In an ordinary ABC dry chemical multi use fire extinguisher is monoammonium sulphate. Other than a respiratory irritant it is non toxic. It is however extremely corrosive to electronics and wire. It's basically salt with baking powder. Each partical is coated in silicone so it does not stick together (caking) that's why it seems like the perfect powder when sprayed and stays suspended in the air. It's also pressurized with nitrogen because nitrogen has no humidity in it also to help it not to stick together on a molecular level. The salts cover the fire and melt and form a layer between the fuel and o2 so it smothers the fire. Due to the salts it will burn if you have a cut just like regular salt would. Your chickens should be ok