i wouldnt put anything from outside in the brooder, you risk getting infested with something ... best way to do it if its a warm day is make a small wire pen out of hardware cloth, set it up with a couple of trays you can wire to the side for food and water, and you can transfer the chicks to it and work the bottom into the dirt under a tree or something .. then in the eve you can transfer them back to the brooder ... at 3-4 weeks, if its warm enough outside and you made the pen large enough, you can add perches and cover it with a sheet, throw a piece of plywood etc on top with a cinder block to weight it down and they can just stay in it as a pre-coop living quarters so you dont have so much to clean, you can just move it around to give them a clean floor daily, wash in the other spot with the hose ... your older birds will get used to them better with this method also ..