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 years 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.