Why are swallows' eyes black? Is this creature evil or not?

The iris of the eyeball consists of five layers of tissue. They are endothelial cell layer, anterior limiting membrane, matrix layer, posterior limiting membrane and posterior epithelial layer. Among these five layers of tissues, there are many pigment cells in the stroma, anterior limiting membrane and posterior epithelial layer, and the amount of pigment contained in these cells determines the color of iris.

The more pigment contained in pigment cells, the darker the iris and the darker the eyeball. The less pigment, the lighter the iris color and the lighter the eyeball color.

Swallow's iris contains a lot of pigment, so its eyes look black.

Swallows are most willing to approach humans, and humans also love this beneficial bird.

Swallows are beneficial birds for human beings, mainly feeding on insects such as mosquitoes and flies. A swallow can prey on more than 500,000 pests and protect crops in one summer. Swallow is a famous summer migratory bird in China. Migrate to the north in early spring and spread all over the country in summer. After breeding, they move south in groups and overwinter in India, Nanyang Islands and Australia.

? Swallows feed on insects, and have always been used to preying on flying insects in the air, but they are not good at searching for insect food in cracks in trees and on the ground. They can't omnivore berries and seeds like grouse and Thunderbird, eat leaves in winter (some conifers don't shed their leaves even in winter), and they can't dig up larvae, pupae and eggs of latent insects like woodpeckers and woodpeckers.