roost \roost\, n. as. hr=ost; akin to od. roest roost,
   roesten to roost, and probably to e. roof. cf. roof.
   1. the pole or other support on which fowls rest at night; a
      perch.
      1913 webster

            he clapped his wings upon his roost.  --dryden.
      1913 webster

   2. a collection of fowls roosting together.
      1913 webster

   at roost, on a perch or roost; hence, retired to rest.
      1913 webster
see also:
roof at roost