Although high levels of salt are bad, there is no where near dangerous levels of salt in cat food...
I have posted this before... So here it is again... https://archive.org/stream/toxicityofsaltfo00mitc/toxicityofsaltfo00mitc_djvu.txt
Based on that study your average chicken at 4.5lbs or 2kg it would take about a tablespoon (8-10 grams) sized does of pure salt in a single dose to be lethal...
There is not that much salt in a regular feeding of even the worst cat food, get a premium cat food that has no added salt and the amount of salt is very low...
The risk of salt poisoning from cat food IMO is nil even if fed as the primary diet, even most poultry feeds have salt as an added ingredient as it's an essential mineral...
Here is a fairly recent list of dry cat food sodium contents...
http://www.felinecrf.org/dry_food_usa.htm
Here is the canned cat food contents list...
http://www.felinecrf.org/canned_food_usa.htm
Looking at that list, average sodium content is well bellow 1%, even using 1% and the 8-10 gram lethal dose it would require the chicken to eat 800-1000 grams or 1.8 to 2.2 pounds of food in a single sitting... Your average chicken only eats about 0.25 lbs of food a day or about 1/10 that amount, and those numbers are are based on the few foods that even have that level of sodium... Use a value like 0.4% that seems to be a common median value for dry cat food, and the chicken would need to consume 4.5 to 5.5 lbs of cat food in a sitting to get a potential lethal dose of sodium... This all assumes a 100% cat food diet, not a treat amount as I suggest that knocks those intake values exponentially lower...
To be blunt, I'll bet chickens scratching around coastal areas ingest more sodium naturally then they would from being fed a treat of cat food...
