Size and Weight:
About Haast's Eagle (Harpagornis):
Wherever there were large, flightless prehisoric birds, you can be sure there were also predatory raptors like eagles or vultures on the lookout for an easy lunch. That's the role Haast's Eagle (also known as Harpagornis or the Giant Eagle) played in Pleistocene New Zealand, where it swooped down and carried off giant moas like Dinornis and Emeus--not full-grown adults, but juveniles and newly hatched chicks. As befitting its prey, Haast's Eagle was the biggest eagle that ever lived, but not by much--adults only weighed about 30 pounds, compared to 20 or 25 pounds for the largest eagles alive today.
Because it relied so heavily on Giant Moas for its sustenance, Haast's Eagle was doomed when these slow, gentle, flightless birds were hunted to extinction by the first human settlers of New Zealand, going extinct itself shortly afterward.