Their most common prey was the same bird the Maori found themselves hunting, the moa. The moa was peaceful, because it could afford to be. Measuring in at up to 6.5 feet and 500 pounds, it was the biggest bird of its era ... until the Maori hunted it to extinction. Only an estimated 160,000 were alive when they arrived, and while their size made them difficult for even humans to hunt, their slow breeding rate and lack of fear of humans who hadn't yet discovered the concept of extinction was a bad combination.
And once their primary food source vanished, Haast's eagle was doomed too. There may have been no more than 1,000 breeding pairs to begin with, and the Maori moving in on their habitats didn't help. Both giant species simply weren't equipped to handle the human era, although there's been surprisingly serious talk about one day cloning the moa so it can eat up New Zealand's sprawling brush. There is hopefully no talk about reviving Haast's eagle outside of a movie wherein the Rock has to fight one, because it would immediately seek revenge by trying to kill us all.