Over an 18-year period (1987–2004), we examined variation in body mass of female Prothonotary Warblers (Protonotaria citrea) captured throughout their nesting cycle. As is typical for many small passerine birds, body mass was greatest during egg laying and decreased throughout incubation and feeding of young. Mass decreased significantly between the onset of incubation and fledging of both first and second broods. Mass loss was gradual during incubation, noteworthy during the first 2 days of feeding nestlings, but did not continue to decrease throughout the feeding period. Mass lost while raising the first brood was regained before initiating the second brood. Mass of female warblers, adjusted for effects of nest attempt, year, clutch size, and day and stage of nesting, increased slightly with age. Body mass of nesting female warblers varied significantly with day of the nest cycle during incubation but not during egg laying or feeding of young. Mass was associated with clutch size during incubation in both first and second broods, but was not associated significantly with brood size when females were feeding nestlings. Frequency of food delivery to nestlings was associated negatively with female body mass. Females typically made more feeding trips per day than males. Feeding rates were correlated among pairs; that is, females with higher rates of delivery were mated to males that made a higher number of trips.