I'm wondering if you got a show line dog. I saw an english setter the other day, medium size but stocky, kinda built like a lab and with a blocky head. Feathering was short like mine though. Sorta like a show E.Pointer I once saw. Didn't look athletic.

Anyhow, every dog is an individual. Some develop sooner some later. You'll will know when you should introduce birds. Your pup will start nosing more, stare at moving creatures, that sorta thing. Took my pup 5 months, her sister two months sooner, other littermates are just getting it (7-8 months). If there is that much variations in one litter then imagine the variations in differing breeds and differing dogs.