Sexual and seasonal variations in the body weights of Eurasian bullfinches Pyrrhula pyrrhula

Bullfinches were generally heavier in winter than in the summer breeding season. This was evident in the population mean weights, and in the weights of individuals caught at different times of year. Only in the breeding period were females significantly heavier than males. This was due to the body reserves accumulated by females prior to egg-laying. At no other times were any weight differences between the sexes statistically significant. Nor were there any significant weight differences between first-year birds and adults, but in the breeding season juveniles were lighter than both older age groups. In general, weights of individuals during November–February were correlated with ambient temperatures, being greater the colder the weather. Daily body weights were best correlated with temperature on the day before weighing, rather than on the day itself. Temperatures on earlier days modified this relationship, but had a diminishing effect with increase in the number of days involved. The implication was that Bullfinches reacted swiftly to a change in ambient temperature and rapidly adjusted the amount of body reserve they carried. During winter months, Bullfinch body weights were not significantly correlated with day-lengths, but rather temperatures apparently had the biggest influence.