Baby chicks need to stay under a heat lamp until they are fully feathered. This takes anywhere from 6 to 8 weeks. The reason they need to be under a heat lamp is that their bodies are not yet developed enough to regulate their own body temperature. Without the external heat source, they would quickly become too cold and die. However, once they are fully feathered, they no longer need the heat lamp and can be moved to a regular chicken coop. It is important to make sure that the transition is gradual, though, as sudden changes in temperature can also be dangerous for baby chicks. Start by moving them into an area that is a little cooler than where they were under the heat lamp and gradually move them into cooler and cooler areas until they are acclimated to the temperature of the chicken coop.