Food Availability as a Determinant of Pairing Behaviour in the European Robin

Abstract

1. Male and female European robins, Erithacus rubecula, defended separate territories from August to January. Pairs formed between January and March. 2. The benefits of early pairing were twofold: first, as 20% of males failed to pair there was strong competition for females; second, although pairing was initially costly in terms of individual foraging success, bachelors invested more time advertising in the long term. 3. Birds on individual territories encountered large food items more frequently than those on pair territories. This decrease in foraging success within pairs was identified as a cost of territory sharing. 4. To test the hypothesis that food availability underlies the defence of individual territories in winter, and the timing of switches to pair territoriality, I manipulated food supply and recorded individual behaviour. 5. Compared to controls, males provided with extra food were forced to repel intruders more frequently, yet advertised for mates earlier and paired earlier. 6. Pair members whose supplementary food supply was temporarily withdrawn spent less time consorting than controls. This suggests that an elevated food supply helped individuals to afford the costs of sharing their food resources, and thus to pair and lay clutches early. 7. The influence of food supply and territoriality on population density and social behaviour is discussed.