unique experience and migration question

[metlhead]

Labor day found me out fishing and I noticed what seemed to be an eclipse drake gadwall swimming around. Some small flocks also came by and landed. No biggie other than I was 41 mies off WP looking for tuna. The flocks were coming from due west. Reason I saw the first was some water commotion had me steer in that direction and my dog was pointing from the bow. Shark attack took one. The birds left but the next flock landed very near. Where from and why?