Thursday, December 12, 2013
By HERB WILSON
Did you know that possibly the most abundant bird in the world is in Maine now? And that the vast majority of Mainers have never seen one?
Wilson’s storm-petrels dip their feet into the ocean surface to attract larval fish and crustaceans to eat.
The bird is Wilson's storm-petrel, a small seabird that is regularly seen from whale watches and fishing expeditions. Occasionally these birds may be seen from shore when onshore winds push them landward.
Wilson's storm-petrels belong to the order of birds called the tube-noses. This order includes three families: the large albatrosses, shearwaters and gadfly petrels, and the storm-petrels. The cylindrical external nostrils on the bill give these birds their order name.
Spending most of their lives at sea with no ready source of freshwater, tube-noses have special glands in the nose that secrete salt. The salt is dribbled out through the tubes onto the surface of the bill.
Wilson's storm-petrels do not breed in the northern hemisphere. Rather, they are now spending their winter on the offshore waters of the western Atlantic. These birds are small, about the size of a swallow (7 inches in length). They may be identified by the short, squared tail and the relatively long legs that cause the feet to extend beyond the tip of the tail.
They have yellowish-green webbing between the toes (good luck seeing that field mark from a bouncing boat!).
Perhaps the best identification feature is the broad, U-shaped white band on the rump that extends underneath to the undertail coverts; the white rump is easily seen against the rest of the brown feathers on the body. From above, these birds have a pale bar on the upper surface of the wings.
Wilson's storm-petrels fly close to the water surface with purposeful, shallow wing beats similar to those of a swallow or small tern. Flight is direct, with steady, shallow wing beats.
Like most storm-petrels, Wilson's storm-petrels feed on the small, drifting organisms called plankton. These include larval fish, various crustaceans including krill, and jellyfish.
Most storm-petrels, including Wilson's, use a fascinating pattering behavior to help them feed. A storm-petrel will hover over the sea and dip its feet into the surface of the ocean three or four times.
This dipping seems to attract larval fish and crustaceans, which then become lunch for the storm-petrel.
Some authors have described this pattering as walking on the water. In fact the name petrel is an allusion to the biblical account of Saint Peter walking on the water.
The storm part of the name comes from the observation that these birds are generally seen by landlubbers only after major storms.
In the Caribbean, Wilson's storm-petrels are called skipjacks. Newfoundland fishermen call them Mother Carey's chickens, a corruption of Mater Cara (Dear Mother), a reference to the Virgin Mary.
Like other tube-noses, Wilson's storm-petrels have a good sense of smell.
It's great fun on a pelagic bird trip when the captain spreads a little cod liver oil on the surface of the ocean.
Within a short while, tube-noses of various sorts find their way to the slick.
Wilson's storm-petrels will follow fishing boats, attracted no doubt by the fishy odors emanating from them. It's not unusual to see 50 or more around a boat.
In their breeding season (our winter), Wilson's storm-petrels breed near high-density plankton areas around the ice in far south and turbulent seas in the subantarctic zone off South America. They seem to breed on virtually every suitable exposed rocky area off the coast of Antarctica as well.
A single egg is laid in a burrow and both members of the pair incubate and feed the chick once it has hatched. It takes 40 to 50 days for the egg to hatch.
Descriptions of their numbers in the southern seas are mind-boggling.
Some observers have observed acres of Wilson's storm-petrels off the coast of South Georgia so dense that the birds seem to be touching each other. Flocks of thousands or tens of thousands of birds are often reported.
Audubon mentioned Wilson's storm-petrels as breeding on islands off Nova Scotia, and other writers extended this claim to include Maine.
The error persisted for many years, until finally corrected by Brewster in 1884. Audubon was undoubtedly observing Leach's storm-petrel, a similar species that nests from Maine northward.
These birds tend to forage at night so are rarely seen during the day.
After the brief breeding season in the subantarctic waters, Wilson's storm-petrels make their way north to more equitable climates to spend most of the year.
They arrive in the Gulf of Maine by early June and mostly depart by the middle of September.
Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at firstname.lastname@example.org.