I used to fly gliders in east Africa, where we had both buzzards and marabou storks (a very large carrion eater). Both were fairly good indicators of lift, but savvy glider pilots always looked below a gaggle to see if they were circling in lift or over a dead zebra. The clue was if they were descending or climbing, obviously.

And here in the Minden area we have flocks of pelicans that gaggle up in thermals. A couple years ago I was about 16,000' above Bridgeport with a dozen or so pelicans in the same thermal. Since there are not many fish at that altitude, I presume they are there for the pure pleasure of soaring. Or maybe not - maybe they were gaining altitude for a long glide.