Why were petrels, in past times, thought to predict a storm?

Because they seem to repose in a common breeze, but, upon the approach, or during the continuation, of a gale, they surround a ship, and catch up the small animals which the agitated ocean brings near the surface, or any food that may be dropped from the vessel. Whisking like an arrow through the deep valleys of the abyss, and darting away over the foaming erest of some mountain wave, they attend the labouring bark in all her perilous course. When the storm subsides, they retire to rest, and are no more seen. Our sailors have, from very early times, called these birds " Mother Cary's Chickens." - Knapp.

The inhabitants of the Faroe islands use them as lamps : they pass a wick through their bodies, which, when lighted, burns a long time, from the quantity of fat they contain. - Blumenbach.