Hi I wonder if someone can help. We have a couple of bird boxes and normally a pair of Blue Tits take residence but this year I noticed the larger Great Tit using it, I think the male is a normally marked Great Tit but the female which spends more time in the nest has similar markings on the body but instead of yellow chest it is grey almost like a Coal Tit but as big at the Great Tit. Is this just a normal colour of females or something like a hybrid? Sadly no pictures yet but will keep trying to get some