Using data on the extent of prebasic molt in known-aged adult and first-winter Western Bluebirds (Sialia mexicana), we assessed the accuracy of the molt limit as a tool for ageing birds. Sixty-nine known-aged birds were examined over the winter 2002–2003 in upper Carmel Valley, California. We also examined 29 breeding males in spring 2004 to assess the accuracy of this method for ageing birds during the breeding season. All birds were aged correctly using molt limit as the defining characteristic. We found that all first-winter birds had replaced 2–8 greater secondary coverts, while adults had replaced all wing coverts. We observed no significant sexual variation in the extent of first prebasic molt. These results indicate that the extent of prebasic molt is a highly reliable means of ageing birds in this population.