"Occasionally the redstart builds a new floor over the Cowbird's eggs, as is often done by the yellow warbler. This procedure is especially likely to take place if the cowbird succeeds in laying its eggs before the redstart's eggs are present.

More often the redstart does not seem to be annoyed by the strange eggs but goes on laying its own. It is unusual for the redstart to desert its nest because of the presence of cowbird's eggs." - Birdzilla