Mystery singer

This little singer has been serenading me for weeks now as I arrive at work before sunrise. Today is the first time I have actually seen it and luckily the zoom on my camera allowed me to get a (slightly distant) picture. 

Currently causing a lot of controversy on the local birding site, the jury is still out on what this lovely little creature might be. I am wondering about warblers and hoping for nightingales. 

Whatever it is, the bird is itself and does not need a human label to sing magnificently in the pre-dawn.