Unidentified Birds

In a field near where i live i keep hearing a loud `sqeaking` type bird call...almost like an owl or sparrow hawk type call.  Yesterday as it was dropping dark about 20/30 birds flew in & went to ground. They were all making this noise as if calling to each other.  It was quite loud.  The birds never took off again.  Looks like they were there for the night.  Even tho it was nearly dark, they appeared quite dark in colour but appeared white underneath their wings.  They were about the size of a crow.  Wondered if they were Starlings but i thought they roosted in trees.  Any ideas anyone????