Tim says:

This is weird, I have been feeding a flock for a few years now, a mother crow has been a regular throughout and her latest batch of chicks, one has this exactly! They are growing daily, lately even faster as cold weather sets in, I call him “Knobs” for the big almost golfball sized tumors on the knees, just like your photo, I know he’s doomed when the freeze sets in, we’re in Canada. I have seen several other birds with strange tumors on beaks, feet, and knees, but none as bad as this guy.
I am curious what this is, what causes it, and if I can save him, he’s very friendly, never leaves his mom. I feed them sardines, fresh water only. Once in a blue moon a little donut or leftover catfood, but I am wary of this. I see other people use them as a garbage disposal at times, and I wonder if this is the cause, there’s something in the food, our food, causing tumors in crows? Swinging away in the dark with this guess but seems like the best answer at the moment.