Diet sources of the endangered Attwater's prairie-chicken in Texas: evidence from ¹³C, ¹⁵N, and Bayesian mixing models

The Attwater's prairie-chicken (APC; Tympanuchus cupido attwateri) was listed as an endangered species in 1967, and since then multiple studies have addressed ecological and human factors that may have limited their population recovery. In this study, we used stable isotopes of carbon and nitrogen (delta C-13 and delta N-15) in conjunction with Bayesian mixing models to determine current diets of APCs at the Attwater Prairie Chicken National Wildlife Refuge (APCNWR), and to compare them with past diets derived from the isotopic composition of feathers from museum specimens (1894-1965) to help elucidate potential factors limiting APC recovery. We collected feathers, blood, and feces from APCs at the APCNWR, and feathers from museum specimens. In addition, potential food items (vegetation, spiders, and insects) were collected from APCNWR and analyzed for delta C-13 and delta N-15. A stable isotope mixing model (MixSIAR) was used to determine diet source contribution to past and present APC populations. delta C-13 values in APC blood were significantly greater in the fall than in the summer. Blood delta N-15 values were significantly greater in the summer than in the winter. delta C-13 values in feathers from museum and contemporary APCs were not different, although delta C-13 values in contemporary feathers were enriched by 0.5 parts per thousand relative to museum specimens. Results indicated that insects were the predominant food source in the summer and fall (61-65%) and C-3 vegetation was predominant in the winter (64%). Using isotope values from feathers, the model predicted that the predominant food source during the period of feather growth were insects (62%) followed by C-3 plants (17-18%), spiders (12-18%), and C-4 plants (2-3%). Feathers from contemporary birds had delta N-15 values 0.9 parts per thousand lower than those from museum specimens, suggesting potential shifts in diets in APCs currently in the wild relative to the past. We hypothesize that the abundance and species richness of various arthropods may have changed at the APCNWR resulting in current APCs in the wild feeding on arthropods with lower delta N-15 values than in the past. Orthopterans comprised about 23% of the APCs diet currently in the wild, while they represented only about 19% in the past.