The Harpy Eagle is well known as a predator on medium sized mammals, but in fact it will take other prey as well, including birds and reptiles.
The most common prey taken by Harpy Eagles are arboreal or primarily arboreal mammals, including howler (Alouatta), titi (Callicebus), capuchin (Cebus), wooly (Lagothrix), saki (Pithecia, Chiropotes), and squirrel (Saimiri) monkeys; two-toed (Choloepus) and three-toed (Bradypus) sloths; opossum (Didelphus); porcupine (Coendu); anteater (Tamandua); olingo (Barraricyon); and kinkajou (Potos flavus) (Fowler and Cope 1964, Rettig 1978, Muñiz-Lopez et al. 2007). Less commonly, terrestrial or primarily terrestrial mammals are captured, including agouti (Dasyprocta), tayra (Eira barbara), coatimundi (Nasua), domestic pigs, collared peccary (Tayassu tajacu) and young brocket deer (Mazama) (Fowler and Cope 1964, Rettig 1978, Bierregaard 1994, Touchton et al. 2002, Muñiz-Lopez et al. 2007).
Birds that have been reported as captured by Harpy Eagles include parrots, curassows, Hyacinth Macaw (Anodorhynchus hyacinthinus), Blue-and-yellow Macaw (Ara ararauna), and Red-legged Seriema (Cariama cristata) (Bierregaard 1994, Muñiz-Lopez et al. 2007). Other reported prey items include snakes, tupinambis, iguanas, and amphisbaenids (Bierregaard 1994).
Monkeys are an important part of the diet in many populations of Harpy Eagles, although in some cases no single species of monkeys is of primary importance. Cebid monkeys, the remains of most of which could not be identified to species, made up ca 37% of the prey remains of a large sample (approximately 83 individuals) of mammalian prey from a nest site in Guyana (Izor 1985). Similarly, cebid monkeys, of six genera, contributed 35% of the prey items of a sample of 10 nests in Amazonian Ecuador (Muñiz-Lopez et al. 2007).
Sloths also are a particularly important part of the diet in many populations of Harpy Eagle. In Amazonian Ecuador, for example, two-toed sloths (Choloepus) alone made up 29% of the prey delivered to a sample of 10 nests, with three-toed sloths (Bradypus) contributing another 5 of the identified prey (Muñiz-Lopez et al. 2007). At the nest in Guyana studied by Rettig (1978), Choloepus and Bradypus contributed 23% and 8% of a collection of 83 individual mammalian prey remains, a sample that was estimated to "represent a large majority of the kills made during the year of study" (Izor 1985). In a detailed observational study of two radio-tagged Harpy Eagles on Barro Colorado Island, Panama (both captive-bred subadults), Touchton et al. (2002) recorded that Choloepus and Bradypus combined accounted for just over half of prey captures for both individuals (52% and 54%, respectively).
Peres (1990) witnessed a successful attack by a Harpy Eagle on an adult howler monkey (Alouatta); the size of this monkey was estimated to be up to 6.5 kg in mass. Rettig (1978) similarly estimated the size of a howler monkey, half of the carcass of which was taken to a nest by a female Harpy Eagle, as 6.3 kg.