On a nearby beach, the dropping tide produces a large mud flat that at this time of year becomes a nursery for various marine birds. This also attracts the predators, usually Bald Eagles, that eye perspective meals from the surrounding trees. Quite often the eagles will hunt in pairs and you know something is happening from the alarm calls of the Canada Geese and seagulls.
The Mergansers seemed pretty calm after their ordeal. I had often wondered why Mergansers had so many chicks and this is probably one of the reasons. It was very easy pickings for the Eagle.