Why do chickens not try to get up if laid on their backs? I've seen a few videos about chickens being laid on their backs for a few seconds after which the chickens stay in that position until they are turned over. Comments around the video suggested that lying on their backs is a very unnatural position for most birds and so they don't have any reflexes or instincts that would help them get back up. But this is only speculation. Why is that really happening? Here's one video demonstrating the effect: < I've been told Chicken hypnotism is related to this.

This phenomenon is called tonic immobility. And those speculations you've mentioned are wrong: they do get back on their feet, only remain in that state for a prolonged time, which depends on the distance and eye contact with whoever laid the chicken on its back, as stated here.

From this experiment, it can be concluded, that this behavior in which the animal is immobile is a defense against predation, because most attackers sense motion primary.