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

When breeding, food availability is essential for optimal reproductive output and is potentially 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 onmore than 11,000 prey items covering seven years.We found a negative correlation between prey diversity and temperature, indicating that Red-backed Shrikes feed on preferred prey items in warmer summers (low diversity)while forced to feed on a larger variety 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 theRed-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. Detailed 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.