'Stress' can bring on illness and death. Racing pigeons are not flying machines but thinking animals who are subject to mental upsets and reactions. They can fret over many things such as the loss of a mate, incarceration in the basket, irregular meals, the 'night out' when sitting eggs or squeakers, and so on. To fret is to worry and to worry unduly invariably leads to loss of appetite. You will have noticed that loss of appetite precedes most pigeon ailments and diseases.
Taken from The Ailments & Diseases of The Racing Pigeon
by Old Hand