I've left my heat lamp at the same height for the past 3 weeks (4.5 week old chicks). My brooder is fairly large and definitely has space for them to move to if they're hot. They'll go all over but come back and sleep kinda on the edge of where the light is (a foot or 2 away from directly under the light). I was thinking "if they're too hot, they'll move" but now I'm wondering if I should raise the light just to force them to acclimate to a colder (still 70+) environment.