Daily energy expenditure during brood rearing of Great Tits Parus major in northern Finland

We present data on the natural variation in daily energy expenditure (DEE, measured by the doubly labelled water technique) of individual female Great Tits Parus major tending their brood near the northern border of the species' distribution. We expected that longer foraging days would result in a higher energy expenditure. Female DEE, visitation rate and the duration of the active period were measured at the nestling age of 12 days. There was a positive relationship between the duration of activity period and brood size. Female DEF was positively associated with the duration of the activity period and the number of young in the nest. Energy expenditure was not related to temperature or feeding rates. Despite the longer foraging day and lower ambient temperatures, female DEE did not differ from the values obtained in central Europe. This result suggests that there are differences in foraging cost and/or mode with latitude.