Your bird dog has been a strong, eager hunter with lots of pizzazz and snap, but now he has lost that snap, speed and stamina. He still eats well, is bright and alert with normal bowel movements. Have you over-trained the dog or is it poor nutrition, heartworms or intestinal parasites? Probably not!
With today’s good quality rations and good parasite control programs the odds are good that these are not your problem. Unless you know you have over-trained either with excess work or electrical stimulation that is probably not the cause. Probably your dog has an unapparent health problem. Although the dog eats well and shows no obvious signs of the cause of his poor performance, it is very likely that there is a physical problem.
Possible causes include tonsillitis, low grade kidney or liver disease, low thyroid enzymes, nocordia or actinomyces infections associated with seeds usually in the respiratory system or abdominal cavity, arthritis, hip dysplasia and heart disease. These and other causes may not be readily obvious and will require appropriate testing and radiographs to diagnose.
It is very frustrating when a good dog’s performance becomes inadequate but there usually is a cause and that cause is usually a health issue. If you notice a change in your dog's temperment or performance, get him medical attention as soon as possible.
Always check your local and state regulations related to dog training and the use of game birds on private and public property.