[vl]

This is interesting. Let’s say 600g Bar-tailed Godwit goes on 13500 km flight and spends very optimistic 200g of fat. Theoretically, if fat is only used for going up, it can climb to 170km (i.e. potential energy). This means that to get to destination in needs to glide by dropping 12.5m per km, or have glide ratio of 80.

Best human gliders have glide ratio of 60. So Godwit still needs to be very efficient glider, or, what is more likely also knows how to use winds and updrafts.

[etangent]

Godwit does not glide at all. Its wings are physically too small to support any kind of gliding flight. It must flap constantly to stay aloft.

[vl]

What this calculation shows is what glide ratio Godwit must have if it spends 1/3 of its body weight very efficiently to gain altitude and reaches destination in completely calm air. By the looks of it it doesn’t have this glide ratio, and I doubt it spends 1/3 of its weight only gaining altitude.

So it means that it must gain altitude and, perhaps, travel speed by alternative means, most likely using updrafts and riding winds.

[etangent]

Your calculation is interesting, but I beg you to go outside and look at how Godwits actually fly. Godwits fly in a straight line using rapid powerful nonstop flaps. They are relatively heavy birds with high "wing loading ratio" and their entire body plan is hyper-optimized for this type of flight and no other. This is especially true during migration when they accumulate lots of fat and their wing loading ratio is particularly unfavorable for soaring. Godwits are among the fastest-flying birds in a straight line, and they have never been observed to engage in any other mode of flight, nor can they. They cannot use regular soaring. They cannot use dynamic soaring. They do not fly like seagulls. Their wing area is simply too small to allow any other type of flight. They also typically do not wait for favorable wind to fly. They just take off and go.

[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 :)