In article , Uri Saovray
writes
Does anyone have an idea of how the birds know where to thermal? Do
they have a vario? Where is it? Where is its capacity?
Assuming they breath while thermalling, then I doubt they use their
lungs as capacity... or maybe they stop and sense the air coming out
their noses.
Just wondering...

Uri 4XGJC

Its a long time since I did any comparative anatomy, but I believe many
birds have air filled cavities in some of their bones.

While on the subject of birds using thermals, the German expedition to
the Andes (1937) decided to bring some raptors birds back to Germany,
the idea being they would find thermals for them. During the voyage back
they were of course kept in cages and fed by the crew. And everyone
else.

When the ship docked in Bremen, the birds had got so heavy they couldn't
get airborne.

From a biography of Hanna Reich.
--
Mike Lindsay