A heart shaped nest hole

A friend rang me this morning to tell me that she had something to show me in her garden. Her house backs on to a golf course with lots of mature trees and recently she has seen several deer so I thought that’s what I was going to see. No deer but instead I saw this sweet little head poking out of the hole in the tree. Apparently there are three but I only saw this one and we didn’t want to stay around too long as the parents were getting agitated.