In some respects the Hebrides lack biodiversity in comparison to mainland Britain; for example, there are only half as many mammalian species. However, these islands provide breeding grounds for many important seabird species including the world's largest colony of northern gannets. Avian life includes the corncrake, red-throated diver, rock dove, kittiwake, tystie, Atlantic puffin, goldeneye, golden eagle and white-tailed sea eagle. The latter was re-introduced to Rùm in 1975 and has successfully spread to various neighbouring islands, including Mull. There is a small population of red-billed chough concentrated on the islands of Islay and Colonsay.