Falconiforms have no obvious evolutionary links with other birds. Currently they are placed between ducks, on the one hand, and game birds on the other; but they bear no clear resemblance to either, while fossil evidence does not indicate intermediate links. The most obvious physical specializations of falconiforms—the cutting, tearing bill and taloned feet—do not indicate close relationships with owls (order Strigiformes) but are the result of similar trends in evolution. Some anatomical work, however, indicates that owls and the family Falconidae may be related.
Few fossils of falconiforms have been found, and those that have may require reassessment. A generalized raptor is known from 50 to 35 million years ago during the Eocene Epoch. The oldest raptorial bird (Lithornis) known is from the late Paleocene Epoch (57.9 to 54.8 million years ago) and may have been a New World vulture (family Cathartidae). Cathartids may have evolved in the Old World, dying out there and surviving only in the New World. Fossil New World vultures include a large terrestrial species (Neocathartes grallator) and a huge vulture (Teratornis merriami) from the Rancho La Brea Tar Pits in California. Because of their apparently ancient origin, cathartids may be regarded as primitive survivors.
Among present-day species, convergent evolution has led to the development of similar traits among different falconiforms. For instance, in Australia, buzzards (Buteo) are absent, but certain large kites have evolved to fit this ecological niche. In South America the buzzardlike harpy eagle represents a climax of one line of evolution filled in the Old World by large booted eagles such as crowned eagles (Stephanoaetus). There has been significant specialization for particular functions in unrelated species; for instance, in the bullet shape of falcons, in the spiculed feet of ospreys and certain eagles, and, perhaps most striking, in the auditory specializations shared by harriers and forest falcons.
Classification
Distinguishing taxonomic features
The main features of falconiforms include the hooked “raptorial” bill; the basal cere, or bare skin, covering the nostrils; the powerful feet with hooked claws; sustained powers of flight and carnivorous habits; and the difference in size between the sexes. Some suborders or families have additional characteristics: the New World vultures (family Cathartidae) have pervious nostrils (i.e., incomplete partition between nostrils) and a rudimentary hind toe; the osprey, a reversible outer toe; the secretary bird, long legs with short, blunt toes; and Falconidae, toothed beak, sharply pointed wings, and a noncentrifugal wing molt. On the basis of inside eggshell colour, the order may be divided into three rather distinct groupings, or suborders: the Cathartae, the Accipitres (including the secretary bird and osprey), and the Falcones.
Critical appraisal
Debate about the classification of falconiform birds centres on two topics: New World vultures (family Cathartidae) as ciconiiforms related to the storks (family Ciconiidae) and the relationship of falcons (Falconidae) to various other orders. Behavioral, morphological, and initial genetic evidence once suggested that New World vultures were related to storks, but recent analyses of morphological and biochemical data challenge this proposition. Proposals that falcons are related to groups ranging from parrots (Psittaciformes) to owls (Strigiformes) and cuckoos (Cuculiformes) continue to surface, but reclassification has not been warranted. Like the New World vultures, the secretary bird may be a ciconiiform. Resemblance of the secretary bird to the South American seriema (family Cariamidae) is likely a case of convergence.
Annotated classification
- Order Falconiformes (diurnal raptors)
- 309 species in 5 families found virtually worldwide; 7 species of the family Cathartidae often classified as ciconiiforms.
- Family Accipitridae (accipiters, kites, hawks, buzzards, harriers, eagles, Old World vultures, sparrowhawks)
- 236 species in 64 genera; some subgroups, e.g., Old World vultures, confined to Europe, Asia, and Africa; small to very large (100 grams to about 10 kg); inside surface of eggshell green; strong hooked talons in all but Old World vultures; squirt droppings; build their own nests.
- Family Falconidae (caracaras, milvagos, forest falcons, falconets, merlin, true falcons)
- 64 species in 11 genera; some genera confined to New World, and others (Microhierax) only Asian; very small to medium-sized (35–1,800 grams); resemble Accipitridae in having hooked beaks, talons, sexual dimorphism, etc.; differ in molt sequence, some anatomical characters, behaviour (droppings fall below perch, head bobbing in some, in all but caracaras no nest is made); inside surface of eggshell reddish.
- Family Cathartidae (New World vultures)
- 7 species of the Western Hemisphere; perforated nasal septum, rudimentary hind toe, large olfactory chambers; large or medium-sized (1.5–1 kg; wingspan 1.2–3.2 metres); do not build nests; inside surface of eggshell yellowish.
- Family Pandionidae (osprey)
- 1 species, nearly worldwide; large, resembles some kites in structure of the sternum and absence of bony eye shield; reversible outer toe, talons rounded not grooved; inside surface of eggshell green; squirt droppings; build their own nests.
- Family Sagittariidae (secretary bird)
- 1 species of central Africa; terrestrial, long-legged, short-toed; nesting and display resemble Accipitridae; inside surface of eggshell green, but certain other characters indicate affinity with South American gruiforms; usually accepted as falconiform but may not be.