Selection on the genetical and environmental components of tarsal growth in juvenile willow tits (Parus montanus)

Abstract

AbstractWe studied selection on tarsus length among first year willow tits Parus montanus in relation to environmental and genetical influences on growth. The main environmental influence on growth was a cohort effect. We also found a substantial heritable component of phenotypic variation for tarsus length (h2 = 0.61), and crossfostering in one year showed no shared environment effect which could account for parent/offspring resemblance. The deteriorating conditions for growth later in the season did not confound our heritability estimates as the time of laying was not correlated to parent size, and no maternal effects operating through egg size were found. We tested for selection during the summer dispersal phase by comparing tarsus length among fullgrown pulli (age 14 days, controlled by repeated measurements of the same individuals later during breeding and the ensuing winter) and the tarsus length of the first year cohort in autumn composed of a mixture of locally born birds remaining within our study area after the dispersal phase and immigrants born outside the study site. Following a season with poor nestling growth, birds with short tarsi were selected against when underweight, suggesting that growth condition is the target of selection. Such selection on those individuals which show the strongest environmental influence on phenotypic variation will reduce the potential for an evolutionary response to selection.