Wild Bird Shelter ?

I have heard of something for sheltering our feathered friends during cold winters.
I think it was referred to as a 'roosting box' that can be used by many birds at once so they can keep each other warm. Something prob like a butterfly house but bigger ???
A picture ,diagram or instructions, anything to give me the idea so I can make some.

Thanks,
§hirley in Smallfrozentownrednecktractorville, WI