Dimorphism
The size and shape of the beak can vary across species as well as between them; in some species, the size and proportions of the beak vary between males and females. This allows the sexes to utilize different ecological niches, thereby reducing intraspecific competition. For example, females of nearly all shorebirds have longer bills than males of the same species, and female American Avocets have beaks which are slightly more upturned than those of males. Males of the larger gull species have bigger, stouter beaks than those of females of the same species, and immatures can have smaller, more slender beaks than those of adults. Many hornbills show sexual dimorphism in the size and shape of both beaks and casques, and the female Huia's slim, decurved bill was nearly twice as long as the male's straight, thicker one.
Color can also differ between sexes or ages within a species. Typically, such a color difference is due to the presence of androgens. For example, in House Sparrows, melanins are produced only in the presence of testosterone; castrated male House Sparrows—like female House Sparrows—have brown beaks. Castration also prevents the normal seasonal color change in the beaks of male Black-headed Gulls and Indigo Buntings.
Read more about this topic: Beak