Sometimes these are called hair worms because they are incredibly thin. They are 1 to 2cm long. There are several species of Capillaria worm but they all live in the upper part of the digestive system, that is, the crop, oesophagus, and proventriculus. They can use earthworms as an intermediate host.
Diarrhoea (usually green), pale yolks, anaemia and birds looking hunched, wings sagging. Death with large infestations.
As with all internal parasites, there are many treatments that will kill these worms. Worming Chickens provides some ideas for worming poultry. Flubenvet is licenced for use in poultry and will kill all common poultry worms.