Latitudinal variation in parental energy expenditure during brood rearing in the great tit

The variation in time and energy allocation of female great tits, Pants maj or, was studied in five different European populations across a latitudinal gradient. Daily energy expenditure (DEE) was measured in fe males tending 12-day-old broods. The number of daylight hours used by the parents to coll ect food for the brood increased with latitude, while DEE and feeding rate per brood tended to level off with latitude. Individual variation in DEE co uld be explained by variation in ambient temperature (-), the duration of a ctivity period (+) and area, but not by brood size, female body mass, brood mass or feeding rate. When the effect of ambient temperature and the durat ion of the activity period on the day of energy expenditure measurements we re controlled for, female DEE still tended to level off with latitude. Temp erature and activity alone can thus not explain the observed pattern. The p resent study suggests that parents at southern latitudes may be under a tim e constraint and do not increase energy expenditure because they have no mo re daylight hours available for foraging, while birds at northern latitudes may be under an energy constraint because they do not make full use of the long daylight period available.