The avifauna of Bermuda include 387 species, according to the Bermuda Audubon Society (BAS) as of February 2015. The 387 species is a remarkable number considering that the island is a mere 53.3 square kilometres. (Additional species known only from the fossil record or speculatively from the time of early exploration are not included.) Ten species were introduced by humans; the mallard also occurs naturally as a non-breeding migrant. Of the 387, 104 are uncommon, 61 are rare, and 145 are very rare, all as defined below. Audubon's shearwater formerly bred in Bermuda but is now only a vagrant; Eskimo curlew is considered extinct.