Mother Carey's chicken

plural Mother Carey's chickens

1. The storm petrel.

   A few blue petrels, black sheer-waters and Mather Caries chickens are all the birds we see.

   1773, James Cook, The Journals, Second Voyage, 27 December

   3 more examples

   Ten or fifteen dark little birds […] these are "Mother Carey's chickens," the "water-witches," Stormy Petrels, which are so familiar to the eye of the sailor, and the sight of which he dreads so much; […]

   1874, Alfred Edmund Brehm, Bird-life, tr. by H.M. Labouchere and W. Jesse, page 866

   There is no doubt that the stormy petrel has learned to imitate the motion of the waves, flying with motions not unlike those of certain fish. […] These strange little "water-witches" or Mother Carey's chickens are familiar to all sea-travellers.

   c. 1917, Royal Dixon, The human side of birds, page 215

   Kemp watched sperm whales and green turtles and hundreds of gulls and petrels – known as “Mother Carey's chickens” – that flew around the ship with piercing screams.

   2004, Nicholas Shakespeare, In Tasmania, Harvill Press, page 36