Northern storm petrels are seabirds in the genus Hydrobates in the family Hydrobatidae, part of the order Procellariiformes. The family was once lumped with the similar austral storm petrels in the combined storm petrels, but have been split, as they are not closely related. These smallest of seabirds feed on planktonic crustaceans and small fish picked from the surface, typically while hovering. Their flight is fluttering and sometimes bat-like. The northern storm petrels are found in the Northern Hemisphere, although some species around the Equator dip into the south. 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. They are colonial nesters, displaying strong philopatry to their natal colonies and nesting sites. Most species nest in crevices or burrows, and all but one species attend the breeding colonies nocturnally. Pairs form long-term, monogamous bonds and share incubation and chick-feeding duties. Like many species of seabirds, nesting is highly protracted, with incubation taking up to 50 days and fledging another 70 days after that. Several species of storm petrel are threatened by human activities. One species, the Guadalupe storm petrel, is thought to have gone extinct. The principal threats to storm petrels are introduced species, particularly mammals, in their breeding colonies; many storm petrels habitually nest on isolated mammal-free islands and are unable to cope with predators such as rats and feral cats.
Taxonomy The family Hydrobatidae was introduced with Hydrobates as the type genus by the Australian born ornithologist Gregory Mathews in 1912. The background is complicated as the family Hydrobatidae had originally been introduced in 1849 with Hydrobata as the type genus by the French zoologist Côme-Damien Degland. Hydrobata had been erected in 1816 for species in the dipper family Cinclidae by the French ornithologist Louis Pierre Vieillot. In 1992 the International Code of Zoological Nomenclature (ICZN) suppressed the genus Hydrobata Vieillot, 1816. Under the rules of the ICZN the family Hydrobatidae Degland, 1849 thus became unavailable as the type genus had been suppressed. This cleared the way for the family Hydrobatidae introduced in 1912 by Mathews. The genus Hydrobates was erected in 1822 by the German zoologist Friedrich Boie. He listed two species but did not specify a type. In 1884 Spencer Baird, Thomas Brewer and Robert Ridgway designated the European storm petrel as the type species. The genus name combines the Ancient Greek hudro- meaning "water-" with batēs meaning "walker". In the past two subfamilies, the Hydrobatinae and Oceanitinae, were recognized within a single large family Hydrobatidae, but this has since been split with the elevation of the Oceanitidae to family status. The Oceanitidae, or austral storm petrels, are mostly found in southern waters (though Wilson's storm petrel regularly migrates into the Northern Hemisphere). The Hydrobatidae, or northern storm petrels, are largely restricted to the Northern Hemisphere, although a few visit or breed a short distance south of the equator. The family Hydrobatidae originally included two genera Hydrobates and Oceanodroma. Cytochrome b DNA sequence analysis suggested that the family was paraphyletic and more accurately treated as two distinct families. A few fossil species have been found, with the earliest being from the Upper Miocene. In 2021, the IOC merged Hydrobates and Oceanodroma into the single genus Hydrobates, as the family was paraphyletic as previously defined. The following cladogram shows the results of the phylogenetic analysis by Wallace et al. (2017).
Species
One species, the Guadalupe storm petrel (O. macrodactyla), is possibly extinct. In 2010, the International Ornithological Congress (IOC) added the Cape Verde storm petrel (O. jabejabe) to their list of accepted species (AS) splits, following Bolton et al. 2007. This species was split from the band-rumped storm petrel (O. castro). In 2016, the IOC added Townsend's storm petrel (O. socorroensis) and Ainley's storm petrel (O. cheimomnestes) to their list of AS splits, following Howell 2012. These species were split from Leach's storm petrel (O. leucorhoa).
Morphology and flight
Northern storm petrels are the smallest of all the seabirds, ranging in size from 13 to 25 cm in length. The Hydrobatidae have longer wings than the austral storm petrels, 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. All but two of the Hydrobatidae are mostly dark in colour with varying amounts of white on the rump. Two species have different plumage entirely, the ringed storm petrel, which has white undersides and facial markings, and the fork-tailed storm petrel, which has pale grey plumage. This is a notoriously difficult group to identify at sea. Onley and Scofield (2007) state that much published information is incorrect, and that photographs in the major seabird books and websites are frequently incorrectly ascribe species. The same researchers also state that several national bird lists include species that have been incorrectly identified or have been accepted on inadequate evidence. Storm petrels use a variety of techniques to aid flight. Most species occasionally feed by surface pattering, holding and moving their feet on the water's surface while holding steady above the water. They remain stationary by hovering with rapid fluttering or using the wind to anchor themselves in place. This method of feeding flight is more commonly used by Oceanitidae storm petrels, however. Northern storm petrels also use dynamic soaring, gliding across wave fronts gaining energy from the vertical wind gradient.
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