While younger birds can probably survive cold temps, until they are fully feathered out (usually at 6 weeks of age) chicks can't properly maintain body temps while sleeping in cold weather. Huddling together is a sign that they may be too cold, and potentially can cause chicks to pile on top of each other, sometimes smothering the ones on the bottom. If you are putting them outside before 6 weeks, a supplemental heat source will increase their comfort and protect them from chilling - just like their mom would.