The Mariana Swiftlet (Aerodramus bartschi) on Saipan lays a single white egg which is incubated for 22.95 days (range 17–30 days). Newly hatched nestlings are naked and weigh 1.11 g (range 1.0– 1.2 g). Nestlings grow slowly, reaching asymptotic weight on day 29 and fledging after 46.8 days (range 40– 55 days). Post-asymptotic weight recession is ±2% and nestlings fledge at slightly above adult weight of 8.01 g. Wing and tail length are >94% of adult size at fledging. Low clutch size, slow chick growth, and extended nestling period are characteristic of other species of swiftlets and may represent food limitation in these diminutive aerial insectivores.