A shows limiting factor analysis for Poecile carolinensis. Areas northeast of the hybrid zone limited primarily by annual mean temperature (dark blue) and mean temperature of the warmest quarter (dark green). Areas west of P. carolinensis limited primarily by precipitation of driest quarter (yellow). B shows limiting factor analysis for Poecile atricapillus. Areas along much of the contact zone itself limited by maximum temperature of the warmest month (light green). Areas south of the hybrid zone limited primarily by annual mean temperature (dark blue). Northern edge of range limited by minimum temperature of coldest month (light gray). Note that some variables are only limiting for one species.