How big is the brooder?
Crowding almost always causes behavioral problems.
If the chicks are fully-feathered and have been acclimated to outdoors then 45 is not a problem. If they're not fully-feathered or have been kept under heat with no exposure to cool outdoor air then they have to have heat still.
This is my brooder -- outdoors, with my Easter hatch in it.
https://www.backyardchickens.com/articles/run-to-outdoor-brooder-conversion.76634/
They were day-old, new hatched chicks with temperatures below 40F expected overnight. That's why instead of just having either the plate or the Big Red Bulb they have dual heat.
In the 50F daytime temperatures they were running around the brooder, ducking back into the warm spots when they needed too -- perfectly fine.
The 6-week-old chicks in their integration pen in the big coop had had all their feathers from 4 weeks (mostly Australorp crosses so fast-maturing), and I didn't take any precautions about them except that their pen was, of course, set up with draft-free shelters available.
I'd booted them out of the brooder when they stopped sleeping anywhere near the heat plate but, instead, chose the corner by the vent.