If you have a large number of birds what we do at the camp is this:
We have two plastic trash cans. A large one and a smaller that fits inside the larger one. We put a layer of ice down in the large can and then put the smaller can inside, bottom on the ice. We put another layer of ice in the smaller can. Then we put a 5 gallon food grade bucket with the brine and birds in the smaller can. Then we fill the whole mess with ice and put the lid on the big can. If it's in a shady place the temp will stay below 40 in the can, just don't open it until you're ready too get the birds out. This works really good for an overnight brine when you don't have frig space or electricity and the temps are cooler.