[4] 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.

[5][6] Based on these studies, the American Ornithologists' Union, which had previously assigned the species to the genus Larus, restored it to Chroicocephalus in 2008.

[15] 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.

[17] It avoids dense stands of conifers, instead choosing more open areas, such as the treed edges of bogs, fens, marshes, ponds, or islands.

[26] 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.

[27] Bonaparte's gulls are known to engage in kleptoparasitism, and have been observed stealing earthworms from foraging dunlins and black-bellied plovers.

[29] 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.

[18] Both sexes help to build the nest,[31] which is a platform of small sticks, sometimes with lichen, moss or marsh vegetation added, measuring about 25 cm (9.8 in) in diameter.

Adults are aggressive in defending their nests, chasing away even large potential predators such as hawks, common ravens and humans.

[32] 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.

[33] 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.

[29] Young birds typically leave the nest within 2–7 days of hatching, jumping out and following their parents to the nearest open water.

[35] 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.

The linguatulid species Reighardia sternae,[39] a tiny, worm-like crustacean, is sometimes found in the gull's air sacs.