This species of warbler is one of the few that now spends the winter, albeit in small numbers, in the UK. When I encountered quite a few yesterday on the edge of the River Exe yesterday I had no way of knowing whether they were true migrants or birds that were going to stay, but I suspect they were probably migrants about to cross the channel. Willow Warblers and Chiffchaffs are hard to seperate but one of the ways, (not always conclusive) is the colour of the legs which are usually balack in Chiffs and fleshy cloured in Willows.  The wings of Willow Warblers extend further on to the tail as well, but with experience you can usually see the difference and I would put my life on these birds being Chiffchaffs. They were feeding on small invertabrates amongst low vegetation with a couple of Wrens, really lovely views and it was very nice to watch them.