25 Oct 2012

Heading South: migrating raptors

By: Jorge Ventocilla

In Panama, through October and November, we should pay more attention to the sky: suddenly, up there, we can see the impressive spectacle of raptor migration, aptly described by a naturalist as “rivers of birds”.

The year was 1520 when Spanish historian Fernandez de Oviedo made the first Spanish description about raptor migration in the New World (actually observed by himself in what would later become Panama). He wrote of how some years in March he would see the whole sky covered in birds, very high above. He was referring to the birds flying over the Caribbean side, that were returning from their migration south.

Those were times of great ignorance about the natural world in Europe. Three centuries later, on May 21st, 1822, in Mecklenburg, Germany, a stork was captured with its neck pierced by an arrow. It was long ago that the mankind in the Old World had more sophisticated and massive means to kill than arrows, so it could have only come from a distant country. Indeed, shortly after it was established that the origin of the arrow was Africa. This strange event was an irrefutable proof of the fact – now universally recognized – that storks spend their winter in Africa. 

The Swedish naturalist Linnaeus carried out the first important research on bird migration. But we would only have a fraction of our present knowledge had it not been for the Danish master Hans Mortensen, who in 1899 thought of identifying birds by giving them them numbered leg rings. Since then, migration is essentially studied with this method: the birds are captured, coded rings are placed on their legs and then the birds are released. Eventually, when observed or recaptured elsewhere, ornithologists will retrieve information and thus decipher the patterns of movement.

Fifteen percent of a total of almost a thousand species reported for Panama, are migratory: they do not nest in the Isthmus and can only be observed during a certain period of the year. Some of them just pass by. Others come and stay here; many of them arrive early enough and leave late enough, to live in Panama more than they do in their hometowns.
And due to the narrowing of the American continent in Panama, good amounts of species converge on the Isthmus during migration. Many follow the Pacific coast, particularly during the southward migration (which reaches its highest numbers between late September and early November). This route makes Panama city one of the few places in the world where you can see, in one morning, numerous species of the three main groups of migratory birds: raptors, shorebirds and songbirds.

Every year during October and November, almost all existing individuals of two species of hawks, Swainson’s hawk (Buteo swainsoni) and the Broad-winged Hawk (Buteo platypterus), along with the noneca (Cathartes aura) – as well as other raptors, but in lesser number – migrate through a narrow air corridor on the Pacific side of our country.

Decades ago and after years of observations, Neal Smith, an ornithologist at the Smithsonian Tropical Research Institute, showed evidence that the number of birds per season was several hundred thousand, and that this joint migration of raptors represented one of the most spectacular and easily observable migration throughout the continent. Counts made by Smith between 1970 and 1982, peaked in 1982 with 981,000 birds.

Sixteen years later, George Angehr (curator of the main exhibition at the Biomuseo), also Smithsonian staff and Audubon Society member, organized another systematic count and the figure rose to 1.6 million raptors. In 2005, in what became “the first global study on transcontinental migratory raptors: Raptors Ocean to Ocean”, more than 40 people, including volunteers from the Audubon Society of Panama, the Smithsonian Institute, the National University of Panama, along with residents of rural communities and volunteers, recorded meticulously the number of birds crossing the Isthmus through a transverse axis in central Panama. The data indicated that about 3 million raptors plowed Panama southward migration in 2005. This staggering number is even more impressive when you consider that the migration of over one million birds occurs only in three other places in the world (Costa Rica, Mexico and Israel).

In addition, this raptor migration is one of the few cases in America in which birds migrate for a long time and in large distances… without eating! They fly by day taking advantage of the warm air currents: the thermals, over which they can glide for long periods and extensive distances without even flapping. They do this to conserve energy, because unlike other birds, they are unable to fly long distances constantly flapping. If night arrives when they are over the capital city, they come down and sleep in the forests of the west banks of the Panama Canal and of the Metropolitan Natural Park.

While the Turkey Vulture or noneca is the largest migratory raptor in the American tropics, their fellow passengers are medium size. It is worth mentioning that apart from those that cross the Isthmus during migrations, we have a whole year resident population of noneca in Panama. The Swainson’s hawk travels from North America plains to similar places in South America, some stay along Central America but hundreds of thousands continue as far away as Argentina (after a journey of nearly 11,000 miles). Birds that pass over Las Cruces, New Mexico, in early October, reach Panama 18 days later and arrive in Argentina during the last week of November.

The Broad-winged Hawk passes over Texas in late September and arrives in Panama 12 or 14 days later, a part of the population remains there and the rest continues to various other parts of South America. These three species return to North America during our dry season and this time crossing Panama could take them only one day – as opposed to the three days in the first trip – because the north wind helps them fly with greater speed.
Five centuries after Fernandez de Oviedo’s report, those mysteries of the world are fortunately and gradually becoming part of our general knowledge. Until a few years ago, there were very few non expert-citizens of the capital city that knew of this seasonal phenomenon, which occurred over their heads. As a friend that studies raptors, Chelina Batista, told me, you can find yourself watching the November parades on Via España, and up there the natural heritage represented by migrating raptors, are marching too, thousands and thousands of birds heading south.