[vl]

I agree with your points. What this calculation shows that godwits must rely on favorable winds and updrafts (which benefit any flying object, including birds, since motion is relative); if this is not the case, they just not going to reach claimed distances given their weight and how much fat they are carrying.

[etangent]

It could also be that the numbers/assumptions input into the equations are incorrect but :)

[vl]

Weight and distance numbers are from Wikipedia, potential energy formula is from school.

You are welcome to recalculate :)