A bird and its song

The bird that sings tunes is back in our garden. I have no idea what kind of bird it is as I have never been successful in spotting it among our trees here. But I have heard it sing short tuneful songs which seem to change every year. Some years ago, I remember distinctly that it kept singing a motif of descending notes that went C Bb A G F. I was fascinated because it wasn't the usual twittering of most birds. Just earlier this morning I heard it sing E A F#.