The ability to accurately determine age of nestlings is useful in studies of reproductive performance of small birds. Because of a resurgence of interest in nest-box studies of reproductive performance of Prothonotary Warblers (Protonotaria citrea), we closely monitored the growth and development of 766 nestlings from 190 nests over the 1999 and 2000 breeding seasons. Distinguishing characteristics of known-age warbler nestlings are provided, including mass measurements, aspects of plumage development, and behavioral notes in sufficient detail that investigators can determine the age of young Prothonotary Warblers with only one or two visits to a given nest.