was a PITA for me to get that sucker easiest way I found was to fly to the city he seems to usually frequent a route near, and check the pokedex. If he isn't there just fly again to the same city, and repeat. It took me 30-40 mins after I stopped trying to fly all over chasing him to get him that way
Also once you get to the route check the pokedex (so wish there was a fast way to check...) to make sure he is there, and after 3 encounters check it again as he might have left (happened to me a few times)
The roaming birds are just like any other roaming pokemon; sometimes they'll stick to a simple route, going from one to the next; sometimes they'll jump from somewhere like route 7 to route 22. If you fly, the birds will jump. However, if you change locations (routes) then the roaming birds won't have a chance of moving to another location. After encountering whatever bird you're trying to chase 11 times (if I recall), then it will go to the Sea Spirit's Den, and is then battle-able. --- I like superheroes because they doesn't afraid of anything.