The early bird gets the shrimp: confronting assumptions of isotopic equilibrium and homogeneity in a wild bird population
Article first published online: 25 MAY 2012
© 2012 The Authors. Journal of Animal Ecology © 2012 British Ecological Society
Journal of Animal Ecology
Volume 81, Issue 6, pages 1223–1232, November 2012
How to Cite
Wunder, M. B., Jehl, J. R. and Stricker, C. A. (2012), The early bird gets the shrimp: confronting assumptions of isotopic equilibrium and homogeneity in a wild bird population. Journal of Animal Ecology, 81: 1223–1232. doi: 10.1111/j.1365-2656.2012.01998.x
- Issue published online: 29 OCT 2012
- Article first published online: 25 MAY 2012
- Received 12 March 2011; accepted 5 April 2012 Handling Editor: Stuart Bearhop
- eared grebe;
- isotopic niche;
- trophic diversity
1. Because stable isotope distributions in organic material vary systematically across energy gradients that exist in ecosystems, community and population structures, and in individual physiological systems, isotope values in animal tissues have helped address a broad range of questions in animal ecology. It follows that every tissue sample provides an isotopic profile that can be used to study dietary or movement histories of individual animals. Interpretations of these profiles depend on the assumption that metabolic pools are isotopically well mixed and in equilibrium with dietary resources prior to tissue synthesis, and they extend to the population level by assuming isotope profiles are identically distributed for animals using the same proximal dietary resource. As these assumptions are never fully met, studying structure in the variance of tissue isotope values from wild populations is informative.
2. We studied variation in δ13C, δ15N, δ2H and δ18O data for feathers from a population of eared grebes (Podiceps nigricollis) that migrate to Great Salt Lake each fall to moult feathers. During this time, they cannot fly and feed almost exclusively on superabundant brine shrimp (Artemia franciscana). The ecological simplicity of this situation minimized the usual spatial and trophic complexities often present in natural studies of feather isotope values.
3. Ranges and variances of isotope values for the feathers were larger than those from previously published studies that report feather isotopic variance, but they were bimodally distributed in all isotope dimensions. Isotope values for proximal dietary resources and local surface water show that some of the feathers we assumed to have been grown locally must have been grown before birds reached isotopic equilibrium with local diet or immediately prior to arrival at Great Salt Lake.
4. Our study provides novel insights about resource use strategies in eared grebes during migration. More generally, it demonstrates the utility of studying variance structures and questioning assumptions implicit in the interpretation of stable isotope data from wild animals.