It's a well known fact that birds bones are very light and filled with
holes, just like we have sinuses in our head bones. I've pondered how birds
might sense rate of climb many times and I now hold the view that they sense
it via the cavities in their bones - this would provide them with a very
sensitive variometer, the capacity being automatically incorporated so to
speak.

Some work was done in the early 70's on this. Whatever a bird's "vario" is, it resides in
their ear. Cutting the 8th cranial nerves (which connect the ear to the brain)
extinguished their ability to sense altitude. This work was done in pigeons, (not soaring
birds, who would be expected to have an even better "vario"). Further work (done in a
pressure chamber) indicated that even a pigeon can sense an ambient pressure change
equivalent to climbing 2 feet.

I'm also convinced that birds soar for pleasure as well as because they
might have to (e.g. Pelicans soar when they are migrating and follow similar
climb/glide patterns to us).

Other research done in the 70's was carried out by a glider pilot in Africa. He wanted to
figure out where all the buzzards went during the middle of the afternoon (all would
disappear every day). They thermaled up, out of sight from the ground. Either for
pleasure,or to cool off?