This study documents patterns of primary feather molt and body mass changes in Least Sandpipers (Calidris minutilla) and Western Sandpipers (C. mauri) in Ensenada de La Paz, México. Both species use the same habitats and complete an uninterrupted primary feather molt between late July and October. Average timing of Least Sandpiper primary molt was from 29 July to 21 September (duration of 54 days), while Western Sandpipers molted from 29 July to 29 September (duration of 62 days). Molt models of individual primaries suggested that the molt of Least Sandpipers starts after all individuals have arrived in the study area, whereas the molt of Western Sandpiper starts before all individuals have arrived; the heavier Western Sandpiper tends to grow more feathers simultaneously than the Least Sandpiper. While Western Sandpipers are heavier than Least Sandpipers at the start of molt, both species decrease their body mass over the molt period. Although Least and Western sandpipers have similar natural histories, they show different primary molt patterns in Baja California Sur.