Everyone is assuming you live where the air temperature is cooler than the body temperature of baby chicks.
If you live in Equatorial Africa or the Bahamas, for example, or somewhere that has a hot climate, say 35C or 90F, your baby chicks will require no supplemental heat because the ambient temperature closely matches their body temperature so they will not lose body heat faster than they can replace it from food calories.
If you live somewhere much cooler than those temps, your chicks, lacking insulating feathers, will gradually lose body heat faster than they can replace it with food calories, so you need to provide a warmup spot just as you would need a heater on a very cold day, but on a warm day that closely matches your body temperature, you wouldn't need a heater.
It would sure save us a lot of guessing if you would just fill in your profile location.