Changing food availability and its effect on the heritability of offspring size in woodland passerine birds.

Climate warming has been associated with widespread body size declines in many vertebrate taxa, but relatively little is known about possible climate warming induced shifts in trait heritabilities. The main goal of the study was to investigate how changing food availability affects evolutionary potential of four traits related to nestlings' body size. We used long-term, pedigree structured data of two woodland passerines living in the boreal zone, the Willow Tit (Poecile montanus) and the Great Tit (Parus major), to study how food availability for their nestlings has changed in time, how this has influenced their morphological traits (viz. wing, tail & tarsus length & body mass) and their heritabilities and evolvabilities. This was done by assessing heritabilities under varying food availabilities using random regression animal models. We found that caterpillar food availability had increased over the 25-year-long study period and that this was accompanied by increases of nestlings' body mass, but not other morphological traits. All traits were heritable in both species, but additive genetic variance, heritability and evolvability were affected by food availability only in the case of the wing length, being higher under low food availability (the Great Tit) or higher under low and high food availability (the Willow Tit). We conclude that changes in food availability seem to have limited influence on evolutionary potential of body size traits in these two passerine birds.