Sabine's gull treated as comprising a monotypic genus; it is only placed within the genus "Larus" when the genus is enlarged. The black bill and notched tail are almost unique within the gulls, as they are shared only with the swallow-tailed gull of the Galapagos. On the basis of this the two species were often thought to be each other's closest relatives, a hypothesis ruled out by a number of behaviour and ecological differences. Mitochondrial DNA studies confirmed that they are not closely related, and the closest relative of the Sabine's gull is now thought to be the ivory gull, another Arctic
Arctic Circle northwards at a speed of about per year. The word "arctic" comes from the Greek word ἀρκτικός ("arktikos": "near the Bear, northern") and that from the word ἄρκτος ("arktos": "bear"). The Arctic Circle is the southernmost latitude in the Northern Hemisphere at which the centre of the sun can remain continuously above or below the horizon for twenty-four hours; as a result, at least once each year at any location within the Arctic Circle the sun is visible at local midnight, and at least once the centre is not visible at local noon. Directly on the Arctic Circle these events