We have an older Isa Brown who's showing distinct signs of illness - she's gone off her food, huddles up on her own, (and when she joins the other three they peck her) and is flicking/twitching her head. She's an older hen - hasn't laid for a year or so but has been in great health. Other information: - she's not lost any weight (she's quite fat since she stopped laying!) - she's still drinking water - her poop is greenish - there are no signs of discharge from the eyes, scabs or any time or coughing - she's been fully wormed and mite-treated very recently - she sleeps in a sheltered coop, on straw - we had a stagnant pond in the back yard and I saw her drinking from this, rather than their clean water bowl (the pond has now been removed) - the other three hens are in good health We've not taken any action except isolating her as we have no idea what the problem is, so don't know how to treat her. Can anyone help as to what it may be? Thanks.