Every year, as autumn begins to paint the northern forests in shades of ochre, the skies above Costa Rica undergo a breathtaking transformation. What appears from a distance as a dark, swirling cloud reveals its true nature upon closer inspection: it is not a storm, but a living river composed of hundreds of thousands, sometimes millions, of birds of prey. This phenomenon, one of the most awe-inspiring spectacles in the natural world, turns the country into the main stage for one of the planet’s most massive migrations.
The Science of the Great Journey: Why and How?
The engine driving this epic journey is the instinct for survival. Fleeing the cold North American winter and the subsequent scarcity of food, these birds embark on a long voyage to the warmer lands of South America. To do so, they must cross the narrow land bridge of Central America.
Costa Rica, due to its geographical position, acts as a natural funnel. Birds spread across a vast territory from Canada to the southern United States are forced to converge in this narrow corridor. But why don’t they simply cross the Caribbean Sea? The answer lies in their flight technique. Raptors are masters of soaring. Instead of constantly flapping their wings—an enormous expenditure of energy—they use thermals: columns of warm air rising from the sun-heated land to gain altitude effortlessly. They circle within these updrafts in groups known as “kettles,” and once they reach a great height, they glide gently southward, losing altitude until they find the next thermal. This flight method is only possible over land, making the isthmus their only viable route.
Ground Zero: The Caribbean Corridor
Although the migration can be observed in various parts of the country, the undisputed epicenter is the Caribbean coast, particularly over the Talamanca mountain range. Here, the geographical and climatic conditions create a superhighway of thermals.
The most famous and scientifically important observation point is the count site in the Kèköldi Indigenous Reserve. Since 1999, ornithologists and volunteers have gathered here each season to count, identify, and study the flow of birds. Their data is crucial for understanding population health and the effects of climate change. For travelers, locations like Cahuita and Puerto Viejo offer front-row seats to witness the passage of thousands of birds from the comfort of the beach or a viewpoint.
The Stars of the Journey: Meet the Migrants
While up to 17 species of migratory raptors have been recorded, the bulk of the contingent is made up of three main species, which account for more than 98% of the total:
- Turkey Vulture (Cathartes aura): The most numerous, recognizable by its wobbly flight and its V-shaped silhouette.
- Swainson’s Hawk (Buteo swainsoni): A traveler of incredible endurance that migrates from the prairies of western North America to the pampas of Argentina.
- Broad-winged Hawk (Buteo platypterus): Famous for forming the densest and most spectacular “kettles,” often with thousands of individuals swirling together.
Traveling alongside them are other fascinating species such as the majestic Osprey, the swift Peregrine Falcon, and the graceful Mississippi Kite.
The Spectacle: An Unforgettable Experience
Witnessing the migration is an overwhelming experience. During the peak of the season, which occurs between late September and early November, the numbers are astonishing. More than 3 million raptors have been counted in a single season, with peak days where the sky is filled with the passage of 200,000 to 500,000 birds. The best time for observation is from mid-morning to the early afternoon, when the sun has warmed the earth and the thermals are at their strongest.
To see the sky literally filled with birds, a river of silhouettes flowing silently southward, is a powerful reminder of the strength and fragility of nature.
Conservation: Protecting the River of Raptors
This global phenomenon faces global threats. Habitat loss both in their northern nesting grounds and their southern wintering areas, climate change that alters wind patterns and thermal formation, and human persecution at some points along their route are real dangers.
The data collected at count sites like Kèköldi is vital. It allows scientists to detect population declines and advocate for more effective conservation policies. Ecotourism, when practiced responsibly, also plays a crucial role, demonstrating that these birds are worth more alive than dead and providing local communities with an economic reason to protect this natural heritage.
Ultimately, the raptor migration is a symbol of the interconnectedness of our continent. Their journey unites Canada, the United States, Central America, and South America in an unbroken life cycle. Protecting this aerial river is not the responsibility of a single country, but a shared commitment of all nations under its path.
