Sexes similar in all plumages. Breeding plumaged adult has entirely black head with narrow white eye-crescents, gray mantle, white tail, white underparts, black bill, and orange-red legs. Nonbreeding adult similar except head entirely white with characteristic dark ear spot behind eye, and gray hindneck.
For a comprehensive review of the conservation status, habitat use, and ecology of this and other Montana bird species, please see Marks et al. 2016, Birds of Montana.
Western Hemisphere Range
Most of Montana is in the main migration path. Bozeman migration dates are May 4 to 15, with peaks on May 10. Fall records are mostly for November (Skaar 1969).
Bonaparte's Gulls prefer larger lakes, but occur on smaller ponds as well (Skaar 1969).
Diet consists of small fish, euphausids, amphipods, and insects.
Single brooded species with three eggs per brood. Incubation period twenty-four days.