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Pretty much what it sounds like. A deep pan is hung or affixed directly beneath the roosts, to catch the droppings. A screen is placed on top to keep the birds out of the water.
When they poop, the droppings go into the water and collect there. This binds the nitrogen and dilutes it, as opposed to it forming ammonia nitrate as it dries in the air.
Affix a valve to the pan so you can drain the water and manure into a bucket, for instant 'manure tea'. This goes into the garden or compost heap. Wash out the pan, refill it and do it again...