I, too, have seen dogs that have lost interest in pointing pigeons but that remained reliable on wild birds, so this may indeed be the case. But there could be other factors involved. If I can get a bit more information I may be able to help:

1. Were you telling the dog to " whoa" as he came upon the birds?

2. Was he wearing an e-collar?

3. Was his yard work done with the aid of an e-collar?

4. Is his yard work complete?

5. Was he doing the bird work in the same place as the yard work?