Winter Fattening Strategies of Two Passerine Species: Environmental and Social Influences

We studied the fattening strategies of two insectivorous passerines, the Robin Erithacus rubecula and the Wren Troglodytes troglodytes, during their wintering stage in a study area of the eastern Po valley (Northern Italy). We analysed the relationship between some environmental (temperature, photoperiod, rainfall) and social factors (density of migrant competitors, territoriality, home range size) on the one hand, and levels of fat reserve on the other. Although average body fat depots differed between the two species, both increased their daily fat stores as the photoperiod shortened with the progress of winter. The amount of fat accumulated by day was more than that required for overnight consumption. Both species also responded to short-term variations in weather conditions: fat depots were inversely related to the maximum temperature on the day of capture. The use of space and the social structure of the wintering populations seem to affect fattening strategies: fat reserves of resident Robins were negatively related to the density of conspecific migrants, early in the season. The same relationship was not observed for Wrens, which were dispersed in extensively overlapping home-ranges. In this species, the home-range size was positively, and fat reserves negatively, correlated with the body size of the bird.