Hi there. You've got some pretty good resources given already. Are your birds under the weather? How old are they and where did they come from?
A chicken can carry Salmonella and NOT be sick with Salmonellosis.
No, it is NOT common for backyard chickens to get it free ranging. But it IS possible. Not my favorite resource as sometimes they have invalid information.. but could be useful to your question..
http://www.poultrydvm.com/condition/salmonellosis
I LOVE Merck as a resource but use the veterinary version.. Not as simple to understand as some resources but accurate.. Says it can be treated with antibiotics but is controversial as it may cause them to become carriers who shed live virus..
https://www.merckvetmanual.com/digestive-system/salmonellosis/salmonellosis-in-animals
https://www.merckvetmanual.com/poultry/salmonelloses/salmonelloses-in-poultry
If the carrier statement is true.. culling is how I would treats the birds. The whole reason I got my own flock was because of eggs being recalled due to Salmonella. Since getting my own birds.. purchased from NPIP participating suppliers.. I collect clean eggs daily and store unwashed in the fridge for 6+ months.. and the're still better than store bought! No one "washes" (if that't what we call rinsing the bloom off) my eggs before eating and no one has ever gotten sick YET.. from my eggs.
But wait, there's more! I harvest my birds as well and believe it or not they stay fresh with no fowl smell or slimy feeling for at least 2 weeks in the fridge! There is a LOT to be said for cross contamination and preventing it.
Most folks find out their birds are carrying salmonella when when of their humans gets ill. I've seen that be way more common than symptoms actually seen in the birds themselves. Even eating raw chicken doesn't cause salmonella.. unless the bird is carrying it. Might cause something else though I doubt it IF FRESH!