Leach’s Storm-Petrel by John Sill © Massachusetts Audubon Society. Courtesy of the Museum of American Bird Art.
John Sill is a freelance wildlife artist living in the mountains of North Carolina. He was the illustrator for the Bird Identification Calendar for Mass Audubon for many years. His work has appeared in Birds In Art at the Leigh-Yawkey Woodson Art Museum, Wausau, Wisconsin, and in Art of the Animal Kingdom at the Bennington Center for the Arts in Vermont. He continues to illustrate the “About” and “About Habitats” series of natural history books for children written by his wife Cathryn.
The Leach’s Storm-Petrel (Hydrobates leucorhous), also known as Mother Cary’s Chicken, is a cosmopolitan species that breeds in colonies on islands in the oceans of the Northern Hemisphere. The Leach’s Storm-Petrel is a brown to black, medium-sized storm-petrel with a long, forked tail and long wings that bend backward at the carpal joint. A large, curved, pale brown wing bar crosses each wing from the carpal joint to the edge of the rump. The wings and tail are darker than the body. The rump is usually white and is shaped like an arrowhead with the point to the rear; it is sometimes split by a median stripe of brown. In colonies of the Pacific Ocean south of the Mexican border, many birds have dark rumps. The nostrils are in a tube that sits on top the black, hooked bill. The legs and feet are also black. The sexes are similar in appearance. Their flight is bouncy, often erratic, low over the water, and often with deep wing strokes.
The taxonomy of the Leach’s Storm-Petrel has been controversial, with up to five subspecies recognized in the past. Currently two subspecies are recognized: H. l. leucorhous, ranging in the Atlantic Ocean from Europe to Newfoundland and south to Massachusetts, and across the North Pacific Ocean from the Farallon Islands in California north to Alaska and along the Aleutian Islands to Japan; and H.l. chapmani, breeding on the Coronado Islands and San Benito Islands off Baja California. More than 8,000,000 pairs breed in burrows and rock crevasses on islands in the Atlantic and Pacific, and several million more nonbreeding birds stay at sea. The breeding islands are generally far enough offshore to be free of mammalian predators.
Colonies vary greatly in size from, for example, one colony on an island off of Newfoundland, which has more than 300,000 pairs, compared to seven pairs in the single, small colony in Massachusetts on Penikese Island in Buzzards Bay, and several more pairs on Noman’s Land off the coast of Martha’s Vineyard. Leach’s Storm-Petrel is an uncommon to sometimes common migrant, usually seen more than 50 miles from shore. The eastern North American birds arrive in April and May; they migrate south in late September to early November. Sightings of Leach’s Storm-Petrels inshore are storm sensitive; 10,000 birds were seen in Cape Cod Bay during a late snowstorm in May 1977. They are often blown into Cape Cod Bay during autumn nor’easters, and even inland during hurricanes.
Leach’s Storm-Petrels are monogamous, but the rare two-egg nests may indicate occasional bigamy or egg dumping. Little is known about the courtship of these largely nocturnal birds. They breed for the first time between ages five and six, and pair formation is thought to occur at the burrow site a year or two before actual nesting. Burrow-site fidelity apparently brings pairs back together in succeeding breeding seasons. Both males and females utter calls that are used in territorial defense and courtship. A chatter call consists of two protracted staccato notes separated by a much longer note and is given in flight at the breeding colony, on the ground, or in the burrow. A purr call is sometimes given in duet between mates and consists of a series of closely spaced notes separated occasionally by a longer note. It is used in courtship and pair maintenance. Harsh screech calls are given when birds are disturbed at their nests. Most calls are given at night.
Leach’s Storm-Petrels nest once per breeding season and raise a single young bird. Pairs sometimes return to their burrows a month or more before breeding. The male is thought to do most of the burrow excavation and nest building during the night. Nests are usually a scrape lined with grass, twigs, mosses, lichen, conifer needles, leaves, and feathers or just about anything that is available. Burrows are generally shallow and less than two feet in length; they may be straight or include a bend. Between late May and July, the female lays a single milky white egg that sometimes has purplish spotting around the large end. Both parents develop brood patches, and both incubate the egg for approximately six weeks until hatching. At hatching, the chick is covered with blue gray down but does not open its eyes for about a week. One adult broods the chick for about a week but rarely thereafter. The parents feed the chick a high-lipid diet, and the chick may eventually weight 1.5 times the adult weight. The chick fledges in about nine weeks and may appear at the burrow entrance for several days before flying away from the colony, after having trimmed down to adult weight.
Leach’s Storm-Petrels forage in open ocean wherever floating zooplankton or swimming nekton are concentrated, usually at upwellings. They forage by pecking while hovering, capturing individual organisms. They sometimes forage while swimming and may forage at night. They drink seawater and have salt glands over the eyes that remove the salt, which they secrete through their tube noses. Their diet includes cephalopods such as squid, crustaceans such as amphipods, small fish, and jellyfish. Breeding adults usually feed within one or two days of travel from their nest. Lipid-rich foods are concentrated in the stomach and regurgitated to feed to the chick.
Leach’s Storm-Petrels occasionally are taken by sharks in tropical waters, and suffer from kleptoparasitism from other storm-petrels, including conspecifics, and also from jaegers. Although they usually nest on islands far from shore, introduced mammals such as dogs, cats, or pigs can cause colony abandonment. Although active at night, they may fall prey to eagles, hawks, crows and ravens. Gulls and owls are also sometimes a problem. Despite these difficulties, Leach’s Storm-Petrels can live into their thirties and produce substantial numbers of young, which, together with their wide breeding distribution and large population size, suggests that this lovely pelagic species is secure.