Editor's Note: Studies of mitochondrial DNA in the subfamily Larinae have suggested that the heretofore broadly defined genus Larus is paraphyletic. Reclassification of this genus now places Bonaparte's Gull in the genus Chroicocephalus. See the 49th Supplement to the AOU Check-list of North American Birds for details. Future revisions of this account will account for this change.
One of the smallest gulls in North America, Bonaparte's Gull is a familiar and often abundant migrant and winter visitor over much of the continent, but it is one of the least known of the gulls with respect to breeding. It winters in large flocks in coastal areas close to human activity, but it breeds solitarily or in very loose colonies in habitats remote from human settlements, around ponds, bogs, bays, and fiords in the taiga and boreal forests of Alaska and Canada. During migration and winter it frequents inland lakes and rivers, and coastal bays, estuaries, and inshore waters. It is the only gull that regularly, indeed almost always, nests in trees. The combination of high latitude, arboreal nests hidden among coniferous branches, and widely dispersed nesting make it difficult to study. It is an unusually graceful gull, dove-like in appearance with a light, buoyant, tern-like flight. In the spring, its lilting calls and delicate pink blush on the breast signal the arrival of its breeding season.
A solitary Bonaparte's Gull may be the only sign that a small colony is nearby. Rarely up to a dozen pairs may be scattered among spruces on an island or bog. Intruders are greeted at a distance of more than 100 meters by overflying gulls which call harshly, and may even swoop and dive at the intruder, while the incubating bird remains hidden in the boughs. This combination of behaviors makes it difficult to pinpoint which tree has the nest.
Bonaparte's Gull nests in sparsely wooded areas and shuns dense, continuous stands of tall evergreens, where there is little visibility of approaching predators. In addition to their nesting habitat, Bonaparte's Gulls may be found hovering over tundra pools or gathering in large flocks at the face of glaciers to feed on insects and small fish by diving from a few meters above the water, by aerial dipping, or by surface-dipping-sitting like a phalarope and jabbing at floating invertebrates.
The breeding season at high latitudes is short, and by mid-July some individuals gather for southward migration, forming flocks of hundreds and eventually thousands as they move along major river valleys to the Pacific Coast, Mississippi Flyway, and Atlantic Coast. On migration and in winter they live in large flocks (sometimes reaching 100,000 birds), and they feed mainly on small fish, taking a large toll, for instance, of hatchery-released salmon fry. They winter as far north as there is ice-free water.
Most studies have been on migration, staging, or wintering areas, but Twomey ( Twomey 1934 ) studied breeding behavior.