Geometric morphometrics and finite element analyses reveal the Haast's eagle (Harpagornis moorei) to be a mixed predator-scavenger

The extinct Haast’s eagle (Harpagornis moorei) was 30-40% heavier than the largest extant eagle. There have been speculations about its evolutionary history and ecology, though there is still no consensus on its feeding behaviour. This study aims at understanding the evolution and ecology of Harpagornis by combining 3D geometric morphometrics and finite element analysis (FEA) on three-dimensional models constructed from CT-data of skulls and talons of Accipitridae. Statistical analyses revealed the presence of two independent modules (beak and neurocranium) and of a strong allometric effect in the skull. Size-free shape analysis of the two modules revealed that Harpagornis’ beak was similar to the eagles, while it’s neurocranial morphology was more like a vulture. In most cranial FEA loading cases, there seems to be a dichotomy between Cathartidae on the one side and Accipitridae on the other. FEA on the skull, nevertheless, indicates that Harpagornis and the scavenging species of our dataset are well adapted to perform a pull-back motion. The talon results suggest Harpagornis was an active hunter. Harpagornis’ talon occupies a position in morphospace close to its closest living relative Hieraaetus (smallest extant eagle), suggesting a phylogenetic constraint on talon shape. However, FEA showed that the talon of Harpagornis undergoes similar stresses to that of other hunting raptors which rely on large-sized prey (e.g. Aquila audax). Neurocranial morphology and FEA, however, clearly indicate a feeding behaviour more similar to vultures, possibly because of the large size of its prey (e.g., giant Moa). Harpagornis’ neurocranial adaptation probably allowed a stronger and faster pull back motion to quickly remove large chunks of meat from the prey, similarly to vultures. Moreover, our results document a rapid evolutionary change, which might have allowed Harpagornis to exploit large sized prey. Harpagornis moorei therefore represents an extreme example of how freedom from competition in island ecosystems can rapidly influence morphological adaptation.