Genetic heritage
A male with long tail feathers may also have genes that provide resistance to parasites. The genes and the resistance they provide are passed on to the mating pair's offspring.
Female's choice
Males with long tails produce more offspring than those with shorter tails. For females that choose wisely, the rewards are better health and survival chances for themselves and their offspring. In short, the females tend to select wisely among the males. How did that come about?
Heritage, selection...
An inherited character which provides good chances of survival and reproduction will be transferred to many birds in future generations.
...and time
Assume, that this has also occurred during several preceding generations. Eventually, the majority of individuals possess the character. Therefore, today's females choose high-quality males – and quality is judged by checking out their tail feathers!
This process is known as "evolution through natural selection". Since the natural selection illustrated by this example is done by females in connection with reproduction, it is also called "sexual selection".
