Body reserves and social dominance in the Great Tit Parus major in relation to winter weather in southwest Ireland

We studied the interactions between;weather, social dominance and body condition (fat and protein reserves) In a colour-ringed population of the Great Tit in the Killarney National Park, Rep. of Ireland in two mild winters (1990 and 1992) differing markedly in rainfall. Rainfall, but not temperature, was a strong predictor of mean daily fat levels (birds were fatter on wetter days) but did not predict muscle score. The birds were also fatter in the wet winter than the dry one, and this was a greater difference than predicted by daily rainfall alone. The effects of rainfall are considered in the context of ultimate and proximate cues to fattening. In contrast, muscle scores were greater in the dry year than the wet. On average, birds known to have survived from the 1990 to the 1992 winter had higher muscle scores, irrespective of their age or sex, in 1990 than those not seen again. In the wet winter, muscle score (but not fat) was positively correlated with a measure of individual dominance; in the dry winter fat was negatively related to dominance, and no relation was found between muscle score and dominance. These results indicate that individual differences in condition reflect differences in individual status, and that experienced environmental conditions strongly influence the relationship between social status and body reserves. This is the first field study of Great Tits relating condition and dominance among individuals rather than among status classes. We suggest that, under some circumstances, muscle may be a better indicator of condition than fat.