Mother Carey's Chickens
Stormy petrels. Mother Carey is Mater Cara. The French call these birds oiseaux de Notre Dame or aves Sanct&aeigh; Mariae. Chickens are the young of any fowl, or any small bird.
“They are called the `sailor's' friends, come to warn them of an approaching storm; and it is most unlucky to kill them. The legend is that each bird contains the soul of a dead seaman.”
(See Captain Marryat: Poor Jack, where the superstition is fully related.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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