E. Coli is a bacterium that is usually present in the bowel of pigeons. With stress, it increases in numbers. It can invade the bowel wall, causing an enteritis (leading to loss of tissue fluid and an interference with nutrient absorption) and the development of green watery droppings. It also produces a toxin, which makes the birds feel unwell. It is often associated with high humidity (which in itself is an acknowledged stress in pigeons), which keeps the droppings damp in the loft. If this is coupled with poor hygiene or poor watering and feeding practices, this leads to a high exposure to the organism.
Taken from The Flying Vet's Pigeon Health & Management
by Dr Colin Walker