The most common ailment developed by racing pigeons in a state of 'stress' is known as 'Escheria Coli'. This occurs when the bacteria in the intestines is brought into a state of imbalance, such as when 'stress' has robbed the bird of its appetite. Thus, an increase in Escheria Coli creates watery, green droppings, sometimes associated with a jelly-like substance. The next stage is 'Going Light' and if something is not done quickly the pigeon dies.
Taken from Ailments & Diseases of The Racing Pigeon
by Old Hand
