Bonaparte's Gull

Overview

Bonaparte's Gull is a small, delicate gull that breeds in the boreal forests of Canada and Alaska, an unusual habitat for a gull, and one of very few gull species to nest in trees rather than on the ground or cliffs. It is common along both coasts of North America in migration and winter, often gathering in flocks over bays, estuaries, and river mouths. Its light build, thin black bill, and buoyant, almost tern-like flight distinguish it readily from larger gulls sharing the same waters.

The species shows a bold white triangular wedge on the outer wing formed by pale outer primaries, bordered by a narrow black trailing edge, a pattern that flashes conspicuously as the bird wheels and dips to pick prey from the water's surface.

Feather Identification

• Wing feathers: Outer primaries are white with narrow black tips, forming the diagnostic pale wedge on the upper wing that is bordered by a thin black trailing edge; under the wing the primaries appear translucent white.
• Size and shape: Primaries and secondaries are notably slender and lightweight, matching the species' small, agile build; wingtip feathers are more pointed than in most other small gulls.
• Mantle and covert feathers: Soft pale gray, unmarked, contrasting with white underparts and rump.
• Head feathers: Solid black in breeding adults (not brown as in Black-headed Gull), reduced to a dark spot behind the eye in nonbreeding plumage.
• Compared to similar species: The wing wedge closely resembles that of Black-headed Gull but Bonaparte's is smaller and lighter overall, with a thinner all-black bill lacking any red base, and a purer black (versus brown) hood.

Plumage Details

Breeding adults show a solid black hood, pale gray mantle, white underparts, and a thin black bill with reddish-orange legs. In nonbreeding plumage the hood is lost, leaving a white head with a small dark ear-spot and a pale gray wash on the nape. Juveniles show brownish mottling on the back and upperwing coverts along with a dark tail band, molting into a gray-backed first-winter plumage that resembles the nonbreeding adult but retains some brown covert feathers and a darker trailing edge to the wing. Full adult plumage, including the crisp black hood, is attained in the second year. The molt into breeding plumage typically occurs in late winter to early spring before birds depart for boreal breeding grounds.

Habitat & Range

Bonaparte's Gull breeds in the boreal forest zone of Canada and Alaska, nesting singly or in loose groups in coniferous trees near ponds, lakes, and muskeg, a striking departure from the ground- or cliff-nesting habits of most gulls. In migration and winter it becomes strongly coastal and pelagic-edge, congregating over bays, harbors, tidal rips, and large inland lakes across much of North America. It is migratory, wintering along both the Pacific and Atlantic coasts as well as the Gulf of Mexico.

Behavior & Field Notes

This species forages with light, buoyant flight, dipping to the surface to pick up small fish, aquatic invertebrates, and insects, often hovering briefly before striking, a technique more reminiscent of terns than of typical gulls. It gathers in large, active flocks over tide rips and river mouths, frequently associating with terns and other small gulls. Nesting is solitary or loosely colonial, with a shallow stick nest built in a conifer a short distance above the ground or water. The call is a soft, nasal, tern-like "cheer" or chattering note, quieter and less harsh than the calls of larger gulls. Its light build, quick wingbeats, and flashing white wing wedge are the best field clues at any distance.