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.
Traditionally, two subfamilies are recognized. The Oceanitinadae are mostly found in southern waters (though the Wilson's Storm-petrel regularly migrates into the northern hemisphere); there are 7 species in 5 genera. The Hydrobatinae are the two genera Hydrobates and Oceanodroma. They are largely restricted to the northern hemisphere, although a few can visit or breed a short distance beyond the equator.
Cytochrome b DNA sequence analysis suggests that the family is paraphyletic and may be more accurately treated as distinct families. The same study found that the storm-petrels are certainly ancestral to the Procellariiformes. The first split was the subfamily Hydrobatinae, with the Oceanitinadae splitting from the rest of the order at a later date.
Morphology and flight
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 Oceanitinadae have short wings, square tails, elongated skulls, and long legs; the Hydrobatinae have longer wings, forked or wedge-shaped tails and short legs.
The plumage of the Oceanitinadae is dark with white underparts (with the exception of the Wilson's Storm-petrel) All but two of the Hydrobatinae are mostly dark in colour with varying amounts of white on the rump. Two species have different plumage entirely, the Hornby's Storm-petrel which has white undersides and facial markings, and the Fork-tailed Storm-petrel which has pale grey plumage. 
Storm-petrels use a variety of techniques to aid flight. Most species will 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 by using the wind to anchor themselves in place. This method of feeding flight is most commonly used by Oceanitinadae storm-petrels. The White-faced Storm-petrel possesses a unique variation on pattering, holding it's wings motionless and at an angle into the wind it pushes itself off the water's surface in a succession of bounding jumps. Storm-petrels also use dynamic soaring and slope soaring to travel over the ocean surface. Dynamic soaring is used mostly by the Hydrobatinae, gliding across wave fronts gaining energy from the vertical wind gradient.  Slope soaring is more straightforward and favoured by the Oceanitinadae, the storm-petrel turns to the wind, gaining height, from where it can then glide back down to the sea.
Storm-petrels nest in colonies on remote islands. Nesting sites are attended nocturnally in order to avoid predators. Storm-petrels display high levels of philopatry, returning to their natal colonies to breed. In one instance a Band-rumped Storm-petrel was caught as an adult 2m from its natal burrow. Storm-petrels nest either in burrows dug into soil or sand, or in small crevices in rocks and scree. Competition for nesting sites is intense in colonies where storm-petrels compete with other burrowing petrels, with shearwaters having been recorded killing storm-petrels in order to occupy their burrows. Colonies can be extremely large and dense; 840,000 pairs of White-faced Storm Petrel nest on South East Island in the Chathams in burrow densities of between 1.18 - 0.47 burrows/m˛; densities as high as 8 pairs/m˛ for Maderian Storm-petrels in the Galapagos and colonies 3.6 million strong for Leach's Storm Petrel have been recorded.
Storm-petrels are monogamous and form long-term pair bonds that last a number of years. Studies of paternity using DNA fingerprinting have shown that unlike many other monogamous birds infidelity (extra-pair matings) is very rare. As with the other Procellariiformes, a single egg is laid by a pair in a breeding season, if the egg fails then usually no attempt is made to relay (although it happens rarely). 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. Meals fed to the chick weigh around 10-20% of the parent's body weight, and consist of both prey items and stomach oil. Stomach oil is a energy rich (its calorific value is around 9600 calories per gram) oil created by partly digested prey in a part of the foregut known as the proventriculus. By partly converting prey items into stomach oil storm-petrels can maximise the amount of energy chicks receive during feed, an advantage for small seabirds that can only make a single visit to the chick during a 24 hour period (at night). Chicks fledge after 50 or 70 days, depending on the species.
Relationship with humans
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. The more specific 'storm petrel' or 'stormy petrel' is a reference to their habit of hiding in the lee of ships during storms. 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.
Threats and Conservation
Several species of storm-petrel are threatened by human activities. Two, the Guadalupe Storm-petrel, and the New Zealand Storm-petrel, are listed as critically endangered. The Guadalupe Storm-petrel has not been observed since 1906 and most authorities consider it extinct. The New Zealand Storm-petrel was also considered extinct for many years but was sighted again in 2003, even so the population is likely to be very small. One species (the Ashy Storm-petrel) is listed as endangered due to a 42% decline over twenty years, and two other species are also listed as near threatened or worse. In addition four species are so poorly known that they are listed as data deficient.
Storm-petrels face the same threats as other seabirds, in particular they are threatened by introduced species. The Guadalupe Storm-petrel was driven to extinction by feral cats, and introduced predators such as have also been responsible for declines in other species. Habitat degradation which limits nesting opportunities caused by introduced goats and pigs is also a problem, especially if it increases competition from more aggressive burrowing petrels.
- Subfamily Oceanitinae
- Wilson's Storm-petrel, Oceanites oceanicus
New Zealand Storm-petrel, Oceanites maorianus
White-vented Storm-petrel, Oceanites gracilis
Grey-backed Storm-petrel, Garrodia nereis
White-faced Storm-petrel, Pelagodroma marina
Black-bellied Storm-petrel or Gould's Storm-Petrel, Fregetta tropica
White-bellied Storm-petrel, Fregetta grallaria
Polynesian Storm-petrel, Nesofregetta fuliginosa
- Wilson's Storm-petrel, Oceanites oceanicus
- Subfamily Hydrobatinae
- European Storm-petrel Hydrobates pelagicus
Leach's Storm-petrel Oceanodroma leucorhoa
Matsudaira's Storm-petrel Oceanodroma matsudairae
Least Storm-petrel, Oceanodroma microsoma
Wedge-rumped Storm-petrel, Oceanodroma tethys
Madeiran Storm-petrel, Oceanodroma castro
Swinhoe's Storm-petrel, Oceanodroma monorhis
Guadalupe Storm-petrel, Oceanodroma macrodactyla (extinct)
Tristram's Storm-petrel, Oceanodroma tristrami
Markham's Storm-petrel, Oceanodroma markhami
Black Storm-petrel, Oceanodroma melania
Ashy Storm-petrel, Oceanodroma homochroa
Ringed Storm-petrel, Oceanodroma hornbyi
Fork-tailed Storm-petrel, Oceanodroma furcata
- European Storm-petrel Hydrobates pelagicus
- ^ Nunn, G & Stanley, S. (1998) "Body Size Effects and Rates of Cytochrome b Evolution in Tube-Nosed Seabirds" Molecular Biology and Evolution 15(10): 1360-1371  Corrigendum
- ^ Harrison, P. (1983) Seabirds, an identification guide Houghton Mifflin Company:Boston ISBN 0-395-33253-2
- ^ a b Withers, P.C (1979) "Aerodynamics and Hydrodynamics of the ‘Hovering’ Flight of Wilson's Storm Petrel" Journal of Experimental Biology 80: 83-91
- ^ Erickson, J. (1955) "Flight behavior of the Procellariformes" The Auk 72: 415-420 
- ^ Pennycuick, C. J. (1982). "The flight of petrels and albatrosses (Procellariiformes), observed in South Georgia and its vicinity". Philosophical Transactions of the Royal Society of London B 300: 75–106.
- ^ Brinkley, E. & Humann, A. (2001) "Storm-petrels" in The Sibley Guide to Bird Life and Behaviour (Elphick, C., Dunning J. & Sibley D. eds) Alfred A. Knopf:New York ISBN 0-679-45123-4
- ^ Bretagnolle, V. (1990) "Effect of moon on activity of petrels (Class Aves) from the Selvagen Islands (Portugal)" Canadian Journal of Zoology 68: 1404-1409
- ^ Harris, M. (1979) "Survival and ages of first breeding of Galapagos seabirds" Bird Banding 50(1): 56-61 
- ^ Ramos, J.A., Monteiro, L.R., Sola, E., Moniz, Z., (1997). "Characteristics and competition of nest cavities in burrowing Procellariiformes" Condor 99: 634–641.
- ^ West, J. & Nilsson, R. (1994) "Habitat use and burrow densities of burrow-nesting seabirds on South East Island, Chatham Islands, New Zealand" Notornis (Supplement) 41 27-37 
- ^ Mauwk, T., Waite, T. & Parker, P. (1995) "Monogamy in Leach's Storm Petrel:DNA-fingerprinting evidence" Auk 112(2): 473-482 
- ^ Warham, J. (1976) "The Incidence, Function and ecological significance of petrel stomach oils." Proceedings of the New Zealand Ecological Society 24 84-93 
- ^ Obst, B & Nagy, K (1993) "Stomach Oil and the Energy Budget of Wilson's Storm-Petrel Nestlings" Condor 95: 792-805 
- ^ Slotterback, J. W. (2002). Band-rumped Storm-Petrel (Oceanodroma castro) and Tristram’s Storm-Petrel (Oceanodroma tristrami). In The Birds of North America, No. 673 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.
- ^ IUCN, 2006. Red List: Storm-petrel Species Retrieved August 27, 2006.
- ^ Sydeman, W., Nurr, N., McLaren, E. & McChesney G. (1998) Status and Trends of the Ashy Storm-Petrel on Southeast Farallon Island, California, based upon capture-recapture analyses" Condor 100: 438-447 
- ^ A contemporary account of the decline of the Guadalupe Storm-petrel can be found here - Thayer, J. & Bangs, O (1908) "The Present State of the Ornis of Guadaloupe Island" Condor 10(3): 101-106 
- Storm-petrel videos on the Internet Bird Collection