Prey diversity is affected by climate and differs between age classes in the Red-backed Shrike (Lanius collurio)

Abstract

When breeding, food availability is essential for optimal reproductive output and is po-tentially one of the main factors limiting breeding success, especially in single brooded long-distance migratory birds. In this study, we examined the diet (as a measure of prey availability) of two Red-backed Shrike (Lanius collurio) populations in Denmark, based on more than 11,000 prey items covering seven years. We found a negative correlation be-tween prey diversity and temperature, indicating that Red-backed Shrikes feed on pre-ferred prey items in warmer summers (low diversity) while forced to feed on a larger vari-ety of species in colder summers. Adults had a more diverse diet and generally fed on smaller prey items than did young birds. Thus, age-and environment-related differences must be taken into account when describing the diet of the Red-backed Shrike. Direct nest observations produced different results for diet composition than did nest and pellet samples, underlining the importance of using different methods in diet assessments. De-tailed knowledge on limiting factors on the breeding grounds, such as food availability, is crucial for mitigating population declines of vulnerable species, such as the Red-backed Shrike.