Mourning Doves (Zenaida macroura) are common throughout much of North America and have been extensively studied. Seasonal changes in body mass are largely unreported and have not been examined in relation to replacement of primary flight feathers. We studied changes in body mass in relation to primary molt of doves captured in southeastern Arizona during 2000 through 2012. Body mass for adult males and females averaged 116.3 g (SE = 0.16) and 109.0 g (SE = 0.18), respectively. The distribution of body mass was similar for all months and was normally distributed. Body mass of adult males was lowest in August and September, and highest in November through January. Body mass of adult females was lowest in June through October and increased from October through March. Changes in body mass of adults generally tracked breeding and nesting activities and to a lesser extent timing of primary feather replacement. Body mass of juvenile doves increased linearly with replacement of juvenal primaries from prior to molt through replacement of P 8-10, while body mass of adult doves decreased from prior to molt through replacement of P 8-10. Adult primary feather replacement started in April and was mostly completed by October with some doves still replacing adult primaries well into December. Primary feather molt of hatch-year doves started in April, depending upon when hatching occurred and was mostly completed by December. The relationship between declines in body mass and progress of primary molt in adults is believed to be due primarily to energetic demands of breeding activities, although primary molt may also have a role.