Birders are well-aware of the recent change in their checklists, which annoyingly relocates the falcons away from hawks, eagles, and other diurnal birds of prey, and puts them deep among the little birds– just before flycatchers. Here is the dramatic rationale behind it.
It used to be that there were a few obvious examples of convergent evolution in the bird world– like meadowlarks and longclaws— and then a bunch of lesser known examples for which this blog is dedicated, in part, to illustrating. However, recent DNA analysis has blown the lid off this idea. It now seems that convergent evolution is practically the norm rather than the exception– that various groups of species once thought closely related (and put adjacent to each other in field guides) are not related after all– that they each evolved, convergently but separately from each other, to serve a similar role in the ecosystem on different continents, though many have since spread out over each other. Near the top of this list of DNA-discovered examples of convergent evolution is the conclusion that Falconiformes (falcons and caracaras) and Accipitriformes (which includes eagles, hawks– both Buteos and Accipiters, kites, harriers, and Old World vultures) are not related to each other. Well, if you go back millions of years they are related in the same way that songbirds and owls are related (which is very very distantly).
Essentially, the story is this. After the breakup of Gondwana over 100 million years ago into Africa, Australia, and South America, hawks evolved in Africa and falcons evolved in South America independently of each other. It was if each place needed a diurnal raptor for their ecosystem, and each got one. Actually, each got a bunch of them– and the parallels include many details and sub-examples at the species level. (Thanks to their ability to fly, both hawks and falcons have since spread around the world.)
From Ericson, P.G.P. 2012. Evolution of terrestrial birds in three continents: biogeography and parallel radiations. Journal of Biogeography 39, 813–824:
It can be postulated that after the break-up of Gondwana in the Cretaceous the terrestrial avifaunas of Africa, South America and Australia were more or less isolated from each other for many millions of years. In each continent different bird groups responded to the local conditions by evolving a range of ecological adaptations. Because of a general similarity in major habitat types between the continents, several specializations evolved in parallel in different phylogenetic lineages. This becomes evident when comparing the ecological adaptations of bird groups in Afroaves with those in Australavis…. Another example of convergence
in ecological adaptations is the parallel evolution of diurnal predators in the two clades, Accipitriformes in Afroaves and Falconiformes in Australavis [which includes species that evolved in South America].
Thus, any similarities between hawks and falcons is due to convergent evolution. Let’s list some of them:
- Shape — similar proportions, often with a longish tail, rounded to pointed wings for either maneuvering or hovering.
- Bare parts — sharp, hooked bills, usually with a yellow cere; large eyes with usually yellow irides– and excellent vision; often yellowish legs with sharp talons for grasping prey.
- Size — generally ranging from a little larger than a robin to larger than a raven, with most about 10″ to 15″ long.
- Plumage — features creamy white, blue-gray, and brown, often with a touch of orange-red; usually dark above, pale below (often finely barred or with a dark belly band), barring on wings and tail, with dark cap, dark around the auriculars or elsewhere between the malar and eyeline; pale throat.
- Habits — being predators, most are fairly solitary; most will sit still for long periods of time, watching for potential prey, and many will soar in circles; many dive or chase prey at high speeds.
- Calls — most have piercing high-pitched or raucous calls, often repetitive; they do not have songs.
There are many species-specific parallels to be drawn; photos of a few comparisons are shown here. The last pairing, the Bat Falcon and the Bat Hawk, is noteworthy because they are each the “bat specialists” of the two groups, feeding primarily on bats at dawn and dusk.