To do it smoothly what you probably want to do is rig the model with bones and each vertex has a weight to the bones painted to it. This is a pretty dense topic though to actually get bone animation working for your model. Typically for games each vertex can be weighted to 3 bones where the sum of all weights is = 1.
For an organic object like a bird though that is probably the best option. If it were something more mechanical like a helicopter you could just separate out, say, the blades, and rotate them around easily. But with a bird you’ll want some vertices (most of the wing) to be transformed 100% and then some (the body) to be transformed 0% and then other verts in between to only be transformed a percentage of those.
Mukund sort of has it right. The verts that are weighted to a bone could be thought of as a vertex set that are transformed by some percentage of the bone. But without that the animation will look very rigid and you won’t get the kind of volume preservation you’d expect with an organic object. I guess it depends on the model… with some models you maybe could get away with rigid vertex sets.