopes track an individual’s diet or habitat use but may be offset from consumed food because feces include gut lining and microbes in addition to undigested food. Just one study has previously investigated this offset for birds. Because feces break down quickly, and most birds have dilute excreta, they may not be well-suited for monitoring diet or habitat use for birds in the wild. Analyzing consumed and regurgitated prey is an alternative approach. Prey remains persist on the landscape much longer, and accumulated remains can provide an archive of predator behavioral trends, as well as regional climate and environmental conditions. However, if regurgitated tissues like bones and teeth are isotopically altered during digestion, this could affect data interpretations and conclusions. This has also not been previously investigated.

To address these unknowns, we conducted a feeding study with one captive red-tailed hawk (Buteo jamaicensis) and one Eurasian eagle owl (Bubo bubo). We collected bird fecal matter as well as consumed murine rodent tissues before and after digestion for 11 days. We analyzed carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes for feces, bone collagen, fur, and muscle, and δ¹³C and oxygen isotopes (δ¹⁸O) for bone apatite. Bird feces had lower δ¹³C values than protein-rich tissues, but trends in δ¹⁵N differed between birds. Fecal δ¹⁵N was significantly higher than most prey tissues for the owl, but lower for the hawk. Feces also tended to be more isotopically variable than prey tissues. Prey muscle, which accounts for most digested food, had lower δ¹³C and higher δ¹⁵N values than both pre- and post-digested collagen and fur. Fecal δ¹³C was ~1‰ lower than prey muscle for the owl but nearly identical for the hawk. Conversely, fecal δ¹⁵N was comparable to prey muscle for the owl but 1.5‰ higher for the hawk. This may be because hawks have more acidic crops and digest more bone than owls. Following digestion, prey collagen δ¹³C was ~1.5‰ higher for both birds, and apatite δ¹³C was ~1‰ lower for the owl. This may reflect leaching of lipids and some proteins from consumed bone. Digestion had no effect on fur δ¹³C or δ¹⁵N or apatite δ¹⁸O values. Researchers should be aware of these offsets when using feces or digested prey remains to interpret diet, habitat, and environment. Shifts of 1-2‰ may hinder some applications.