Bald spots on chicks' wings

[Mos]

Hi!
All 5 of my chicks have more or less visible bald spots in the exact same place.

They are 2 weeks and I've not witnessed any of them pulling out feathers or down of each other or themselves, which makes me wonder why they are getting these bald spots. Is this normal? They really enjoy flying up and down from branches I've put in their brooder, could their down have been worn off from their wings flapping together quite a lot? Or is it more likely one or more chicks is pulling it out? It just seems so weird to me that they all have it in the same place..
Anyone know what it might be from and whether I should do anything about it or not worry too much about it?

 

[Wezdin]

Do you use a heater plate in your brooder? if so the fuzz might have been singed off from that.

 

[Mos]

Ahh I do!! I will raise it more right away! thank you!!