Hi All. 2 years ago, Park John and I observed a Adult Bald Eagle and a Greater 
Black-backed Gull at Eastern Neck Island do this same behavior. The Canvasback 
would pop up, and then the Eagle would pounce. The Canvasback would then dive 
again and slowly but surly got wore out. Diving and diving over and over again. The 
Greater Black-backed Gull was joining in on the killing. What he would do was grab 
the Canvasback by the neck and hold its head down under water and also take feather
 plucking jabs at it. This joint effort killing took about 20 to 30 minutes. It was a horrible
 way to watch something die, but this is the way of the world.

Sean Candles