It was a drama I had only seen once before, but this time it played out in a rather different manner.
A Bald Eagle is just too big to be able to hover over one spot except by flying into a rather high wind. And that is the way I saw it before: in a high wind, an eagle hovered over a merganser and tried to capture it. This time, the high wind brought a four-year-old Bald Eagle to try to capture a mallard, or probably, its chick. Given the appearance and the noise of the wind, the mallards may have been unaware that they were being targeted.
What happened next was unexpected. An osprey appeared, attacked the eagle, and drove it off. Why did the osprey do it? I haven’t a clue. But, the mallards swam off as if nothing had happened. 
Drama on the high seas
Any more thoughts on why the osprey dove in and attacked the eagle?
Finn, I have only see an osprey attack an eagle once before. Five years ago, an eagle tried to steal the fish from a male osprey. The wife then attacked the eagle and drove it off. That does not seem to be the motivation here. I am mystified. (Note added: see my later comment, below.)
Osprey will chase an eagle away as they are a threat to the young fledglings. A friend saw an eagle kill a young osprey that just learned to fly; it managed to separate it away from it’s sibling and an adult. The ones I watch will scream and take off to protect in two scenarios; when an osprey encroaches on it’s territory, and when an eagle shows up. Three actually, I watched one chase two Great Blue Herons until one of them turned on the osprey and started chasing it.
So I suspect the osprey had no interest in the water birds, simply that an eagle was nearby, was distracted and could be harassed.
I agree with Derek. I was watching an osprey nest last week when one of the parents suddenly left the nest and moved east as if on a mission though I saw nothing until a few minutes later when the osprey began to tangle with an eagle several hundred meters off that direction. I realized then that the osprey have a large “safe zone” around their nest, much larger than I had thought. Wonderful images, Alistair
Three correspondents suggest that the osprey’s behaviour was territorial. I suspect that they are correct.
One pointed out that a Nova Scotia study of osprey and eagle conflicts suggests that “In general, Ospreys and Bald Eagles were tolerant of each other.” Certainly, I only occasionally see conflicts even though I live within two kilometres of a nest of each. Leaving aside an eagle stealing food from an osprey, a question remains as to what prompts such less common conflicts. While the osprey wasn’t trying to protect the mallards, the attack was certainly a consequence of the eagle’s attack on the mallards.
The osprey’s timing was probably driven by eagle distraction. Maybe there would be many more attacks if the other species weren’t always on guard. On this occasion, the eagle was preoccupied by hunting mallards.
Further, during bird attacks, altitude seems to be paramount: always attack from above. The eagle was not only distracted, it was near the surface allowing the osprey to get the upper hand, er.., wing. I attach another shot showing the osprey above the eagle.
So, territorial, yes. But, might the timing be driven by the osprey taking advantage of the eagle being both distracted and near the surface. If so, it isn’t so much that the two are “tolerant of each other” as it is that they are wary of each other.
This morning on the Nelson waterfront I watched an Osprey (with its mate watching) half-heartedly attack (from above) a Bald Eagle perched across the river. It looked like it could be the same eagle (not quite adult). The Osprey took a few short dives within a couple of meters but the eagle never budged. It did not look like the eagle had any prey.
I also suspect the relationship to be not so tolerant, more of an uneasy truce. I think if this eagle is not an established local breeder, it will get a rougher ride from the local Ospreys (and local eagles).
I suspect that some individuals are more cantankerous or more anxious to impress a mate and will take any chance to harass a potential threat.
Max, I am partial to the hypothesis of the cantankerous osprey. I note that it was in the same area and about this time last year that I watched an osprey attack a heron and drive it into the Lake. Now, a heron hardly poses a threat to an osprey, so maybe I just happen to live near an ill-tempered osprey. That is good for pictures, but not good for other birds. I wonder if we could train ospreys to take an interest in geese.
I would have to do some reading but I think GBHE could pose a threat to Osprey nestlings. I know some Bald Eagles can’t stand having GBHE near a nest, and will vigorously chase and even kill them even though they probably don’t pose much of a threat to an adult, and there are probably easier and safer prey to catch for an eagle.