Monotypic. Plumage varies, non-breeding (SEP-MAR) or year-round and breeding (APR-AUG). Interestingly, breeding is duller while non-breeding has yellow and/or olive coloration. M/F differ, with female duller than male but drawings on BOW indicate female in basic may have some yellow tinging male lacks. BOW also notes "Determining sex of fall adults is difficult in the field, but males average less yellow", so perhaps best to avoid for non-breeding.