Luckily, I was walking quietly, stalking butterflies, when I realised there was a family of young Chiffchaffs flitting through the shrubs and umbellifers near me, intent on discovering and devouring as many insects as they could find.
I wasn’t entirely sure they were Chiffchaffs (Phylloscopus collybita), as many young warblers look very similar and there wasn’t an adult around singing the distinctive Chiffchaff song. But they were in an area where I had heard that song many times before, and a couple of helpful folks on the Facebook birding page I belong to have confirmed my identification.
Apparently, one of the tell-tale signs is their dark legs, and they also have ‘relatively short primary projection compared with Willow warblers’. I had to google that one, and discovered: ‘Primary projection is the distance between the tip of the tertial feathers and the tip of the primary feathers, usually given as a ratio of the projection to the tertial length, vis. 1/2 tertials double the primary projection which is typical of Chiffchaff, whilst 3/4 (projection three quarters of tertial length) is more typical of Willow Warbler.’ If you’re still baffled, there’s an excellent visual comparison here, and I found a great BTO video, comparing the two species, on Youtube.