Can anyone tell me what this might be ?

[poulet]

https://www.backyardchickens.com/forum/uploads/7478_dscf0003.jpg
She has lost these feathers while she was sitting on eggs (about three weeks ago ) . I thought it may be normal but am now worried as gradualy all but one of her chicks have died with no apparent cause ?,
I also saw today that another one of my chickens has started go go bald like her too.
She sleeps seperately to the other chickens but should I confine her ?
what could this be ?

 

[chickenlady]

Check for mites during the day and then again at night. some mites only feed at night time.

 

[poulet]

how do I check for mites are they like fleas? could that kill chicks ?