Bird of prey and may be raised to an order of their own, Cathartiiformes. The secretary bird and/or osprey are sometimes listed as subfamilies of Acciptridae: Sagittariinae and Pandioninae, respectively. Australia's letter-winged kite is a member of the family Accipitridae, although it is a nocturnal bird. The nocturnal birds of prey – the owls – are classified separately as members of two extant families of the order Strigiformes: Below is a simplified phylogeny of Telluraves which is the clade where the birds of prey belong to along with passerines and several near-passerine lineages. The orders in bold text are birds of prey orders;
The Origin of Birds exquisite detail the fossil remains of several extinct birds, including "Hesperornis", "Ichthyornis", "Archaeopteryx", and ""Archaeornis"", the name used commonly at the time for the Berlin specimen, which was then thought to represent a separate genus. Throughout this section are many finely detailed and labeled renderings of different parts of the skeletal anatomy of these birds, as well as other groups of extinct reptiles and some modern birds. Heilmann's attention to detail in his artwork is inspired at least in part by his dissatisfaction with anatomical renderings of these animals in scholarly works of the time, which he deemed as "unsatisfactory"