Hi,
This is a very common problem with dogs. The reason why he is doing it is it smells good to him. Animals digestive tracks work very fast so feces often carries the smell of the meal it once was. Therefore attracting your dog to it. This is a habit that must be broken because birds carry a large number of parasites as well as other illness that can be transfered via the stools
Breaking the habit isn't always easy and obedience training is a must. You can spray it with bitter apple and he will learn that stool has a unpleasant taste. This is going to be difficult with the number of birds but if you are constant it will break the habit. You can get a water hose and attach a sprayer to it to spray the feces in a large area. It may take a few weeks but most dogs learn quite quickly that it is nasty. There are other products you can try such as Forbid which can be purchased at your local pet store. Since he is eating the birds stool you may want take him to a obedience training class and teaching her the word OFF will be very helpful in curbing the behavior. Your local pet store offers training for a nominal fee.
Here are a few links for learning to train your dog:
How and Why to Train Your Dog
101 Dog Training Tips
Obedience Page - Basic Training
Walter
Breeder
Breeder and pet health consultant. Have spent a number of years with Behavior Training &health care.