Bonaparte's gull

Bonaparte's gull (Chroicocephalus philadelphia) is a member of the gull family Laridae found mainly in northern North America. At Template:Convert in length, it is one of the smallest species of gull. Its plumage is mainly white with grey upperparts. During breeding season, Bonaparte's gull gains a slaty-black hood. The sexes are similar in appearance.

Taxonomy and etymology

Bonaparte's gull in immature (first-winter) plumage, showing the tail band and brown wing pattern cited in Ord's description of the species

When George Ord first described Bonaparte's gull in 1815, he gave it the scientific name Sterna philadelphia, and the English name 'Banded-tail Tern';<ref name="Ord"/> the description clearly identifies it as a bird in first-winter plumage, while "The slender and tern-like form of the bill probably induced Mr. Ord to put it in Sterna".<ref name="Lawrence"/> Most later taxonomists assigned it to the genus Larus, a longtime catch-all for most of the gull species. However, in 1858, George Newbold Lawrence moved the species to the genus Chroicocephalus,<ref name="Lawrence"/> and some taxonomists followed suit.<ref name = "Brewer"/> Subsequent molecular DNA studies have shown that this species fits neatly into a clade with other Chroicocephalus "hooded" gulls, and that it is in a close-to-basal position in that grouping.<ref name = "Pons"/><ref name="Černý"/> Based on these studies, the American Ornithologists' Union, which had previously assigned the species to the genus Larus, restored it to Chroicocephalus in 2008.<ref name = "AOU"/> It is monotypic across its range.<ref name="IOC"/>

The species is named after Charles Lucien Bonaparte, a French ornithologist (and nephew to the former French emperor Napoleon Bonaparte) who spent eight years in America, contributing to the understanding of the taxonomy and nomenclature of birds there and elsewhere. Its genus name, Chroicocephalus, is a combination of the Greek words Template:Lang, an adjective form of Template:Lang meaning "colour", and Template:Lang meaning "head". This refers to the dark heads that most gulls in this genus show during the breeding season. The specific epithet philadelphia is a Latinized adjective meaning "from Philadelphia", a reference to the location from which the type specimen was collected.<ref name = "Sandrock"/>

Description

Bonaparte's gull is among the smallest of the gull species; only little gull and Saunders's gull are smaller.<ref name = "Harrison"/> Adults range from Template:Convert in length, with a wingspan of Template:Convert and a weight of Template:Convert.<ref name = "AllBirds"/> There is no difference in plumage or bare part colour between the sexes,<ref name = "Howell"/> though males tend to be heavier than females.<ref name = "CRC"/> Bonaparte's gull is smaller-bodied, smaller-headed, and smaller-billed than the other common hooded gulls of North America.<ref name = "Thompson"/> The adult has grey upperparts and white underparts, with, unlike other Chroicocephalus gulls, the grey extending up the rear of the neck to just below the head; its wingtips are black and white striped above with the outer four primary feathers white with a black tip, and the other primary feathers grey with a black tip; the whole wing is very pale below. In breeding plumage, it has a slaty black hood, which it loses in non-breeding plumage. Its short, thin bill is black, and its legs are pinkish-red in winter, orangish-red in summer.<ref name="Olsen"/><ref name = "Dunn"/>

Bonaparte's gull in fresh juvenile plumage; Whitehorse, Yukon

The juvenile plumage, usually only seen on the breeding grounds, is scalloped brown on the back, and patchy brown on the head; they moult into first-winter plumage in August mostly before migrating south.<ref name="Olsen"/> First year birds are more like the adults with a grey back, but retain a brown bar across the forewing, blackish tips to the secondary feathers, and a black tip to the tail; they have the same plumage in winter and the following summer, but the summer plumage is paler due to wear and fading. Fewer than 5 percent of Bonaparte's gulls acquire a dark hood in their first summer, and on those that do, the hood is duller than on breeding adults.<ref name="Olsen"/> Two-year old birds mostly gain full adult plumage, but a few retain signs of immaturity such as brown spots on the forewing and dark tips to some of the tail feathers.<ref name="Olsen"/>

The most similar potential confusion species is the Old World black-headed gull, which each being a regular, though scarce or rare, winter visitor within the wintering range of the other. They can best be distinguished by the smaller size of Bonaparte's gull, its slenderer, darker bill, pale grey rear of the neck, pinker legs, and in flight, by the conspicuously whiter underwing, lacking the dark uderside of the primary feathers of black-headed gull.<ref name="Olsen"/> Little gull can also be confused in immature plumages; it is even smaller than Bonaparte's gull, and the dark bar on the forewing of year-old birds is black, not dark brown. Adult little gulls, with their uniform pale grey upperwing and blackish underwing, are far less likely to be confused.<ref name="Olsen"/>

Distribution and habitat

Bonaparte's gull breeds in boreal forest across southern Alaska and much of interior western Canada, as far east as central Quebec and south to within Template:Convert of the Canada – United States border.<ref name = "Dunne"/> It avoids dense stands of conifers, instead choosing more open areas, such as the treed edges of bogs, fens, marshes, ponds, or islands. It typically nests within Template:Convert of open water.<ref name = "Sandilands"/> It winters along the coasts of North America, and in the Great Lakes.<ref name = "Dunne"/>

Vagrancy

It is a rare but annual visitor to western Europe and the Azores, where it generally associates with black-headed gulls.<ref name = "Beaman"/> In Great Britain, the number of records increased from averages of six per year at the start of the 21st century to around 12 per year by 2018–2022; the reason for this increase is unknown.<ref name="French"/> Individuals often return to particular sites over a number of years; one at Bamburgh in Northumberland returned for its 12th autumn in 2024.<ref name="Fox"/> In 2017 a breeding pair was found in northwestern Iceland.<ref name="Nielsen"/> Bonaparte's gull is an occasional non-breeding vagrant to the Hawaiian Islands,<ref name="Pyle"/> and vagrants have also been recorded west to Japan and south to Brazil and Senegal.<ref name="Olsen"/>

Behaviour

They are migratory and most move southeast or southwest to coastal waters, also the Great Lakes. They are graceful in flight, more like terns than most other gulls.

Feeding

A Bonaparte's Gull eats a sandworm in the Saco Bay, ME.

Like most gulls, Bonaparte's gull has a varied diet, with prey items changing over the course of the year, and from year to year. During the breeding season, it is largely insectivorous.<ref name = "Braune"/> It is known to quickly congregate in large numbers to take advantage of termite dispersal flights, circling over the emerging swarm and hovering briefly to take the insects in flight.<ref name = "Cowan"/> It also gathers in large numbers to feed on the eggs of spawning salmon, alighting on the water and, if necessary, diving to take drifting eggs.<ref name = "Moyle"/> During migration and into the winter, insects are first supplemented, then replaced by other food items, including fish, small crustaceans, mollusks, euphausiids, marine worms, and other invertebrates.<ref name = "Braune"/> At least one immature bird has been recorded as having fed on walnut meat.<ref name = "Frohling"/> Bonaparte's gulls are known to engage in kleptoparasitism, and have been observed stealing earthworms from foraging dunlins and black-bellied plovers.<ref name = "Payne"/>

Breeding

Once they leave the nest, chicks follow their parents to the nearest water.

Bonaparte's gulls begin breeding at two years of age; they are thought to be monogamous.<ref name = "Sandilands"/> The breeding season begins in mid-June.<ref name = "Baicich"/> Courting pairs perform swooping display flights, calling loudly and diving at each other, and then drop down to perch on a branch. Crouched and facing each other, with neck and crown feathers erected and wings slightly raised, they scream at each other with bills opened wide, bobbing up and down as they do so. This display can continue for several minutes before ending abruptly; afterwards, the birds may sit quietly together for some time before separating again.<ref name = "Twomey"/>

They raise a single brood per year,<ref name = "Baicich"/> nesting singly or in loose colonies of Template:Nowrap, with nests spaced at least Template:Convert apart. Unlike most other gulls, Bonaparte's gulls typically nest in trees — almost exclusively conifers, most often black spruce, but also northern whitecedar, tamarack and jack pine.<ref name = "Sandilands"/> Both sexes help to build the nest,<ref name = "Swanson"/> which is a platform of small sticks, sometimes with lichen, moss or marsh vegetation added, measuring about Template:Convert in diameter.<ref name = "Sandilands"/> Most nests are placed Template:Convert above the ground, and within Template:Convert of open water, though they have been found low as Template:Convert, as high as Template:Convert, and as far as Template:Convert from open water. They are only rarely placed on the ground, except in the far north where trees are absent. Adults are aggressive in defending their nests, chasing away even large potential predators such as hawks, common ravens and humans. There is some evidence that human activity in the area of their nests may cause decreased productivity — they are known to travel as far as a kilometre (about a half mile) to mob people, — but, contrarily, some choose to nest near human habitation.<ref name = "Sandilands"/>

Bonaparte's gull nesting at the top of a black spruce in Churchill, Manitoba

The female lays a clutch of Template:Nowrap, with most nests containing Template:Nowrap. The eggs are subelliptical, slightly glossy and smooth, measuring Template:Convert<ref name = "Sandilands"/> and weighing Template:Convert.<ref name = "Heydweiller"/> They range in colour from pale to medium green, olive or buff, and may be variably marked with spots, blotches, or scrawls of brown, grey, violet, or black; these markings may be fine and evenly distributed over the entire surface of the egg, or thick and concentrated towards the egg's larger end.<ref name = "Baicich"/> The eggs are incubated by both parents for Template:Nowrap.<ref name = "Sandilands"/><ref name = "Baicich"/> Studies have shown that breeding populations can be significantly impacted by bad weather. In one multi-year study in Churchill, Manitoba, more than half of the eggs laid in observed nests were blown out of those nests during storms; only 42% of eggs survived to hatching.<ref name = "Jehl"/>

Hatchling Bonaparte's gulls are semi-precocial, emerging from their eggs covered with natal down feathers, and with eyes already opened. The down is yellowish-buff, greyer on the flanks and belly, and somewhat pinkish on the breast, with dark brown mottling on the upperparts and head. The bill is blue-black with a pinkish base, and the legs and feet are pinkish-buff.<ref name = "Baicich"/> Young birds typically leave the nest within Template:Nowrap of hatching, jumping out and following their parents to the nearest open water.<ref name = "Swanson"/> Both parents tend the young.<ref name = "Baicich"/> The time it takes the young to fledge is unknown.<ref name = "Sandilands"/> Bonaparte's gulls may live to 18 years.<ref name = "AMNH"/>

Voice

Template:Listen Bonaparte's gull has a voice described as nasal and raspy, with calls variously transcribed as Template:Not a typo or Template:Not a typo.<ref name = "Peterson"/>

Conservation

Because of its extremely large range and its increasing population, Bonaparte's gull is listed as a species of least concern by the International Union for Conservation of Nature.<ref name = "IUCN"/> In North America, it is protected by the Migratory Bird Treaty Act.<ref name = "USFWS"/> Because it occurs in Europe and Africa only as a vagrant, it is not protected by the Agreement on the Conservation of African-Eurasian Migratory Waterbirds.<ref name = "AEWA"/>

The species is known to be preyed upon by peregrine falcons.<ref name = "Hunter"/>

Like many birds, it hosts a number of internal and external parasites. The linguatulid species Reighardia sternae,<ref name = "Holl"/> a tiny, worm-like crustacean, is sometimes found in the gull's air sacs.<ref name = "Ward"/> The acanthocephala worm Corynosoma bipapillum is sometimes found in the bird's posterior gut,<ref name = "Schmidt"/> as is the intestinal worm Echinostomum spinulosum.<ref name = "Gilbert"/> Gigantobilharzia lawayi, a schistosome (or blood fluke) may be carried in the capillaries.<ref name = "Farley"/> Externally, it is known to carry several species of lice, including Actornithophilus funebre, Degeeriella atrimarginata, Degeeriella punctata, Menopon species, and Philopterus gonothorax.<ref name = "Peters"/>

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  Ground nesting in Katmai National Park, Alaska
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  Showing the white underwing
• 
  In flight, showing the upperwing pattern
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  Vagrant in UK

External links

• Template:BirdLife
• Template:Avibase
• Template:InternetBirdCollection
• Template:VIREO
• Template:IUCN Map
• Bonaparte's gull - Larus philadelphia - USGS Patuxent Bird Identification InfoCenter
• Template:Xeno-canto species
• Template:Field guide birds of the world