This has been an informative read.

I think that given the range and distance that both petrels and sailors travel, that it would be unwise to try to limit the term used to a particular species.

Anyone - including the experts who have ever tried to see these birds at sea - let alone to specifically identfy these tiny birds from a pitching vessel - will testify as to how difficult it is. There are ways to do this especially on the odd calm day, but the average sailor would be unlikely to make these distinctions.

I suspect that any small black and white petrel seen 'walking' on the surface of the water would be Mother Carey's Chickens.