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Almost all the time there are no birds in the room. The mean concentration is, therefore, aproximately zero birds per cubic metre.On the other hand, the concentration of birds outside is higher than this.Diffusion takes place to reduce the difference in concentration.On average birds are more likely to fly in.However, once there's a bird in any normal size room the concentration gradient will be reversed and the bird is more likely to leave than to be joined by another bird.