I think this is a 'she', and I think she has just started making a nest in one of our home-made blue tit nest boxes. They have a wide range of calls; one in particular I hear when the pair are close by and it seems they are chatting to each other while they hunt for food. Today though, this bird's call was different, and I think she was calling her mate from afar. After a few minutes he arrived safely and proceeded to chide him.