By Alex Jahn, Doug Levey, and Victor Cueto
Linked paper: Bird migration within the Neotropics by A.E. Jahn, V.R. Cueto, C.S. Fontana, A.C. Guaraldo, D.J. Levey, P.P. Marra, and T.B. Ryder, The Auk: Ornithological Advances.
As earth-bound, bipedal creatures, our view of the world contrasts sharply with that of the miniature winged beasts we call songbirds, capable of crossing vast oceans, deserts, and mountains numerous times during their fast-paced lives. Unsurprisingly, the journeys of songbirds, as well as those of larger birds such as storks, defied the comprehension of observers for centuries. Recent years have witnessed the advent of miniature tracking devices that allow us to track even some of the smallest of migratory birds, such as swifts. And rapidly advancing molecular techniques such as next-generation sequencing are shedding new light on the deep and often complex evolutionary histories of these journeys. Indeed, we are privileged to live at a moment when the combination of emerging technologies and increasing accessibility to historically remote corners of the globe drives research on bird migration forward at a faster pace than at any previous time. As a result, we are learning many surprising details about how and why birds move across our planet, even in places where birds were thought to migrate little, if at all — places like the tropics.
For centuries, most naturalists were based in Europe, making forays to accessible parts of the Neotropics for a year or two at a time. Those who were based in the New World, such as John James Audubon, primarily worked in North America. In the 20th century, with the advent of commercial aviation and the expansion of highways, naturalists gained greater access to historically remote regions. Then, in the early 21st century, miniaturized tracking technologies that could be deployed on songbirds started to reveal exquisite details of the movements of songbirds across the entire year. One of the most notable discoveries was that some Nearctic-Neotropical migrants don’t just go to the tropics to hang out in a few hectares, essentially taking it easy. Instead, they can be extraordinarily restless, moving hundreds of miles across their winter range. Even more surprising, ornithologists are realizing that migration is both common and complex, even in species that spend their entire lives in the tropics. For birds, there is no Margaritaville.
Why was this unexpected? Most ecologists thought of tropical ecosystems as without seasons, leading to the conclusion that birds had no reason to move about these (seemingly) constantly lush ecosystems. Combined with the relatively subtle nature of bird migration in the tropics, where many migratory birds occupy the rainforest canopy and occur at lower densities than many of their north-temperate cousins, this resulted in a general consensus that tropical birds basically don’t migrate. Yet thanks to a combination of traditional and emerging techniques and technologies, we are now beginning to realize that many tropical birds are moving much more than we thought. Altitudinal migration (migration between elevations over a relatively small geographic area), for example, is now known to be common along Neotropical mountain ranges such as the Andes. And longitudinal migration, in which Neotropical birds primarily migrate east-west, is apparently not uncommon.
In a comprehensive review of the state of knowledge on bird migration within the Neotropics, we find that while scientists have already learned a great deal about how birds migrate across this region that extends from the Chihuahuan Desert of Mexico to the Cape Horn in Chile, bird migration there is likely much more common than currently appreciated. Just how many migratory bird species exist in the Neotropics, the region with the planet’s highest levels of avian diversity, is still anyone’s guess. Addressing this, while also attaining a better grasp of the drivers of their movements and clarifying the strategies they employ on migration across this vast region, is an enormous but achievable challenge. Emerging techniques and technologies, as well as a rapidly growing network of citizen science initiatives, are already shedding light on the rich and complex annual cycle of Neotropical birds, as witnessed by recent papers describing previously unknown migrations in such places as Amazonia, Patagonia, and the Atlantic Rainforest. No doubt, many more exciting discoveries lie ahead in this cradle of avian diversity.
“Why was this unexpected? Most ecologists thought of tropical ecosystems as without seasons, leading to the conclusion that birds had no reason to move about these (seemingly) constantly lush ecosystems”… umm, not for those of us who live here! We are very aware of the seasonal changes in Belize, between wet and dry, changes in photohours, which fruits/seeds are produced when (often multiple times in a year), population trends (for example toucans trending towards higher elevations), and the nuances these factors play with regard to ecosystem, whether pine woodland, broadleaf jungle, lowland savannah, cayes and coasts, mangroves etc
Completely agree! However, its likely that many ecologists based in the US and Europe didn’t fully appreciate just how seasonal tropical ecosystems (at least those in South America) really are. As stated by Zimmer, “It is not uncommon…to hear the statement made that South American birds do not migrate…” J. T. Zimmer 1938. Notes on migrations of South American birds. Auk 55:405–410.
Another answer to Hilmy’s question is that the tropics are extraordinarily diverse, including a range of elevations, latitudes, soil types, geographies (including tropics globally), all of which includes many highly seasonal and heterogeneous habitats that birds have become adapted to exploit. A lot of what we know about the tropics comes from a few well studied sites (e.g., Costa Rica, Belize, Panama), which do not necessarily represent “the tropics”. Belize, in particular, is at a high enough latitude to be highly seasonal, so quite distinctive from habitats in Amazonia, for example, and adding in the enormous range of elevations in the Andes Mts. (not to mention the Pantanal, Caatinga, and other highly seasonal habitats in South America) adds enormous heterogenetiy, just within the Neotropics. Birds have evolved an incredible array of strategies, including migration, to take advantage of these patterns of changing food abundance, predators, etc.