Dr Johathan Cheah Weng Kwong was tern-watching in August 2006, making observations on a breeding colony of Little Terns (Sterna albifrons) in the north of Singapore when he took a picture of an adult bathing.
It was a hot day and the bird was in the water splashing its wings about to get the feathers wet. It was obviously bathing. But then it may also be cooling its chicks.
According to Gochfeld & Burger (1996), “Terns not infrequently alight on the water to bathe, but swimming is a remarkably uncommon occurrence among this family of web-footed birds.” During periods of incubation and brooding, the adults counter heat stress by moistening their belly feathers and then return to their nests where the water droplets help cool the eggs or chicks.
Gochfeld, M. & J. Burger, 1996. Family Sternidae (Terns). In: del Hoyo, J., A. Elliott & J. Sargatal (eds.) Handbook of the birds of the world. Vol. 3. Hoatzin to Auks. Lynx Editions, Barcelona. Pp. 624-667