Does high social rank position affect winter survival of Willow Tits Poecile montanus negatively under prolonged conditions of extremely low ambient temperature?

Abstract

January 2010 was the coldest for more than 30 years in south Norway, with ambient temperatures in parts of central Norway between -30 and -40 oC. The January survival rates of individually colour-ringed Willow Tits Poecile montanus were studied by comparing the numbers of survivors from 10 flocks under the extremely low ambient temperatures in 2010 with that of 8-9 flocks under milder conditions in the two previous years. All flocks consisted of an adult mated pair and two to four unrelated juveniles, and in the flock hierarchy males generally dominated the females and the adults dominated the juveniles of their own sex. As expected, relatively fewer low-ranked juveniles survived January 2010 than in the previous years (0-20%, mean 13% vs 0-89%, mean 56%). But also the January survival rate of the top ranked males, the alpha birds, of the flocks in 2010 was lower (40%) than that during the two previous winters (88% and 89%). Because the top-ranked adult males have about 10% higher day-time oxygen consumption rates than the lower ranking birds in the same flock, the energetic requirements are higher for the alpha males than for the other flock members. This extra energy demand may have negatively influenced their survival during the long extremely cold period.