Yeah, you got weight paint issues. Example: There are index finger bones weighted to the tip of the middle finger.
If you select each one of these vertex groups, go into edit mode, select the middle finger and press “Remove” to remove them this fingernail will stop acting up. But this is not the only finger with weight issues.
I’m also learning rigging and one thing I noticed is that usually auto weight doesn’t work well when either the armature is not positioned correctly/has too few bones or the mesh has topology issues. Common issues are not creating a mesh well-positioned for rigging or using too few edge loops at joints.
Good topology for a good rest pose is one where fingers, eyelids, mouth are as wide apart as possible so auto weight doesn’t accidentally pick geometry it’s not supposed to pick. In your case that means keeping fingers well apart and palm of hand splayed. It also has as few “baked-in” joint deforms as possible: straight fingers, straight spine, straight neck, all joints bent only enough to make sure they bend in the correct direction once rigged. The first post of this WIP thread of mine is a good example of someone committing both mistakes at once.
Also, use at least 3 edge loops in joints so they can keep their shape when deforming:
The middle edge (highlighted in blue) will keep the volume while the other two loops will do the actual bending.
And try to make your bones touch the tip of whatever they’re controlling. Your fingertip bones don’t go all way to the tips of the nails.
A good armature will have joints where where joint loops are. Fingers are easy, select the middle edge loop of a joint, place the cursor to selected (space bar + Snap cursor to selected), go into edit mode for the armature, select the respective bone head/tail overlap and place them at the 3D cursor location.
Because I’m using auto weight as a learning tool sometimes the model doesn’t have enough bones to keep shape when deforming. Example: I’ll leave just a spine bone and a breathing control bone in the thorax of this model, but in real life you’d have an entire ribcage there.
Because it’s a very large boneless area auto weighting leads to the thorax picking weights from shoulder and wing bones. So I created two temporary bones (children to the spine) in the place a group of real bones would be. In the end I’ll assign the weight of these bones to the spine and delete them.
@skuax Being new to all of this sometimes I have trouble remembering proper terminology, and not being a native English speaker doesn’t help. So when in a hurry I make stuff up. 