Postnatal dispersal and drivers of successful recruitment in resident <i>Poecile gambeli</i> (Mountain Chickadee)

Abstract Many animals disperse from their natal sites as juveniles to settle in new locations where they may eventually breed. Estimating distances of such postnatal dispersal within and across populations, as well as identifying factors affecting recruitment success, is important for understanding the evolutionary consequences of dispersal. We investigated patterns of postnatal dispersal and identified predictors of successful recruitment in highly resident Poecile gambeli (Mountain Chickadee) using data on 326 recruits of 5,226 total fledglings detected at winter feeders (recruitment into winter flocks) and nest boxes (recruitment into the breeding population) over 12 years at 2 elevational sites in the northern Sierra Nevada, USA. Like most Parids, chickadees dispersed close to their natal sites (median distance: 644 m). Dispersal distance was not associated with fledgling mass, but females dispersed significantly longer distances than males. When only considering dispersal distances based on recaptures at nest boxes, birds that fledged earlier dispersed significantly shorter distances. Successful recruitment both into winter flocks and into the breeding population was associated with higher fledgling mass and earlier fledging. Over the study period, only 13 birds were detected dispersing from one elevational site to the other, and 12 of these birds hatched at low elevation and dispersed to the high-elevation site. Our results suggest that earlier fledging timing and higher fledgling mass are both critical for successful recruitment in P. gambeli and confirm that fledgling mass is a key measure of individual quality in which even small differences in mass (~2%–3%) may have important implications for recruitment and, therefore, fitness.