Whats wrong with this pigeon?

I found a pigeon on my fire escape in Detroit. It can't fly or walk and holds its head at a weird angle. It's wings are not broken (I have chickens and know how to check) so I think it must be a neurological thing. Maybe it ran in to a window, or has a sickness which is causing it? Otherwise it seemed totally healthy. It has green droppings. I have had it for 3 days, I have kept it in a clean fish tank filled with towels and I have food and h20 constantly available. I cover the fish tank with a blanket (I leave an open area for air)so the pigeon feels secure. Today I plan on trying to hand feed the pigeon some bran cereal mashed with water because it has lost some weight. Can anyone give me some info on what might be the problem? What should I do to provide care? I can not bring the bird to vet. It's not an option because I hardly have any money at all right now and absolutely can't afford it. Any info would be great! Thanks :)