As an emergency measure coarse straw (not feed quality) will allow the water to drain through better than bark chippings, but the first couple of inches will sink into the mud, so you'll need much more than you'd think. All our poultry are under cover at present - in old pig pens, a cattle yard, pole barns .... I'd much rather they could free range but it's chucking it down again right now and the ground is so saturated the grass is rotting in standing water on our flatter fields.