I've always used them, and like that there's good air flow in summer.
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Yes, it's a mess! The sill in front is too low, they kick out bedding all the time. And the 'roof' isn't sloped enough, so birds sit on it and make another mess. The shelf they sit on has big openings, so it's easy to clean off. Each box has an empty feed bag in it, with shavings or hay on top as bedding. There's a row of them at the back wall, and another two in the annex coop section, all 24" above the floor.
Mary