I know about it only because I spotted one of the female chaffinches in our garden a few weeks ago with what looked like a clump of bird seed stuck to her right foot. Having had a look through the binos I realised it wasn't something she'd trodden on as I'd first thought, but a growth on her actual foot. Remarkably, it doesn't seem to bother her and she hops about feeding and moving quite happily and can fly without impediment too. It just looks awful.
So what with Bumble (as we've taken to calling the little chaffinch) and Poppet (our dunnock with the wasted leg), I feel we are fast becoming a refuge for birds with unwell legs and feet! At least they know they can come here and always find an easy source of food, which is important in this current cold snap.