Food availability affects diurnal nest predation and adult antipredator behaviour in song sparrows, Melospiza melodia

Food and predators are thought to have independent effects on avian reproductive success. Several studies have now shown that extra food can reduce nest predation rates. Most hypotheses suggest that this is due to changes in adult and/or nestling behaviour. A critical test as to whether behaviour is key, is to examine when predation events occur for birds with and without extra food. If behaviour is important, extra food should lead to fewer nest predation events during the day, when birds are active, but have no effect at night. We conducted a food supplementation experiment on song sparrows and tested for food effects on timing of nest predation events, adult antipredator behaviour and nestling begging behaviour. Food supplementation significantly reduced the relative frequency of daytime nest predation especially during incubation. During incubation, fed females had shorter foraging bouts and longer bouts on the nest, and these behaviours were associated with a reduced probability of being preyed upon. During brooding, we found trends (P < 0.1) towards lower daytime predation and greater adult nest attendance for fed birds than for unfed birds, but nestling begging did not differ with food treatment. Our results show that food availability does influence nest predation through its effect on antipredator behaviour. We highlight the importance of examining adult behaviour and vulnerability to predation at both stages of the nesting period because behavioural changes with extra food were extreme at the egg stage and were more effective at reducing nest predation than were behavioural changes at the nestling stage.