The order Falconiformes is a group of about 290 species of birds that comprises the diurnal birds of prey. Raptor classification is difficult and the order is treated in several ways.
Traditionally, all the raptors are grouped into four families in this single order. However, in Europe, it has become common to split the order into two: the falcons and caracaras remain in the order Falconiformes (about 60 species in 4 groups), and the remaining 220-odd species (including the Accipitridae – eagles, hawks, and many others) are put in the separate order Accipitriformes. A prehistoric family known only from fossils is the Horusornithidae.
The idea that Falconiformes should be divided into many orders comes from the suggestion that the order may not share a single lineage that is exclusive of other birds. The most controversial but best-supported suggestion is that Cathartidae are not Falconiformes but are related to the storks, in the separate order Ciconiiformes. However, morphological evidence supports the common ancestry of the Falconiformes, and the Strigiformes may be very close to the Falconiformes as well.
The American Ornithologists' Union provisionally reintegrated the New World vultures (family Cathartidae) into Falconiformes in 2007. This goes against the influential Sibley-Ahlquist taxonomy, in which all the raptors are placed into Ciconiiformes, but the Cathartids are considered to be outside the lineage that includes other raptors. While the latter may be correct, the "Ciconiiformes" sensu Sibley and Ahlquist are a paraphyletic, artificial assemblage and one of the weakest points of their classification scheme.
Karyotype analysis indicates that New World vultures are indeed distinct, and the Accipitridae stand apart from all other falconiform birds in that their microchromosomes show a high degree of merging to medium-sized chromosomes, which is unique in birds (de Boer 1975, Amaral & Jorge 2003, Federico et al. 2005). Whether this has any bearing on the validity of the proposed Accipitriformes is still a matter of dispute, but it at least proves that the accipitrids are a monophyletic group.
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