I should explain that this picture, like most that I take of the birds at the bottom of my garden, is taken from my bedroom window using a manual focus Nikon 300mm f/2.8 AIS lens with a coupled 1.4x converter which is mounted on my Canon 7D camera. The combination gives me the equivalent of a 670mm lens. However, with the Birch tree the best part of 18 metres away from the camera this warbler still looks rather small in the viewfinder!

Below is the full-frame version of the same photograph to show you what I mean -

And it doesn't help when the constant movement of the hyperactive little bird means that the thin branch is in constant motion  - In the final picture you can see how much the branch springs up as the warbler launches itself.

Despite the blurring of the bird I think the shot confirms the light colouring of its legs, something that helps distinguish the Willow Warbler from its relative the Chiffchaff. If I've got the ID around the wrong way please let me know!