Tim writes: I have posted these two photographs as interesting anomalies, because most birdwatchers learn that the easiest way to tell a (non-singing) Chiffchaff from a Willow Warbler is by leg colour. Chiffchaffs have dark legs and Willow Warblers have pale legs. But not always. The top bird here is a Willow Warbler with dark legs, and the lower bird is a Chiffchaff with noticeably pale legs, and certainly paler than this Willow Warbler.
Here are some typical examples with normal leg colour so you can see what they usually look like. First Willow Warbler with pale legs: https://www.flickr.com/photos/timmelling/48030968412/sizes/l/
And a Chiffchaff, with dark legs: https://www.flickr.com/photos/timmelling/49722831691/sizes/l/
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Interesting photos Tim.
Chiffchaffs frequently perform ‘tail dipping’ don’t they whereas willow warblers rarely do – so that is a useful distinguishing feature and something only a few field guides mention.
Tail-twisting is a recognised identification feature used in Sweden.