During the evolution of the hawk species, one particular branch, the males of the Ecuadorian Squirrel Hawk, started attacking other animals to satisfy their own vanity.
The male Ecuadorian Squirrel Hawk would build a nest in the traditional manner. It would then have to attract a mate. The unusual method of attraction used by this bird was not a display of hunting prowess or an elaborate dance. The male hawk would clinically remove the tail from any mammal it could find and then hang these tails from the nest to try and attract a female hawk. Over the years, the squirrel hawk must have deduced that squirrel tails worked the best and so decided it could hunt rats, mice, and other rodents for food, but squirrels should be left alone as their tails were more important than their meat for the preservation of the hawk species.
The male hawk would place the tails in fetching arrangements designed to impress the female hawk. Some hawks would drape the tails over the sticks in the nest to make the nest more comfortable for their potential partners. Other hawks would hang the tails from the nest, where they would sway in the wind and catch the eye of any passing females.
The unusual behaviour of these birds has also led to a change in the appearance of Ecuadorian squirrels, whose tails are, on average, 65% shorter than in other squirrel species. These squirrels also sit on their tails when at rest unlike other squirrels whose tails stick out behind them when they are sitting still eating a nut. It’s also believed the Ecuadorian Ground Squirrel may have evolved from particular families of Ecuadorian Squirrels who lived close to hawk’s nests and who were attacked more than other squirrels.
Extract from Animals Evolution Forgot