Cornell University ornithologists have discovered that even after the female barn swallow has made a long term commitment to a male, the female continues to look for additional sexual partners.
And it's not like personality counts for much for the females. The females judge potential significant others based on the reddish color of the males' breast and belly feathers.
A faded breast and the female sneaks out of the nest and secretly copulates with the better looking male.
"The bad news for male swallows is the mating game is never over," said Cornell researcher Rebecca Safran.
Thanks to all of this furtive funny business half of all male barn swallows typically care for at least one chick fathered by another bird.
Presumably, darker breast feathers signify a stronger, healthier, more prestigious barn swallow.
Add it all up and we see the evolutionary advantage of the mullet.