Poo on chick bottoms, feathers look odd/thin on bottom and under wings

For the last three days my larger 5 week old chicks (all seemingly of the same breed) have poo sticking to their bottoms. There are 14 of these with the same problem and they are the larger chicks in the group.

It looks like their feathers are either not yet in, or have been picked out on their bums and under their wings.

The smaller chicks are fine - no similar issues.

I have been washing them off, but the problem persists.

Is it chiggers? Lice? Something more dire?

My local extension office said "I don't know anything about chickens." Very helpful.

 

Thank you everyone. I will try the olive oil today.

They are on Chick Starter feed and we give them grass clippings (plus they joyfully lunge at any moth that dares enter the coop. I have been putting vitimins in their water until yesterday.

I removed the brooding lamp last weekend since they have been under it for 4 weeks and it is hot in Tennessee. I can put it back in...

These chicks are about 4 times larger than the other ones - Does growing so darn fast cause problems?

Trying to et pictures today...

 

After cleaning and oiling the formerly mentioned posteriors, I noticed evidence of the feathers having been pulled out (bloody spots where they are pulled). Is this bugs?

I can put the little guys out in the yard during the warm part of hte day and dust the coop, but what can I put on the birds to make them more comfortable?

Shoudl I separate the plucked babies from the others?

They are alert and running around, though they are not as spry as their smaller friends.