Little emphasis has been placed on the influence of season and age on morphological measurements of swifts, which makes comparative mensural studies difficult in the Apodidae. This study provides information about the importance of morphological variation in measurements to the biology of the Biscutate Swift (Streptoprocne biscutata). I studied individuals captured at four colonies in southern Brazil. Mass varied with age, time of day, and season. Birds captured at dawn weighed 115.5 ± 9.9 g (n  =  1,320). Mass was greater in fall and spring and lower in summer and winter. Subadults weighed less than adults during summer, fall, and winter. The wing measured 206.3 ± 5.2 mm (n  =  812), and the tail measured 69.9 ± 4.6 mm (n  =  802); both showed seasonal variation in length. The tarsus measured 25.9 ± 0.6 mm (n  =  695), and the exposed culmen measured 9.94 ± 0.36 mm (n  =  658). The Biscutate Swift has significant seasonal variation in several morphological measurements. This variation should be considered in studies comparing populations of this species, as well as those of other apodids.