Storm Petrels: Seabirds that 'Walk' on Water

Storm petrels have many unusual qualities and habits that separate them from other seabirds. Not the least of those characteristics is the appearance they give of walking on water.

A storm petrel is any member of about 20 species of seabirds that constitute the family Hydrobatidae, sometimes also called Oceanitidae, in the order Procellariiformes.

Etymology
The storm part of storm petrel (recorded c. 1833 in the Oxford English Dictionary) comes from the fact that the bird tends to live far out at sea unless storms drive it to land. The bird is also called stormy petrel (c. 1776).

Petrel (1703, pitteral in 1676) is believed to be derived from St. Peter because the bird's habit of pattering on the surface of the water while feeding makes it seem to be walking on the water, as St. Peter did in a biblical account (Matt. 14:29).

The same story underlies the family name Hydrobatidae, which is based on Latin hydro- ("water") and -bates ("walker").

The order name, Procellariiformes, is based on Latin procella ("storm").

An old English sailors' name for the storm petrel is Mother Carey's chicken (1767), probably a salty alteration of Mater Cara ("Dear Mother," that is, "Virgin Mary"). Seafarers traditionally entrusted their safety to the Virgin Mary.

General Description
Storm petrels range in length from 5 inches to 10 inches. All are dark gray, sooty black, or brown, often with white in or near the tail.

The wings have rounded tips, and the toes are webbed except for the small, elevated hind toe. The tail is medium to long and, in different species, has a square, wedge, or forked shape.

On the upper surface of the medium-length bill, nostril tubes are united into an organ by which the birds can dispose of the salt that they take in when they drink sea water. The birds blow or "sneeze" the salt out of the tubes.

Storm petrels are built for prolonged flying. They have thin legs that cannot support their weight for more than a few steps on a solid surface. Therefore, besides storms, only breeding brings them to land.

Storm petrels breed in colonies on remote islands and islets along the coasts of North and South America, in the eastern and southern Atlantic, and throughout the Pacific. Coming ashore at night to avoid being preyed on by gulls or other large seabirds, they nest in burrows they dig in soft soil or in crevices in rocks. The female lays a single whitish egg that both parents incubate.

Storm petrels are categorized into two broad groups based on structure, behavior, and geography: southern storm petrels and northern storm petrels.

Southern Storm Petrels
Most of the storm petrels that breed in the southern hemisphere have, compared with the northern species, shorter wings, shorter toes, and longer legs. The tail tends to be square shaped.

The southern species are the most notable "water walkers." While feeding, they spread their wings, lightly dip their feet into the surface of the water, and use their bills to pick up plankton (minute marine organisms).

Wilson's storm petrel (Oceanites oceanicus), for example, is a water walker that breeds on islets along the Antarctic continent. It spends the winter in northern oceans, especially the Atlantic.

The white-vented (or Elliot's) storm petrel (Oceanites gracilis) is another water walker. It is endemic to the Galápagos Islands, in the Pacific Ocean about 600 miles west of Ecuador, South America.

Northern Storm Petrels
Most of the storm petrels that breed in the northern hemisphere have, compared with the southern species, longer wings, longer toes, and shorter legs. The tail is usually forked or wedge shaped.

The northern species usually forage by swooping over the water, occasionally landing on the surface.

Leach's storm petrel (Oceanodroma leucorhoa), for example, is a northern species. It breeds on islands in the northern Atlantic.

Several other Oceanodroma species inhabit the northern Pacific. One of these is the fork-tailed storm petrel (Oceanodroma furcata), which breeds in various parts of the Pacific, including the eastern Pacific from Alaska south to northern California.

The British storm petrel (Hydrobates pelagicus) breeds on islands and cliffs along the coasts of Europe.

Some northern species, like their southern relatives, patter on the water. For example, Tristram's storm petrel (Oceanodroma tristrami) is a northern species that breeds in the central Pacific from Hawaii to Japan. It usually catches its prey, including crustaceans and small squid, on the wing, but sometimes it feeds while pattering on the surface of the water.