The storm-petrels are seabirds in the family Hydrobatidae, part of the order Procellariiformes. These smallest of seabirds, relatives of the petrels, feed on planktonic crustaceans and small fish picked from the surface, typically while hovering. The flight is fluttering and sometimes bat-like.
Storm-petrels have a cosmopolitan distribution, being found in all oceans. They are strictly pelagic, coming to land only when breeding. In the case of most species, little is known of their behaviour and distribution at sea, where they can be hard to find and harder to identify.
Storm-petrels nest in colonies on islands, arriving in the evenings. One white egg is laid in a burrow in turf or soft soil. Both sexes incubate in shifts of up to six days. The egg hatches after 40 or 50 days; the young is brooded continuously for another 7 days or so before being left alone in the nest during the day and fed by regurgitation at night. Nestlings and parents leave the nest together after 60 or 70 days.
The name "petrel" is a diminutive form of "Peter", a reference to Saint Peter; it was given to these birds because they sometimes appear to walk across the water's surface. Early sailors named these birds "Mother Carey's Chickens" because they were thought to warn of oncoming storms; this name is based on a corrupted form of Mater Cara, a name for the Blessed Virgin Mary.
There are two subfamilies. The Oceanitinadae are mostly found in southern waters; there are 7 species in 5 genera. These have short wings, square tails, elongated skulls, and feed by hopping and fluttering on the water. All but one species have white underparts; the exception is Wilson's Storm-petrel which is also the only one to regularly migrate any great distance into the northern hemisphere.
The Hydrobatinae are thought to descend from an ancestor of Wilson's Storm-petrel. The 14 species in 3 closely related genera have longer wings, forked or wedge-shaped tails, and walk on the water rather than hop. All but two are wholly dark in colour. They are largely restricted to the northern hemisphere, although a few can visit or breed a short distance beyond the equator.
They are all highly pelagic seabirds (feeding in the open ocean), and all of them have their nostrils enclosed in one or two tubes on their straight, deeply grooved bills with hooked tips.
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