I knew a guy who had a mastiff that I swore was half labrador. She was a smaller dog for a mastiff and was extremely athletic. That dog absolutely LOVED playing fetch and was an awesome tracker. She readily tracked and bayed deer and was one of the few dogs I'd had the pleasure of watching track a wounded deer.
That was the ONLY mastiff that I knew of though that was like that and she was a purebred. Most purebred mastiffs are not gamey at all and have no real desire to track any animals. They're quite content to lay there and watch the game animals run across their field of view.
Your mastiff was a mastiff/boxer mix you say? That explains the prey drive. Boxers have an extremely high prey drive and are natural hunters. They aren't normally used to hunt birds because of their tendency to eat the game they track too but they're still a very high prey drive dog. Mix that with a mastiff and you just might get a mastiff mix that loves to hunt.
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