Coal Tit fat-storing patterns during the non-breeding season: the role of residence status

According to the optimal body mass hypothesis, resident individuals, having priority of resource access and better knowledge of food availability in an area, should carry smaller fat reserves than transients whose resource predictability is lower. We tested this prediction in a free-living population of Coal Tits Parus ater in sub-alpine coniferous forest during three winters. We examined the role of residence status in determining the daily patterns of fat accumulation of individuals using time of day, temperature, season, year and body size as covariates. Only time of day, residence status and temperature significantly affected daily variation of fat score in Coal Tits. Fat scores were hi.-her in transients than residents, although within residents, juveniles did not carry significantly more fat than adults. Our results show that fattening strategies are associated with residence status in the Coal Tit, in support of the hypothesis that transient individuals facing lower food predictability carry larger body fat reserves. The finding that within residents, patterns of fat accumulation did not differ between adults and juveniles, suggests that residence per se, involving higher food predictability, rather than age-related dominance or hoarding behaviour, determines fattening strategies in wild Coal Tits.