Occasional influx of lean and aggressive Great Tits Parus major during autumn migration at Utklippan

Abstract

On October 13^th and 14^th, 2014 there was an influx of Great Tits Parus major to the island of Utklippan, 15 km off the coast of south-eastern Sweden. During these two days, 158 Great Tits were trapped, ringed and weighed. Most of them were very light; 90.4% of the males and 96.5% of the females had a body mass below the estimated lean body mass (males = 17.3 g, females = 16.3 g). The mean body mass during the influx days was 1.5 g (males) and 1.56 g (females) below these lean body mass values. The birds had probably left the eastern coast of the Baltic Sea the day before and then met unfavourable weather with rain and strong head-winds. Probably they had been disoriented and forced to fly for at least 8 hours or even up to 20 hours over open sea before reaching Utklippan. Many of the Great Tits were very aggressive and they attacked other species and killed them by pecking at the head to eat the brain of the victims.