This video from Washington state in the USA says about itself:
Bonaparte’s Gull feeding
One of about 400 Bonaparte’s in outer Quartermaster Harbor, Vashon Island. Most were roosting on the water while a few were feeding. This bird has completed the transition from winter to breeding plumage, but many were still in transition. Taken on 1 May 2011.
From Club300 in Denmark, 26 June 2017:
Breaking breeding news: Bonaparte’s gull is breeding in Iceland, new breeding bird for the western Palearctic!
Read more, in Danish, here.