The earliest species of a group of hawks, the buteoninae, are believed to have appeared in South America approximately 17 million years ago, from the same ancestor who spawned a group of birds which includes the American eagle, one of the symbols of the United States, according to a recent study conducted by a team of biologists from the University of São Paulo/USP. At that time, North and South America were still separated. The slow formation of Central America, during the following millions of years, provided these birds with some land on which they could rest and feed themselves while going on their migratory flights. Later on, over the course of many generations, the hawks flew even farther to the north. Approximately 5 million years ago, they arrived in North America and started to colonize it, thus originating other species. Then they migrated to another strip of land, the Bering Strait, which was rising to the west of North America. Approximately 1.5 million years ago, they arrived in Asia, Europe and Africa, a route which was the opposite of the one followed by the human species, which had emerged in Africa a long time after the hawks had arrived there. Nowadays, Antarctica and Australia are the only places where there are no hawks.
“Migration is important for the diversification and survival of species of hawks and probably of other animal groups, as some specialists have already indicated in less encompassing studies,” says biologist Fábio Raposo do Amaral, who headed this study. The objective of the study was to investigate the evolutionary history of hawks to reconstruct their routes which took them all over the world. Birds that do not perceive the onset of migration, such as decreasing luminosity and temperature, run the risk of dying during severe winters. Nonetheless, there are no fixed rules. There are species of buteoninae hawks which never leave, while in other species, the entire population leaves during winter, in flocks with hundreds of individual birds in search of warmer places with abundant food.
In the Galapagos
Sometimes, remaining in the same place ensures survival. If the buteoninae hawks that live on the archipelago of Galapagos left in search of new lands, they would probably die of exhaustion over the sea while flying over one thousand kilometers until the coast of Ecuador. These birds probably arrived in the Galapagos a mere 300 thousand years ago, taken by an unexpected air current or a storm and then never left again, because the winds did not help. These birds fly over hundreds of kilometers a day without ever tiring because they glide, just like vultures do, by taking advantage of the hot air that comes up from the surface of the earth; they would not get very far if they had to flap their wings. Genetic analyses performed by Raposo indicate that the ancestors of the Galapagos hawks might have been migratory species just like their kin species, the Buteo swainsoni hawk, which migrates every year from the south of Canada and the United States to Argentina. The hawks that remained in the Galapagos lived in isolation to the point that they became one of the only buteoninae hawk species confined to islands, the Buteo Galapagos species, which only lives on the archipelago. “In this case,” says Raposo, “the birds that kept quiet survived.”
Just like the hawks he studied, Raposo flew around the world trying to understand the evolution of this bird species. He also learned to be careful about things which seemed the same but are actually different. In 2003, he had planned to investigate the appearance, the differentiation and the kinship of ten species of hawks living in forests, most of them with black feathers on the chest and white feathers on the abdomen. It was a modest and comfortable plan. The genetic analyses, however, showed that appearances could indeed be misleading.
Different species can have the same plumage not as a result of kinship but as a result of distinct evolutionary paths that led to a common characteristic which offered some advantage in the battle for survival in the jungle. “In evolutionary terms,” says Amaral, “black and white plumage appeared various times among the buteoninae, possibly as a result of natural selection in forest environments.” He gradually included other species into his work, species which seemed distant. By 2008, he had built up a phylogenetic tree, also referred to as the tree of life, of the 53 species of one group of hawks, which are part of a bigger group comprising 237 species.
EDUARDO CESAR“The morphological similarity is not always a good guide to establish evolutionary history,” concludes Raposo, after having compared nine stretches of DNA from 105 samples of blood, muscles or feathers of 53 buteoninae species. His research work was conducted under the supervision of Anita Wajntal, founder of the University of São Paulo/USP’s research group on bird genetics. These results place the buteoninae hawks as a group that was probably formed a very long time ago. In relation to other birds, the ancestors of hawks, from which these and other groups originated, are also ancient: they probably appeared approximately 50 million years ago. Macaws, parrots and toucans probably appeared 30 million years ago, according to the conclusions of the research group, nowadays coordinated by Cristina Yumi Miyaki.
DNA analyses led to the re-classification of the group, with genera that had changed names because they proved to be far apart in evolutionary terms, unlike previous impressions. After the analyses, the ten species studied at the beginning of the research work, and which had seemed to be close, were spread through six of the 17 genera of the buteoninae classification tree that Raposo and other biologists from Brazil, the United States and Austria presented in an article to be published shortly in the Molecular Phylogenetics and Evolution journal. Upon analyzing the study, Alexandre Aleixo, a biologist at the Museu Paraense Emílio Goeldi museum and an expert on birds, pointed out that Amaral and the other authors – validated no less than five genera of buteoninae hawks already described but previously considered non-valid, and had to describe two new genera,” According to Aleixo, “this is the biggest change in the group’s taxonomy in the last 80 years and shows how current taxonomy can reflect incorrect arrangements from the evolutionary point of view.”
In cities and mangrove swamps
The buteoninae hawk group includes birds in a variety of sizes, habits, and diets. In South America, even though there is more diversity, most of the species are smaller and feed mainly on insects and small vertebrates. The more recent species are found in North America, Asia, Europe and Africa. Most of them are bigger and devour squirrels, rodents, carcasses, or other birds. The Brazilian species include the ‘gavião-carijó’ (Buteo magnirostris), commonly found in urban areas. “I see a pair of gavião-carijós nearly every day in the trees here at USP,” says Raposo. A group of ‘gavião-asa-de-telha’ (Parabuteo unicinctus) also lives in the forests surrounding USP’s Cidade Universitária campus. This bird, which has a red spot on a brown wing, was believed to be extinct in the State of São Paulo until very recently. Another species, the half-a meter long ‘gavião-pombo-pequeno’ (Leucopternis lacernulatus), with a white body, black chest and wings, is found only in the Mata Atlântica rain forest. Another species, the ‘águia-cinzenta’ (Harpyhaliaetus coronatus), which is one meter tall and has a two-meter wing span, is found only in open spaces, especially in the Cerrado tropical savanna. Both birds are classified as endangered species.
“This study provides substantial insights on little-known phenomena, such as the evolution of migration in hawks and the biogeographical relationships of hawks and environments in the Amazon Region,” says Frederick Sheldon, director of the US’s Natural Science Museum of Louisiana State University, where Amaral conducted some of his genetic analyses. Among other findings on the evolution of these birds, Amaral found a peculiar group of three species of buteoninae hawks that live only on river banks, lakes or in coastal environments – one of these species lives on the banks of the Amazon River, while another even more specific species lives in the mangrove swamps that line the coastal regions from Venezuela to the State of Paraná and only eat crabs. When comparing habits and genes, he concluded that these species probably have a common history, from the time when the Amazon River basin was a huge lake fed by sea water, more than 5 million years ago.
Sistemática molecular, biogeografia e evolução da plumagem dos gaviões sub-buteoninos neotropicais (nº 04/14840-1); Modality Doctorate grant; Advisor Anita Wajntal – USP; Grantee Fábio Sarubbi Raposo do Amaral; Investment R$ 66.673,15
AMARAL, F.R. de et al. Patterns and processes of diversification in a widespread and ecologically diverse avian group, the buteoninae hawks (Aves, Accipitridae). Molecular Phylogenetics and Evolution. In press, 2009.