"Even after they have paired with a male, female North American Barn Swallows still comparison-shop for sexual partners. And forget personality; the females judge males by their looks--the reddish color of the males' breast and belly feathers. If the male's red breast is not as dark as other males in the population, the female is more likely to leave him and then secretly copulate with another male, according to a Cornell University study featured on the cover of the journal Science (Sept. 30, 2005)."
Link: Full article from Cornell Labs
Barn Swallow image © Michael Allen McDowell