a simple solution - kill them. blow their little heads off with your rifle and sell their remains at the market to earn money. you may want to shoot a few birds out of the sky, they occasionally drop an egg when they`re shot. if you manage to shoot the egg out of the sky, you can sell the eggs at the market as well. don`t worry if you`re a bad shot, you could hit an oil field and earn some additional ka-ching. 