Male and now female chaffinch have some kind of growth on their feet...

Hello!

I am worried about one of my two pairs of chaffinches.

I noticed the male had something on his left foot. Almost as if he had stepped in chewing gum or something. Looked like string or something like that. Now I see that the female has it too :-(

I would be grateful for your thoughts.

Best wishes 

Vicky

ps I have two pairs of greenfinches!