if there is traces of undigested grain in the droppings it would indicate hypermotility of either one or both of the pigeons stomachs(proventriculus and ventriculus). Which usually indicates that they have some form of infection or inflammation. Now inflammation of this area can be signs of many things such as Salmonella, coccidiosis, worms, e.coli, yeast, herpes, hexamita and chlamydia. So i would suggest if you rate the bird, do the best thing and get the birds droppings analysed, if you do not rate the bird and its the only one ill, its nature way of telling you survival of the fittest.