When it has a range like that, smaller dose is for smaller birds or emaciated birds. Larger dose is for larger birds OR severe cases.I don't know if the medications suggested are the most appropriate but I liked the site because it has all the dosages.
I think the dosages must be correct. But I suspect when it mentions per bird it refers to the average (well-fed) racing pigeon.
For instance, I checked the official Baytril site and the recommended dose (oral) is 10 - 20 mg per Kg.
So for a 500 g pigeon it is 5 - 10 mg.
However, an emaciated feral does not weigh 500 g so the doses could be on the high side.
Having said that, I think I have read that regarding Baytril, even administering 5 or 6 six times the recommended dose produces no adverse reactions.