Chicks need temps around 90 for a week or two, then temps in the 80's is fine until they are feathered out, at which point, they can take quite cold temps, as they are acclimated to them.
The bulb could produce too much heat in 90F weather, frankly. Be careful. It just doesn't take much to cook them. Give them space to get away from the heat circle. This discussion is foggy and lacks clarity if one doesn't speak about the ambient air around the brooder. If you'll have them out in the garage or shed, it will be plenty warm, it sounds like during the day, as is. Just some supplemental heat at night when it drops into the 70s.
Raising 70 degree ambient air is easy and doesn't take much. Raising 50 degree ambient air takes a bit more. Hope that helps.
Watch the chicks. Their behavior is Far Far better than any thermometer. If they pile up ad cheep, they are cool. If they hold their wings or hide from the heat circle and pant, they are too too warm.
Edited by Fred's Hens - 8/31/12 at 7:40am