Bonaparte's Gull Life History

Habitat

Bonaparte’s Gulls breed in openings in the boreal forest of Canada and southern Alaska. Here they select marshy areas along or near the edges of lakes and streams. Their tree nests in the taiga are unique among gulls, which nest on the ground as a rule. Because much of their foraging in spring and summer is on flying insects, they feed mostly over marsh and open water, but they also forage over trees if insects are present, sometimes hawking them in company with Common Nighthawks or Black Terns. Bonaparte’s Gulls migrate in flocks across much of North America, and can show up in almost any aquatic environment if the migration is interrupted by bad weather: sewage treatment plants, lakes, ponds, rivers, and reservoirs are all used. During winter, this species is exceptionally versatile: flocks may occur well inland, even on the Great Lakes if not frozen, but are equally at home well offshore, even at the interface of the Gulf Stream and Labrador Current many miles from land. In between, they winter on rivers, bays, and beaches, anywhere prey is abundant, and they may move great distances during winter in search of prey. They may roost on a lake or at sea, on the water’s surface or on kelp beds, or in sheltered waters, inlets, or coastlines. In the past, when raw sewage was piped into oceans, Bonaparte’s and other small gulls often congregated at outfall sites; they are still found at hot-water discharges from power plants.

Food

Like most gulls, Bonaparte’s are opportunists that feed on protein-rich invertebrates and small fish. They are rarely seen at landfills, where larger gulls scavenge garbage. Bonaparte’s diet includes many organisms too small or swift for larger gulls to capture, such as zooplankton and midges. Most of the diet in the nonbreeding months consists of small fish, which they take by dipping and diving, much like terns. Where small fish are reliably present each year—such as emerald shiners in the Niagara River—Bonaparte’s appear in great numbers and feed intensely all through the day, for weeks at a time. In western North America they eat salmon eggs and small fry, spawning Pacific herring, and several species of shad. In marine environments, prey such as small fish and tiny crustaceans (especially krill, amphipods, and copepods) are often concentrated by upwelling currents or by predatory fish, and Bonaparte’s gorge themselves on this type of prey. Along coastlines, they might pick fly larvae from storm-tossed seaweed, pull marine worms from mudflats, or pluck small crabs or snails from the water’s edge. Like other gulls, they steal (or “kleptoparasitize”) food from other birds including Red-breasted Mergansers, Horned Grebes, and Razorbills, but they also simply capture fish fleeing from these diving birds. In gatherings of shorebirds, they sometimes steal from yellowlegs or plovers. On wet agricultural fields, or during plowing, Bonaparte’s materialize to take earthworms, grasshoppers, locusts, ants, and beetles (and their larvae). Breeding birds take advantage of the great bloom of insect life in the boreal forest in the warmer months, capturing flying ants, termites, bees, and many sorts of aerial insects in flight. In Alaska, they take shrimp from along the edges of glaciers. They have even been documented taking ripe berries.

Nesting

Nest Placement

Nests are near water in conifers such as spruce, tamarack, Douglas-fir, hemlock, or jack pine, near the trunk. These tree nests are usually between 6 and 40 feet high, but nests up to 56 feet are known. In some areas, Bonaparte’s regularly nest on or near the ground, especially on top of dead marsh vegetation.

Nest Description

Tree nests are built of bark, twigs, and branches, lined with lichen and moss. They vary in size, being about 10-15 inches across and 3-5 inches tall, with a shallow central depression about 1.5 inches deep. Nests of Bonaparte’s Gulls have also been documented on the ground or on top of rushes near the ground, made of twigs, mosses, lichens, grasses, sedges, earth, and leaves.

Nesting Facts

Behavior

Bonaparte’s Gulls are especially agile, equipped with a fine bill, lithe body, and narrow pointed wings, all of which permit them to select small prey items quickly and precisely and maneuver capably in tight flocks, ideal in situations where prey are highly concentrated. They mostly forage by dipping and diving from the air, but they are equally skilled hunters on land, where they take invertebrates in farm fields as if they were shorebirds; or in shallow waters where they pivot and spin like phalaropes, stirring up prey from the bottom. One of their most mesmerizing techniques is “conveyor belt” foraging, seen in the Great Lakes, Atlantic, and Pacific: large numbers fly upwind, just above the water’s surface, dipping down to seize prey such as small fish. As they reach the end of the food patch they fly upward, where the wind catches them, bringing them quickly to the end of the queue, where the process starts over. Bonaparte’s Gull pairs court mostly on the nesting grounds, where they pair with partners in flight and perform postures and calls when perched, including “long calls,” with bills pointed skyward, much like larger gull species. Pairs select a nest site soon after reaching the breeding ground and defend the area around the nest from predators and other gulls. They fly swiftly and erratically to stoop on or chase out intruders. Sometimes, nests are close enough together (a few hundred feet) that an area of several nests could be considered a colony. Both sexes build the nest, incubate eggs, and tend young. After breeding, Bonaparte’s gather for migration in highly social, tight flocks, rather like those of Franklin’s Gull, and they normally forage as cohesive flocks as well, less dispersed than the larger gulls tend to be.

Conservation

Numbers of Bonaparte's Gull have probably increased over the past 100 years. Partners in Flight estimates a world population of 260,000 and rates the species a 10 out of 20 on the Continental Concern Score, indicating it is a species of low conservation concern. As with many marine species, Bonaparte’s Gulls are vulnerable to toxic materials such as heavy metals in their environment and to changes in prey populations resulting from climate change.

Credits

Burger, Joanna and Michael Gochfeld. (2002). Bonaparte's Gull (Chroicocephalus philadelphia), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

Kushlan, J. A., M. J. Steinkamp, K. C. Parsons, J. Capp, M. A. Cruz, M. Coulter, I. Davidson, L. Dickson, N. Edelson, R. Elliott, R. M. Erwin, S. Hatch, S. Kress, R. Milko, S. Miller, K. Mills, R. Paul, R. Phillips, J. E. Saliva, W. Sydeman, J. Trapp, J. Wheeler and K. Wohl (2002). Waterbird conservation for the Americas: The North American waterbird conservation plan, version 1. Washington, DC, USA.

Partners in Flight (2017). Avian Conservation Assessment Database. 2017.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

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