Naef-Daenzer B. & Grüebler M.U. (2008) Post-fledging range use of Great Tit Parus major families in relation to chick body condition. ARDEA 96 (2): 181-190

The timing of breeding and the chicks’ fledging condition are major fitness-relevant traits in the great tit Parus major. Yet, the proximate mechanisms relating these traits to differential survival and recruitment are largely unknown. We analysed the range use of great tit family groups during the first 20 days after fledging, i.e. the period of post-fledging dependence, in relation to the juveniles’ body condition. Radio-telemetry and colour marks were used to track 25 families with 107 chicks. The post-fledging home-ranges of families with fledglings of good condition (average body mass 19 g) were c. 3 times larger than of those with poorly nourished (average 15 g) young. The rate of movements exceeding 50 m per hour was also positively correlated to the chicks' mean fledging mass. We hypothesise that the fledglings’ body condition resulted in large differences in flight performance, and consequently, spatial behaviour of the family groups. The slow movements and the close grouping of chicks of poor body condition likely increase their vulnerability to predation. In the late season, fledglings of poor condition had significantly lower food intake rates compared to those of good condition. We suggest that the relevance to survival and thus, fitness of the fledglings’ physical condition, is via the behavioural and metabolic competence, which in turn may affect flight performance, range use, energy intake rate, and ability to escape predation.