I have reviewed your comments and have been very interested in watching your videos of the intruder eagle.
Territorial battles are quite common amongst eagle, loons, and most birds that are not colonial nesters. I have seen knock down drag out fights between all sorts of birds. And I have been surprised that we have not seem more behavior of this sort at this nest site, since it is in a prime location—relatively protected from weather and close to foraging areas. They are also common after a nesting failure.
There are a number of eagle nests in the area, and there are young adult eagles that are attempting to establish territories. This territorial battle is a good sign for a number of reasons.
First it demonstrates clearly that the pair are quite attached to the nest site and are defending it again intruders. This indicates clearly that the pair has not abandoned the nest site after the failure.
As many of you noted that the confrontation was between two males. In reviewing the videos it’s not clear to me which is the territorial male and which is the intruder. It is possible that we could be witnessing a mate switch. It is often the case with birds that they are more attached to the nest site and territory rather than each other.
If you can, please review the video from the beginning and see if you can see a difference between the males, and see as they change back and forth on the nest if you can tell if it is the same male at the beginning of the confrontation as at the end. It appears to me that one of the birds had more white feathers showing on the body.
Another reason this is positive sign, is that it shows that the eagle population as a whole in increasing and that young strong birds are trying to establish their own territories—often they will try and take over another rather than establishing their own. This younger cohort of eagles, are extremely important to the population as a whole. Since eagles live so long and reproduce relatively slowly, it is vital for a stable or growing population to have a large group of younger birds available to fill if a breeding adult leaves.
Finally, even though this battle looked intense to our eyes, birds, like dog, people, and most animals do every thing they can to avoid physical danger. In watching one of the videos a saw a lot of calling, dive bombing, and little if any physical contact. If you think about people, generally people yell at each a great deal before getting into a physical alteration—same with birds.
Keep up the great observations.
Wing Goodale, BioDiversity Research Institute