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The eagles Parham photographed no doubt brought the Red-tail to their nest intending not to raise it, but to feed it to their own nestling. However, when it was deposited into the aerie, the hungry and disoriented fledgling immediately began begging for food alongside the eaglets. The confused parent eagles mistook the hawk as one of their own and began treating it in kind. Though surprising, such behavior can occur when the wrong species ends up in a nest. That’s because most adult birds cannot recognize their own chicks from others—a vulnerability that brood parasites exploit by laying eggs in other species’ nests.
I couldn't find any more info on this beyond the original article.
If you do search for the photographer, Parham Pourahmad, he seems to have been winning many awards and accolades, so you may want to look at his other pics if you like this.
As to why the eagles eventually turned on the hawk, I couldn't find anything conclusive, but I found a before and after set of stories about another incident in California last year, and an older one in British Columbia that had a better ending, so by looking at a success and a failure, we can deduce a few things.
The eagles grow a bit slower than hawks, but we don't know from the story how big these eagles were compared to the hawk. The eagles are larger and more aggressive, and the hawk may have gotten injured, either through competition for food or fighting with the adoptive siblings. Also, if food being delivered to the nest was limited to begin with, the eagles may have outcompeted the hawk for what was available. Once injured or at the beginning stages of starvation/dehydration, it only gets more and more difficult to get food from the eaglets. Reading the behavior descriptions in the success article, it seems a very strong personality would be required to survive even in a fairly "safe" eagle's nest.
The articles have some great pics and info, so give them a look!
CA Before Article
CA After Article
BC Successful Article