Apparently Harpies do not have legs? But honestly, no idea about Hamstrings on a bird, but bird legs/feet can bleed. So not sure if this is a bug or not.

ham harpy
Your target must have legs in order for you to execute this move.

Harpies now have legs.

But they don't really use their legs to fly.
Because of that, hamstring doesn't work particularly well against them.

Is that true for all birds that fly?

Shifter forms haven't exactly been all made in the most consistent of manners.

Not every air form has legs. Weird, right?

Of those that DO - hamstring won't mess with air forms that don't fly using their legs as much as it does with air forms that do use their legs.