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.