Taxonomy and Evolution
The Sabine's Gull is usually 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 is almost unique within the gulls, characteristics 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 this, and the closest relative of the Sabine's Gull is now thought to be the Ivory Gull, another Arctic species. The two species are thought to have separated a long time ago, around 2 million years ago.
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