Why do chickens sink their head down into their shoulders?

it is generally a sign that they are not well.  Have you checked her over for lice, are her poos ok?  Is she eating and drinking ok,  if she has been bullied she may not be getting her fair share?
She could also be moulting which makes them a bit under the weather.

Sometimes when they have had antibiotics it affects their digestive system because good bacteria is killed as well as bad bacteria. As she has the runs, this may be the case with your girl. You could try putting her on something like Avipro plus to help restore the correct balance in her gut. It wouldnt do her any harm anyway.
Jenny

I was told that it means they're in pain with something ... having lost Gertrude last week, Yoda is now a little off colour. But no abdominal swelling at with Gertie 

My vet charges less for chooks consultation than the cats so hopefully yours will charge less than the dog and if anti-b's etc are needed they shouldnt cost too much. Last vets trip for me was £12.40 for the consultation and enough anti-b's to last a week (they gave me double the amount I needed as well in case I had problems getting it into her).

Keep an eye on her for the next 24 hours, if she isnt eating and drinking I would take her to the vets. Have you checked her crop in the morning to make sure it is empty and also how her breath smells in case there is a problem there? If just today she holds herself funny it could be an off day, but if it persists I would always go down the vet route if I cant identify and treat the problem myself.

Thanks Caralou and Joyfull.  I will try these in the morning.  As I said earlier I felt her crop and then compared to my other two and there wasn't any difference.  Will try under her wing in the morning.  I have a terrible cold at the moment so might not be able to detect nasty smelly breath but I will try.  I will keep you posted on progress.  Fingers crossed everyone please!

Hi everyone,
just to let you know that I took Nugget to the vet this afternoon.  I couldn't find anything obviously wrong with her but she was still sitting in a sad waywth her head on her shoulders.  The vet fees were not too bad, £9.77 for the consultation and about £6 for the antibiotic injection that she had.  The vet wasn't really sure either and did a full check over so we are keeping our fingers crossed that she is ok in the morning.