3 Week old chick laying on side with one wing raised?

[SapphireVL]

I have a very mixed (various breeds) flock (14 chicks) who are 3-4 weeks old. I let them out for the first time today in a little pen and while the sun was warm, the occasional breeze felt slightly cool. So just in case, I put their brooder (Brinsea Eco-Glow) warmer in the pen with them so they wouldn't get chilled. They were outside for maybe an hour. I put them back in their brooder and they all seemed fine. It's been a few hours, so I checked on them and most of them are laying on their sides under the heat lamp with one wing raised. What does this mean?

 

[springvalley123]

without a picture, if the wing is stretched out, I'd say they're sunbathing. Are their mouths open (panting) as if they're too hot?