Chicks Sleeping On Top of Brooder Plate

These chicks are about 4 weeks old, "Rustic Rock" and "Americana" aka Easter Egger chicks. I've noticed the past week or so, they no longer sleep under the brooder plate. They pile up on top of it. I've raised chicks before and I would think that means they don't *want* the heat but I'm not quite sure they are feathered enough to fully go without. One for sure is a little slower feathering than the rest and I will post a picture of her because she is the least feathered. The rest are a bit further along (pics 2&3). They make happy cheeps most of the day (the runt one "Pepper" has begun flying at/pecking at her sisters a bit but it seems like normal pecking order stuff- no blood drawn) but by and large they do not seem distressed or unhappy at all.

Just wondering if they can just go without the brooder plate altogether at this point. Chicken pics incoming....