at roost

from The Collaborative International Dictionary of English v.0.48

Roost \Roost\, n. [AS. hr[=o]st; 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]