Here's a note I got from Dr. Jim Siegel on my staff about the nest visitor yesterday. Jim has studied sanderlings, scrub jays, great blue herons, brown creepers, hermit thrushes, and a few others pretty intimately, including their territorial, reproductive and feeding behavior. Observing eagles is a hobby for him, so here are his thoughts:
"Birds are instinctual creatures. If the male is bringing in fish without young in the nest (Feb 4) , he is provisioning the female - feeding her as if she cannot leave the eggs or young and making sure she is fit and healthy to reproduce. The fish may have attracted the sub-adult (Feb 5)
I am not convinced the sub-adult is a fledgling from 3 years ago just hanging out and visiting its parents. Fledglings of long-lived predatory birds have to disperse widely from home or they will be constantly competing with their parents who may live for decades in the same place. Every young born cannot stay home and wait for their parents to die; they may end up waiting for years. They will compete with their parents (and the new nestlings) for food and space every year. And every year more young competitors are reared. The gene line loses in that case.
Eagles don't live in family groups - none of the predatory birds do. But jays and crows do and these birds try to live at the borders of their parents territory, hoping someone will either die or they find another unoccupied territory nearby that they already know well due to its proximity to their natal territory. Longevity: 6-7 years for jays and up to 20 years for crows. Most small songbirds only live 3-4 years at most.
If the sub-adult bird (I don't know perhaps 2-3 years old?) acts juvenile enough, the male or even the female may act as if the sub-adult is the bird they, in their tiny bird brains, are thinking (?) they have hatched out of the eggs that have not even been laid yet. They may feed him or tolerate him as if he was their 2015 fledgling! This before those 2015 fledglings even exist. Its hormonal not rational.
That is why you get chicken hens brooding kittens, and songbird feeding gaping goldfish mouths in a backyard pond. And lions that try to care for young antelope. Maternal and paternal Instinct takes over and strange things happen.
The other possibility is that subadult is trying to displace the adult male or female bird. It is not likely, but it is possible, that the sub-adult is in better physical condition than one of the adults, and although sub-adult in plumage, it is masculine or feminine enough in its genetic fitness to convince the rival bird and its opposite sex, that it is a potential viable territory holder and mate.
I am learning stuff from these birds all the time."