Dickcissels [Spiza americana] are a fun sparrowish bird of the prairies and are considered (ignorantly) as an agricultural pest on their wintering grounds. Populations declined greatly in the 1960s and 70s, but are somewhat stable now.
In preparation for fall migration, Dickcissels begin assembling in larger and larger flocks that gradually coalesce into flocks of thousands. Winter roosts can number into the millions of birds. – Cornell Lab of Ornithology’s All About Birds