This thesis concerns with the relationship between genetic and environmental attributes and their influence on offspring growth and survival in willow tits (Parus montanus). It also investigates the occurrence of directional or stabilising selection on body size during different life stages or times of the year. Since passerines exhibit determinate growth, early development is crucial to future survival, and tits have only about two weeks to finish their skeletal growth. Final body size may thus be sensible for environmental variation during growth, which may influence further survival or fitness. Selection forces may act differently during different life stages or seasons.
I found that there was a substantial heritability (h2) in tarsus length for willow tits, which varied between 0.46-0.89 during different growth conditions. The lower h2 was caused by a higher environmental variation (VE) during unfavourable growth conditions. It was mainly offspring from large parents that did not grow as well as their genetic potential allowed and became smaller than expected, and they were also undernourished. In years with unfavourable growth conditions there was selection against small juveniles in the autumn, which probably reflects the bad condition in birds with a poor growth. Selection on juvenile tarsus length in autumn did not lead to a change in the population mean, partly because h2on tarsus length was low in years with selection.
Temperature was the most important environmental factor, explaining 33% of nestling non-genetic growth. Temperature both influences nestling thermo-regulation and their food availability. However, in an experiment where nestlings were provided additional food, growth of feathers was enhanced with additional food. I suggest that phenotypic plasticity in feather growth may be a life history character, because of willow tit's social system during winter.
Adults survived winter almost twice as good as juveniles. Among juvenile females, those surviving winters had smaller wing length than non-survivors. None of the other body characteristics were linked to survival within sex and age categories. Juvenile females are the lowest ranked individuals in willow tit winter flocks and may experience limited access to food or be forced by more dominant birds to the outer reaches of trees where predation risk is higher. A shorter wing may increase manoeuvrability, which would be advantageous, both when foraging along outer reaches of trees and in escaping predator attacks.
Stockholm: Department of Zoology, Stockholm University , 1999. , 31 p.