On Thursday morning I got off the train at Farnborough North and was surprised to hear the alternating two-note song of a chiffchaff as I walked over the bridge.
It was back in the spring that I first noticed one of these small, non-descript warblers belting out its distinctive song from high up on a power line, and was so intrigued that I took the trouble to look it up in BWP-CE. It was the repetitive two-note song and the fact that it was sung from high up that convinced me that it was a chiffchaff (Phylloscopus collybita). Over the next few weeks I would regularly hear two or even three of them each morning as I walked the half-mile through the woods to Frimley, but I never again managed to see any of them. The song is so distinctive that I am sure that I would remember if I had heard it before, but I am fairly sure I haven't. The two-note phrase is repeated between about five and ten times and then is followed by a lower and quieter brrr-brrr-brrr sound.
During the summer I noticed that the chiffchaffs were no longer singing, or were only doing so much less frequently than in the spring. Hence my surprise to hear one singing again at the start of autumn.