Thanks to 
The willow warbler (Phylloscopus trochilus)
One spring, around the beginning of May, I heard the barely audible voices of birds overhead. I began to look for who publishes them, and I saw. These birds jumped together on the branches.
To us, these birds seem the same, that the male, that the female, but for birds everything is completely different, their vision is different from ours.
One bird flapped her wings strongly, and in general it was more like games.
The male kept trying to climb the female, but he succeeded with great difficulty. The female moved briskly and did not give in. The male must show his strength, prove that he is strong. So birds choose the strongest males in order to pass on the strong genes to their future children.
