I have searched through the forum a few times and see a lot of discrepancy between all of the recommended heights. So far I've come up with a minimum of 7" and if you are planning on going over 18" you need it to be a lot taller so that they will not hit their heads when startled. For below 18" the idea is that they will not gain enough momentum to do serious damage to themselves. If there is anything I misinterpreted about that let me know. My real question is if 8-9" height will accommodate the jumbo variety of coturnix. I'm not sure how large they are, so it makes it difficult to decide. I really don't know if I will be getting jumbo coturnix, but I figured that it wouldn't hurt to make the cages large enough for them just in case.