When using any pesticide, always read and follow the label directions. See links to publications below for managing pests of poultry or penned animals.
Extinguish Professional Fire Ant Bait contains an insect growth regulator (a chemical called methoprene), and The Environmental Protection Agency (EPA) has approved it for use in many sites, including zoos, rangeland, and cropland.
While not specifically labeled for use around chickens, use of this product in this situation is not prohibited and the active ingredient methoprene is not known to have any effect on vertebrates. Because of its low toxicity, methoprene is exempt from the requirements of food tolerances as established by the EPA. Methoprene is used in food animals such as cattle, where it is fed to the cattle in order to control horn flies in the manure. It also has uses on stored grains and other food commodities. Therefore, a methoprene product can be the proper choice to control fire ants in your free range chicken situation.
It is always a good idea to exclude the birds in the area where the bait has been applied for 24 hours. Exclusion from the treatment area will give fire ant workers a chance to pick up the bait and take it back into their nests (studies have shown that fire ants pick up most of the bait in 12 hours). However, incidental ingestion of the bait will not harm the birds. Otherwise a granular bait product must be applied in such a manner to make it unavailable to free ranging poultry, such as applying it around but not inside the pen.
It is important to note that this product will take about a month before the maximum level of control is achieved (see E-628, Broadcast Baits for Fire Ant Control), and by following a regular program of two bait applications a year, you can be relatively fire ant free.
Find more information about fire ants in eXtension's Imported Fire Ant Resource Area.