How does skin prevent entry of micro organism in the body

The skin functions as a barrier to prevent the entry of microorganisms.

A hawk eats by finding prey on the ground. Better eyesight in a certain individual would increase it's likelihood of finding food. There is already competition among a population for food, which is generally limited. The individuals with better eyesight would be more likely to survive. The frequency of the better eyesight trait would increase, assuming that the individuals with this trait are more likely to reproduce and the offspring were more likely to survive.  If the birds with better eyesight also had weak wing muscles, they likely wouldn't be able to catch food and survive.