I have the answer. Pigeons feature an amazingly complex feature. I mean, they not only have variable wing span, but their wings can just morph during flight (till now no human aircraft manufaturer has succeeded in doing such a fine mechanism). During gliding, true, they use some kind of really high dihedral configuration (just a lazy way to keep a constant heading), but they tend to alter this dihedral during high speed flight.

Thank you GOD for creating such beautiful aircraft !!!