growth on chick's head-any idea what it is?

[brittabake]

I have 15 chicks that are ~4 weeks old. They are mixes of Old English Game and various bantams (a result of my neighbor's spare rooster discovering my flock and moving in...lol..another story!) Anyway, all the chicks are active and healthy. But, I noticed this growth on one of their head's today. Only one chick is affected and he/she seems fine otherwise. These chicks have never been around any other chickens except for my flock and the rooster and no one else has anything like this. The bump is hard and crusty, well defined, no discharge, doesn't seem to bother the chick. Anyone have any idea what this might be? https://www.backyardchickens.com/forum/uploads/31730_img_7321.jpg
https://www.backyardchickens.com/forum/uploads/31730_img_7318.jpg
https://www.backyardchickens.com/forum/uploads/31730_img_7320.jpg

Thanks!