Zompist Bboard Again

What is a kestrel? It's a bird, of course, but what kind of bird? To most people it's one of these, but in two separate instances - in the glossary at the end of the Rogue vade mecum, and in a fantasy novel I read many years ago - it appears to be a seabird, which in the second instance flies in flocks. Is this a different usage which has somehow fallen under the radar of the search engines?

Fun fact: in some older versions of English it was known as the windfucker, from its habit of hovering into the wind when looking for dinner.

I've definitely heard "kestrel" used to describe a seabird. I suspect it refers to one of those truly nasty predatory gull-like birds, like a skua.

Maybe the authors are misremembering petrel?