From Sialis.org:
Bluebirds: Parasitism in bluebird nestboxes with properly sized holes is not common. Cowbirds occasionally parasitize nests in nestboxes. The female is capable of squeezing through 1.5″ hole, however they probably prefer a larger hole (1.75″ or maybe a Peterson hole). Keith Kridler has found Cowbird eggs right beneath a bluebird nestbox entrance hole, and wonders whether the tight squeeze will sometimes “pop” an egg out of the female as she enters the box.
Cowbirds are tricky…detailed information on them can be found on the Sialis website.
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