Pardon this input from an uninformed intruder to this subject, but why
couldn't
birds simply sense how strong the lift is by the amount of stress it puts
on
their "airframe". When I pump iron, I'm all too aware of the amount of
effort
required. Surely birds can do the same.

Ted Frost
Soaring Society of Boulder

The stress would only change when acceleration was present (as in entering a
thermal, we use our bums for for that - transitting from an area of intense
sink to and area of lesser sink: e.g. no thermal). In a steady climb the
stress wouldn't change and would be the same as in normal turning flight. I
suspect that Martin's observation about the birds choosing a more extreme
dihedral has more to do with lack of effort than sensing the lift intensity
- a high dihedral result and a high value of lateral stability, but poor
efficiency of the wing.

Rgds,

Derrick Steed