I went to the National Maritime Library in San Francisco and learned more about Mother Carey this weekend.

From The Dictionary of Nautical Literacy, by Robert McKenna, International Marine/McGraw Hill, 2001:

"Mother Carey's Chickens, sailor's term for storm petrels or similar seabirds. In folklore, Mother Carey is the woman in charge of Fiddler's Green, the last home of the souls of drowned sailors. She sees to it that her guests are comfortable and, from time to time, lets them visit the upper world. When such visits occur, the souls take the form of seabirds known to the sailors as storm petrels. For this reason, no real sailor will ever harm a storm petrel."

From Superstitions of the Sea, by James Clary, Maritime History in Art, St. Clair, Michigan, 1994:

"Perhaps the most famous sea bird is the black storm petrel also known as Mother Carey's chickens, alamottie, storm fish, or litter Peter. Believed to appear before a storm as a warning to mariners, as its Latin name, Procellaria, infers, it did in fact warn sailors of a coming tempest by gathering under the stern of their ships. The severity of the storm was supposedly calculated by the number of birds sighted. The storm petrel commonly seen in great numbers in all seas of the world from coast to coast appeared as a close companion of the seafarer, as they were known to tirelessly follow ships to feed on the garbage thrown overboard. Still, early mariners saw the bird as a bad omen because of its black coat of feathers and, because it was often sighted hundreds of miles from land, seldom tiring, resting, or feeding. Its storm-foreboding character is described in this verse:

Oh, stormy, stormy petrel!
Thou art a bird of woe,
Yet would I thou couldst tell me half
of the misery thous dost know

Although befriending man by warning him of approaching storms, man's unjustified contempt for the bird was exemplified in this verse:

Thus doth the prophet of good or ill
Meet hate from the creatures he serveth still;
Yet he ne'er falters; so, petrel, spring
Once more on the waves with thy stormy wing.

[My apologies, I neglected to copy the references as to where these verses come from (radriano)]

Because they were always seen on the fly and rarely noticed on land, it was believed that the little bird hatched its eggs beneath its wings, and never rested

Mother Carey was the fable wife of Davy Jones of the sea, both of whom were mythical maritime deities. To retire to "Davy Jones' locker" is the sea term for drowning. Storm petrels were often referred to as Mother Carey's chickens because they were believed to be the spirits of dead sailors.

Besides being able to swim, petrels possess the unique attribute of supporting themselves with flapping wings while rapidly striking the water with their webbed feed, which has given cause to them being compared to St. Peter walking on the water."

And finally, from Salt-water Poems and Ballads, by John Masefield, The MacMillan Company, Publishers, New York, 1913, the poem mentioned earlier in this thread:

MOTHER CAREY
(As told me by the bo'sun)

Mother Carey? She's the mother o' the witches
'N' all them sort o'rips;
She's a fine gell to look at, but the hitch is,
She's a sight too fond of ships.
She lives upon an iceberg to the norred,
'N' her man he's Davy Jones,
'N' she combs the weeds upon her forred
With pore drowned sailors' bones.

She's the mother o'the wrecks, 'n' the monther
Of all big winds as blows;
She's up to some deviltry or other
When it storms, or sleets, or snows.
The noise of the wind's her screamin',
'I'm arter a plump, young, fine,
Brass-buttoned, beefy-ribbed young seam'n
So as me 'n' my mate kin dine.'

She's a hungry old rip 'n' a cruel
For sailor-men like we,
She's give a many mariners the gruel
'N' a long sleep under sea.
She's the blood o' many a crew upon her
'N' the bones of many a wreck,
'N' she's barnacles a-growin' on her
'N' shark's teeth round her neck.

I ain't never had no schoolin'
Nor read no books like you,
But I knows 't ain't healthy to be foolin'
With that there gristly two.
You're young, you things, 'n' you're lairy,
But if you're to make old bones,
Steer clear, I says, o'Mother Carey,
'N' that there Davy Jones.