of birds is determined by the ratio of the two leg bones tibiotarsus and tarsometatarsus and by their strength. At Patagornis this ratio is almost 70%, meaning that the taxon was very agile, a trait common among the smaller Patagornithines. Research from 2005 therefore showed that the animal had a maximum speed of 50 km/h, about the same speed as the living rhea. The anterior margin of the fenestra antorbitalis of Patagornis is moderately sloped, while at Andrewsornis it is strongly sloping and at Andalgalornis only slightly sloping. Alvarenga & Hofling describe the dorsal portion of the nostrils as "very conspicuous". The symphysis mandibulae is slightly curved, with the apex of the beak end uncurved. On the best preserved Patagornis skeleton, BMNH-A516, the ulna preserved large quill knobs on the posterior end that suggest large wing feathers. These feathers were theorized to have been used for assistance in running, as implied by its ecology and limb anatomy, or as a shield like on the extant Secretary bird. The former theory is much more likely based on the anatomy of the quill knobs themselves and their presence in the related Llallawavis which has a similar agile anatomy. The ungual phalanges preserved in Patagornis and its distant relative Mesembriornis are large, curved, and thin laterally, likely being used to stab prey based on those of modern predatory birds. Ear anatomy In 2015 during their study on the ear anatomy of phorusrhacids, Degrange et al also studied the internal ear anatomy preserved in Llallawavis, Patagornis, & several modern birds. They discovered that the hearing of Patagornis was very poor & had the smallest hearing range in the Cariamiformes studied. The semicircular canals of Llallawavis were much more elongated compared to the short canals of Patagornis, and with the greater body mass, it was inferred that there were more sluggish head movements in this taxon, with enhanced sensitivity to low-amplitude motions. Classification Due to the great preservation of Patagornis and its relatives, comparisons with other taxa and detailed classification are much easier compared to other phorusrhacids. In 2003 during their redescription of phorusrhacidae, Herculano Alvarenga and Elizabeth Hofling created a new subfamily, Patagornithinae, with Patagornis as the type genus. Patagornis is similar to its relatives Andrewsornis and Andalgalornis in that they all are medium-sized phorusrhacids with slender, lightly built bodies, long and narrow mandibular symphyses, and long and slender tibiotarsi and tarsometatarsi. However, a