I agree with Miss Lydia - lime used around the area, under the pen or in any place that they play(water fowl are great at digging holes in the puddles and causing the topsoil to wash out over time due to filling/dumping playpools, etc) and it will actually keep your yard/dirt healthy. But I wouldn't use the lime until you can move the birds to another spot. Lime can be caustic and eat away at little feet, bad for mucous membranes (eyes, respiratory, etc). Lime will help break down the bacteria in the soil that causes the stink. Adding more of a 'mulch' type compound will eventually build up and you'll have to physically scrape it and add it to the weedbin, but in a quick fix it would work as long as you put lime down first. You want to get rid of the bacteria and not give it a place to hide so I'd try a little yard/garden lime from the feed store and see where that gets you. I have the same problem - not wet all of the time, but it can get humid and the still air holds in the smell, it seems. I pick up the fowl house and move it to the other side of the yard every few months.