In Viêtnam, most White‐nest Swiftlet Collocalia fuciphaga colonies (80% of the colonies in Da Nang and 58% in Khanh Hoa provinces), and all large ones, were found in caves with wide openings where the sea entered far inside and waves may wash away numbers of nests. In dry caves above sea level, air with a relative humidity below 70% may cause many nests to fall from the walls. Breeding success was strongly influenced by nest harvesting and was especially low in the third and fourth clutches. The best harvesting method is to collect the nests twice a year, and the first harvest should occur only when the first 10–15% of the nests have eggs. The second harvest should be made when second clutch fledglings have left the nest. The later the first harvest occurs, the lower are the ability of the swiftlets to rebuild second nests and the productivity of second clutches.