Featherless Bellies on Cornish Cross

[MOlstad]

I have a flock of six Cornish Cross, this is my first flock of Cornish Cross, but I had Cochin's in the past. The birds are about 4 weeks old now, and this morning when I fed them I noticed that they have feathers missing on their bellies. They looked fine prior, but I'm sure the feathers didn't fall out over night. I haven't seen any feathers in the coop. The coop had straw for bedding for a while, but I pulled all of it out this morning thinking that it might have been irritating their bellies. The birds are fair bound and I only have until the 24th of this month to get them looking better. Is there anything I can do to help them along? Is straw a poor bedding choice for them? I attached a picture, this one appears to be the worst one.


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Sorry about all the questions, first post and first flock, I had issues with Turkey's earlier in the year and want to catch the problems early.

 

[Poultry parent]

chickens have feather tracts (places where feathers grow) and featherless tracts (places where they don't) so it's fine.