Laughing Gull, Leucophaeus atricilla
These handsome mid-sized gulls nest along the eastern and Gulf coasts of the United States and south along both Mexican coasts and in the Caribbean; they winter at their breeding grounds, except those furthest north, and south into Central America and northern South America. Their name derives from their high-pitched cackling calls. I've never seen one where I live, in Northern California, though there are scattered reports of them here.
The top three photos here show Laughing Gulls in breeding plumage on the Texas Gulf Coast.
Above and below, Laughing Gulls doing their charming courtship ritual on the shores of Tampa Bay, where they are abundant.
The bird above, photographed on the shore of Long Island Sound in September, had already molted to non-breeding plumage, but was likely to migrate further south; they are rare winterers along the Atlantic coast north of New Jersey.
They reach adult plumage in their third year; the bird shown above, wintering on the Pacific Coast of Costa Rica, shows the first winter plumage, with gray upperpart feathers partly replacing the juvenal brown.