Interactive effects of food and age on breeding in the Montagu's Harrier Circus pygargus

We evaluated the direct and interactive effects of food and age on reproduction in the Montagu's Harrier Circus pygargus , to test whether variation in food supply was likely to affect age-specific breeding probability or success. Younger females were more frequently non-breeders than older females. When breeding, older females laid earlier, produced larger clutches, failed less often and had higher number of fledglings than younger females. Probability of breeding was higher, laying was earlier, and clutch size and number of fledglings per pair increased with increasing food abundance. A significant interaction between food and age was observed in both breeding probability and breeding performance: older females were more likely to breed than younger females when food abundance was low, and younger females performed less well in good food conditions than older females. Overall, differences between age groups were most marked in extreme food conditions, regardless of the quality of the conditions.