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.
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            He clapped his wings upon his roost.  --Dryden.
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   2. A collection of fowls roosting together.
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   At roost, on a perch or roost; hence, retired to rest.
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