Silver Birches and Catkins

Catkins from one of our Silver Birches, which are now bare of leaves. We thought all our Silver Birches were the same when we bought them, but we’re now seeing that 3 or 4 are different and producing catkins. As yet, I’ve not found out why some have them and the others don’t. I do know that male catkins come out in autumn and stay throughout winter and female ones in spring. On the same tree – but I don’t know why some have catkins and others don’t.