A method of stable carbon and nitrogen isotope analysis in assessment of the diet of birds of prey

Abstract

Currently, most studies on the feeding of birds of prey are based on analysis of food remains in their pel� lets. This approach has a number of advantages, and the majority of researchers consider that the results it produces are sufficiently reliable for evaluating the qualitative composition of food objects consumed by these birds (Shepel', 2008). Moreover, this method is also used to evaluate the quantitative composition of their diet (Osmolovskaya, 1948; Kalyakin, 1989). However, there are problems associated with such a quantitative analysis. First, the diet of these birds may include prey of different sizes. Smallsized prey spe� cies are usually swallowed as a whole, and the numer� ical ratio of their skulls fairly accurately reflects the diet composition. It is also possible to reconstruct the size and weight of some prey objects from skull mea� surements (Pagels and Blem, 1984; Shepel' and Shokhrin, 1985) and, hence, to improve the accuracy of quantitative analysis by taking these data into account. If prey is relatively large, the bird tears it apart and often consumes only soft tissues, which are com� pletely digested. Therefore, identification of prey by its remains is impossible (this is especially true of the diet of chicks). Even when the remains of largesized prey do occur in pellets, it is very difficult to recon� struct its size and weight. Second, it is practically impossible to determine the time of regurgitation, while collection of only fresh pellets means that 80- 90% of the material is excluded from analysis (Shepel', 2008). An alternative method for studying the diet of birds of prey, based on analysis of stable carbon and nitrogen isotopes, provides the possibility of much more accurate quantitative assessment of its composi� tion. Various applications of stable isotope analysis in ecological research are discussed in a number of pub� lications. A detailed description of this method, with emphasis on its use in soil ecological study, is given in the review by Tiunov (2007). Here, its advantages and drawbacks are considered as applied to assessment of the diet of birds of prey.