In other parts of the world, migrating raptors face very different conditions from those encountered in the hot, cloudless climates of the Middle East. In general, the atmosphere contains more energy in tropical regions than at higher latitudes, which enables raptors to migrate in some conditions through the tropics that would ground them elsewhere. For example, at Talamanca in Costa Rica, thermals can form over shallow water in the early morning, which at that time is warmer than the nearby land, enabling soaring raptors to set off earlier in the day. There may also be sufficient updrafts even during rain, enabling some birds to continue migrating in the wet. In Panama, corridors of rising air produce long clouds, and for much of the time raptors fly through the cloud base, taking advantage of the 'thermal cloud streets' that enable them to glide for tens of kilometres at a time. They also use the updrafts created by winds hitting the central mountain spine. Counts at Panama are inevitably underestimates, partly because the birds are often hidden by cloud, and also because they often fly higher than visual range. Some fly over the central mountains at 4-6 km above sea level, while others may overfly the mid-afternoon storms at even higher altitudes (Smith 1985a). Unpublished counts near Panama City (Ancon Hill) in 2004-2005 produced a total of around
2.5 million birds (K. Bildstein).
The Veracruz totals represent the largest migration stream of soaring birds seen between North and South America (and the largest known stream of soaring birds from anywhere in the world). The overall autumn total in 2002 amounted to nearly
6.6 million raptors, including nearly 2.7 million Turkey Vultures Cathartes aura, 2.4 million Broad-winged Hawks Buteo platypterus and 1.1 million Swainson's Hawks
B. swainsoni (the latter species exceeding 1.2 million in 2003 and 2005). Up to 25 other raptor species have been seen at this site (Table 7.1), and several other soaring bird species, including more than 28 000 American White Pelicans Pelecanus erythrorhynchos). In addition, however, other raptors are known to migrate down the western side of the Americas at similar latitude to Veracruz, and further east, yet others also migrate down the Florida peninsula across the Caribbean Islands to South America. No good estimates are available for these other routes, but the Ospreys Pandion haliaetus counted on passage through southeast Cuba (La Gran Pedra) in autumn would represent about 90% of the known eastern North American population. It seems that the total raptor migration between North and Central-South America could well exceed seven million birds, which makes the known Eurasian-African total of two million seem erroneously small.
Was this article helpful?