Diet composition influences annual breeding success of Montagu's Harriers Circus pygargus feeding on diverse prey

The aim was to evaluate whether breeding density, age structure and breeding success of Montagu's Harriers were related to variations in diet composition. Breeding data were collected over an eight-year period (1991–98) in Madrid, central Spain. Diet was evaluated through pellets and remains collected near nests. Diet diversity was calculated using Shannon's index. Diet diversity was negatively correlated to the proportion of hares Lepus granatensis in the diet, positively to the proportion of insects, and not significantly to other prey types. Breeding parameters varied significantly between years, although annual variation in breeding parameters in Madrid was lower than in populations where diet is less diverse. Nest failure was the most important variable explaining variation in productivity, and was primarily caused by food-related factors (abandonment and starvation). Nest failure was less frequent in years where diet diversity was lower. Hares appear to be the preferred prey for Montagu's Harriers in Madrid, and other prey were used mainly when hares were less abundant or less available (with insects being the main alternative prey). Years when diet was less diverse (i.e. when hares were dominant in the diet) were associated with a higher productivity, and with a lower between-pair variation in this parameter