If a bird at sea level requires a speed of 30 mph to have enough lift to fly, how fast must it fly for the same lift at 20,000 ft?

rho v^2 = constant

what is density rho at ground and at 20,000 feet?

At sea level 0.0765 lb/cubic ft

At 20,000 ft 0.049 lb/ cubic ft

.0765 (30)^2 = .049 (v^2)

v^2 = 1405
v = 37.5 mph

The point is

Lift = lift Coef * area * (1/2) rho v^2
the lift coef and geometry are the same

Thank you very much for your help!