Yes. As the air they displace pushes down upon the place to force an equilibrium of weight. I also think mythbusters broke this myth in a show too. So yes, the plane is heavier with birds on it, flying or not.
They do. If i sing i normally mimic voices, so i sing in the accent of whoever is singing. But take... The Beatles. They sound english.
No, it comes out of about 15 anyway. The nipple has 15-20 small "holes" for milking. The extra ones just make them a bit bigger.