I think the birds are disappearing because of the pollution in the air. Maybe birds aren't able to live in the air. Because of this, naturally, I am upset that they are leaving.
If the birds continue to disappear, it will greatly impact our ecosystem. This is because birds control their prey population, which is usually fish, birds, worms, etc. so they will overpopulate and die out because they will not have enough food to prey on. Another reason is that their predators won't have food, so they will die out as well.
Hope this helps! :-)