A nest?

I have noticed some strange objects on a tree near to me. There are about 4 or 5 of them and they appear to be made of mud and are wrapped around smaller branches of the tree. I'm not sure if they are part of the tree (dont know what tree it is) or whether something made them. They are quite small - probably the size of a cricket ball, maybe a little larger. I will try and get a picture but in the meantime does anyone know what they could be - do you think they could be nests?