Bird species often demonstrate intersexual selection, perhaps because - due to their lightweight body structures - fights between males may be ineffective or impractical. Therefore, male birds commonly use the following methods to try to seduce the females:

• Colour: Some species have ornate, diverse, and often colourful feathers.

• Song: Male birdsong provides an important way of protecting territory (intrasexual selection).

• Nest construction: In some species, males build nests that females subject to rigorous inspection, choosing the male that makes the most attractive nest.

 

Males are more colorful or ornamented than females in most, but not all, bird species. Understanding this phenomenon requires a basic grasp of the evolutionary forces that shape the behavior and morphology of individuals and species. Charles Darwin developed much of the theory that helps explain this. He proposed that traits promoting survival in individuals are favored by the process of natural selection, whereas traits that help the individuals of just one sex (usually the males) compete for mates are favored by sexual selection. Sexual selection is responsible for many of the features unique to one sex in a given species. These features can be divided into two general categories: those acting as weapons that allow males to fight for access to females (antlers on deer, for example) and those acting as ornaments that attract the attention of females, such as long tails on birds.