Stable isotopes of carbon and nitrogen can provide powerful tools for estimating the trophic positions of animals and determining the source or the primary producer of a food web. I used stable isotopes analysis of carbon (δ13C) and nitrogen (δI5N) to investigate the trophic position of burrowing owls (Athene cunicularia) in agricultural and natural habitats and trophic relationships of a community of vertebrate predators in the Morley Nelson Snake River Birds of Prey National Conservation Area (NCA), located in southern Idaho.
Burrowing owl populations have declined across much of North America owing to loss of habitat. However, burrowing owls show affinity for nesting near agriculture in some portions of their range, including s. Idaho. I used analysis of 13C and ‘$N to investigate burrowing owl food habits and trophic relationships in agricultural and natural habitats in the NCA. δ13C did not differ between natural and agricultural habitats and indicated carbon sources in burrowing owl diet contained primarily C3 plants. Conversely, δ13C differed between nestling and adult owls, which may indicate that adults provisioned nestlings with a different diet than they consumed. Burrowing owl δI5N values depended on both habitat (i.e., natural or agricultural) and group (i.e., samples from 20 day old juveniles. 30 day old juveniles, adult females or adult males), although owls nesting in natural habitat generally had higher δI5N values than owls nesting in agricultural habitat Owls in natural habitat potentially fed on more kangaroo rats (Dipodomys ordii), scorpions (Hadrurus spadix) and spiders (Infraorder Mygalomorphae) and fewer montane voles (Microtus montanus) and crickets (Gryllus spp.), which may help explain elevated δ1SN values for owls nesting in natural habitat.
My results corroborated Moulton et al. (2005, 2006), who used traditional food habits analysis and found that burrowing owls nesting in natural and agricultural habitats feed on different prey species in each habitat. As adults in natural areas had higher δI5N values, this may be further evidence that adult owls consumed different prey than they used to provision nestlings. Food webs, of which burrowing owls are a part, for both natural and agricultural habitats were similar despite the introduction of irrigated agriculture into a naturally arid landscape.
I also examined trophic relationships of a community of vertebrate predators in the same area. The NCA has a rich diversity of predators, including sixteen raptor species and an array of mammalian predators. It presents a unique opportunity to examine trophic ecology of predators that may use the same prey resources. I compared my results from analysis of I3C and I5N with results from traditional food habit study methods from Marti et al. (1993). I collected 272 samples from 14 species of vertebrate predator. Predators had a relatively narrow range of δI5N with only 2% separating the majority of the species; therefore, the vertebrate predators that I examined occupied a similar trophic position. The food web in the NCA is based on a combination of C3 and C4 plants and illustrates that a mixture of plant species is supporting a community structure of herbivores, omnivores, and predators, rather than a particular species of shrub, forbs, grass, or crop plant. My findings were consistent with the results from Marti et al. (1993), who found, when prey were identified to the class level, mean dietary overlap among vertebrate predators was 82%. As in Marti et al. (1993), results based on stable isotopes analysis indicated that most species clustered into four principal groups, while two species (coyotes, Canis latrans and great homed owls, Bubo virginianus) were sufficiently dissimilar and were excluded from other groups. By pairing stable isotope technology with traditional food habit study methods, my study provides a more complete view of trophic relationships among vertebrate predators.