Yellow Warbler (Dendroica petechia) Science Article 14
Extra-pair mating has been proposed as a source of sexual selection responsible for secondary sexual traitsthat are common among socially monogamous birds, although supporting evidence is scant. In the socially monogamous yellow warbler, males are larger than females, and unlike females, have extensivereddish streaking on their breasts.
STEPHEN M. YEZERINAC and PATRICK J. WEATHERHEAD, Proc. R. Soc. Lond. B (1997) 264, 527-532