Interesting link Emma, thanks for posting it! Fortunately the cowbirds haven't taken over here, but I've been watching the starling population explode.
@chebeagueschool check this link
http://www.wqed.org/birdblog/2008/05/page/2/
it explains why you have seen plain eggs. The cowbird sneaks in the catbirds nest and lays her eggs. Read the section on Cowbird vs catbird.
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