Order: Procellariiformes. Family: Hydrobatidae
The storm-petrels are relatives of the petrels, and are the smallest of sea-birds. They feed on plank tonic crustaceans and small fish picked from the surface, typically while hovering. The flight is fluttering and sometimes bat-like. There are 21 species worldwide and 1 species which occurs in Vietnam.
Storm petrels are the smallest of all the seabirds, ranging in size from 13–26 cm in length. There are two body shapes in the family; the Oceanitinae have short wings, square tails, elongated skulls, and long legs; the Hydrobatinae have longer wings, forked or wedge-shaped tails and shorter legs. The legs of all storm petrels are proportionally longer than those of other Procellariiformes, but they are very weak and unable to support the bird’s weight for more than a few steps. The flight is fluttering and sometimes bat-like.