Because the birds apparently warned of storms the sailors called them Mother Carey's Chicken's. Mother Carey is a corruption of "Mater Cara" which is was a name for the Blessed Virgin Mary. So Mother Carey's Chickens meant the Virgin Mary's Chickens.

The Stormy Petrels do not go to land except to mate and nest, so at sea the birds hide in the lee of passing ships during storms. So if suddenly there were a bunch of storm petrels along side your boat, it was a hint that a storm was approaching.