SELECTION ON THE GENETIC AND ENVIRONMENTAL COMPONENTS OF TARSAL GROWTH IN JUVENILE WILLOW TITS (PARUS-MONTANUS)

We studied selection on tarsus length among first year willow tits Par us montanus in relation to environmental and genetical influences on g rowth. The main environmental infleunce on growth was a cohort effect. We also found a substantial heritable component of phenotypic variati on for tarsus length (h2 = 0.61), and crossfostering in one year showe d no shared environment effect which could account for parent/offsprin g resemblance. The deteriorating conditions for growth later in the se ason 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 i mmigrants born outside the study site. Following a season with poor ne stling growth, birds with short tarsi were selected against when under weight, suggesting that growth condition is the target of selection. S uch selection on those individuals which show the strongest environmen tal influence on phenotypic variation will reduce the potential for an evolutionary response to selection.