Dimorphisms

Most people will be familiar with sexual dimorphism, famously explained by Darwin as a consequence of sexual selection. Many birds provide striking examples of sexually dimorphic plumage.

Wood duck pair (Aix sponsa)

 

Baltimore Orioles ( Icterus galbula)

In other cases, like the white-throated sparrow, there are two colour morphs, but both sexes can have a tan- or white-striped head. In this species, the head stripe colour is curiously linked with a personality difference (the genes for color and personality are linked on an inverted bit of chromosome that can't recombine). Tan-striped birds tend to be more placid, and make better parents, while white-striped birds are more aggressive, and promiscuous. 

 White-throated sparrow (Zonotrichia albicollis)

 An amusing irony is that white males that spend more time courting other females and less time mate-guarding, are cuckolded at a higher frequency than the tan males. Similarly, white females that spend considerable time engaging in rumpy pumpy away from the nest are subject to higher levels of conspecific brood parasitism (where other females dump eggs in an absent mother's nest) than the stay-at-home tan females. Both white males and females pay the price of extra-pair copulations by running the risk of bringing up someone else's children. 

Why, you might ask, do both colour morphs seem to remain at about 50% in populations of white-throated sparrows? A probable answer is that opposites attract, and tend to produce more offspring than two parents of the same colour morph. Two tan-striped sparrows may be dedicated parents, but they get bullied into making do with paltry territories, while two white-striped birds would spend so much time being asserting themselves that their offspring don't get as much to eat.

References:

L. Y. Huynh;D. L. Maney;J. W. Thomas 2011 Chromosome-wide linkage disequilibrium caused by an inversion polymorphism in the white-throated sparrow (Zonotrichia albicollis) Heredity 106: 537-546.

R. W. Knapton;R. V. Cartar;J. B. Falls 1984 A Comparison Of Breeding Ecology And Reproductive Success Between Morphs Of The White-Throated Sparrow Wilson Bulletin 96: 60-71.

R. W. Knapton;J. B. Falls 1983 Differences In Parental Contribution Among Pair Types In The Polymorphic White-Throated Sparrow Canadian Journal Of Zoology-Revue Canadienne De Zoologie 61: 1288-1292.

E. M. Tuttle 2003 Alternative reproductive strategies in the white-throated sparrow: behavioral and genetic evidence Behavioral Ecology 14: 425-432.