Wyorp Rock is right - if they've only been on your property a month then cocci is a very likely cause - there are several different strains, so even if a bird develops immunity to the cocci where they are raised they don't automatically have immunity to other types that they may encounter when they are moved from one home to another.
Although it is dangerous, and can be fatal, cocci is easy enough to treat. Making certain that your birds can only drink the treated water is absolutely essential. Ensure that there are no puddles / plant saucers with water sitting in them etc. that they could drink from, and if you give them anything to eat that you normally add water to (for example pellets made into wet mash, or bread soaked in water) then make certain that you use medicated water, not plain tap water. In fact, if you want to be certain that your poorly boy is getting enough food and medication inside him then giving him a bowl of wet mash would be a good idea - for some reason many chickens seem to love their pellets this way, and it is a good way to encourage a sick bird to eat and drink. (It won't hurt for all your birds to have some - you will be surprised at how much they will love it!)
Good luck with the treatment - let us know how it goes.
Although it is dangerous, and can be fatal, cocci is easy enough to treat. Making certain that your birds can only drink the treated water is absolutely essential. Ensure that there are no puddles / plant saucers with water sitting in them etc. that they could drink from, and if you give them anything to eat that you normally add water to (for example pellets made into wet mash, or bread soaked in water) then make certain that you use medicated water, not plain tap water. In fact, if you want to be certain that your poorly boy is getting enough food and medication inside him then giving him a bowl of wet mash would be a good idea - for some reason many chickens seem to love their pellets this way, and it is a good way to encourage a sick bird to eat and drink. (It won't hurt for all your birds to have some - you will be surprised at how much they will love it!)
Good luck with the treatment - let us know how it goes.