What's wrong with this pigeon?

I'm a gardener and see my fair share of wildlife. Whilst on break I saw this rather sad looking pigeon. With bird feed in hand I rather foolishly tried to approach causing them to fly away but not before taking this photo (I've cropped it). It didn't look happy, It's head seemed wet with some kind of substance (blood, rain?) and we'll...it looks ill. I'm curious if anyone knows what disease or issue they have? Thank you, I know my photo isn't particularly helpful.