The name petrel is derived from St. Peter, because in flight they appear to be walking on water. This is merely an illusion - a consequence of their fluttering flight very close to the surface of the water, with their feet dangling below their bodies. This flying behaviour is a reflection of the birds' feeding method of scooping invertebrates from the ocean's surface waters. Storm-petrels are the smallest of the sea birds, reaching no more than 23 cm in length and only 84 g in mass. Their plumage is always dark in colour, varying in shade from black to grey. Their nostrils are conspicuous and are fused over the top of the bill.