Discover the fascinating science behind bottlenecks in bird migration

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Discover the fascinating science behind bottlenecks in bird migration

After successive humid days in the Andes in central Colombia, the November morning provided rain-free warmth. And how the Swainson’s Hawks Buteo Swainsoni react. Hundreds of broad-winged, spread-tailed shapes flanking a ridge surrounded by tropical forest swirl upward. Their simultaneous release of pent-up migration needs manifests itself conversely as Fairground Helter-Skelter.

The migrants take advantage of an ascending column of air and soar hundreds of feet into the sky until they are elevated enough to relax on an energy-efficient glide towards a distant ridge. Here they will repeat the trick, with each rise and fall representing another important step on their 9,000 km journey from the North American breeding grounds to the Argentine pampas meadows. For me, however, the show is over: I can finally breathe out.

The experience is exciting – but so is the underlying science. Traveling over long distances costs migratory birds a lot of energy. Although New World raptors and songbirds migrate between roughly the same locations, their strategies differ radically. Songbirds fly by flapping their wings quickly, mainly at night to avoid both predators and overheating. However, broad-winged raptors travel during the day and save fuel by flying high.

Swainson’s Hawk, Copyright Glyn Sellors, from the Surfbirds Galleries

With air currents, they gain buoyancy before continuing and preserve fat reserves by minimizing flutter. Sometimes the slope of the raptors rises and the winds push up the mountain slopes – famous for the famous Hawk Mountain, which extends over a 500 km ridge that surrounds the US states of New Jersey and Pennsylvania. Such helpful updrafts occur when the wind is blowing. However, calm conditions require an alternative approach. Birds circle upwards on the thermals – pockets of warm, rising air that arise when the sun warms the land surface differently – before they drive away with hardly a flap of a wing.

These collections of soaring birds are known as “cauldrons”, as if the protagonists were steaming upwards from the spout of an imaginary container. Although the cluster appears to behave as a unit, appearances are deceptive. Raptors are usually solitary animals for whom cooperation makes little ecological sense. The spectacle is based on chance and not on coordination: birds independently follow the migration path of least resistance and examine the sky for people who have already discovered the next free voyage.

While songbird migration routes usually follow straight lines – the imperative is to fly the shortest possible distance – geography regulates the routes of the birds of prey, dividing them into concentrations. Famous migration bottlenecks often arise from the opportunities offered by mountain passes or gorges such as Organbidexka in the French Pyrenees, where currents direct bird travelers through narrow airspace.

Swainson’s Hawk, Costa Rica, Copyright Henry Cook, from the Surfbirds Galleries

However, geography can also pose obstacles to the migration of birds of prey – and none is more powerful than the open sea. Waters slowly and evenly give off heat and thus prevent the formation of thermals over them. Without help from the air, raptors have to prepare for the next land mass. This is not only energetically costly, but inherently dangerous: lack of energy means drowning. Accordingly, birds of prey embrace the coasts in order to benefit from the thermals generated by the land for as long as possible. This can lead to raptors congregating in seemingly surprising locations – witnessed the 2.1 million birds counted over the Panama City land bridge on November 2, 2014 – as well as famous promontories on either side of sea routes such as the European Strait from Gibraltar.

Where sea and mountains stand side by side, the mother of all hiking bottlenecks can arise. The largest raptor flyway in the world is located between the mountains of the Sierra Madre and the Gulf of Mexico in the narrow coastal plain of the Mexican Veracruz. Around five million birds of prey flock south along this corridor between September and November. The spectacle is known as the “River of Raptors” and includes pretty much the entire world population of Swainson’s Hawks.

The fact that pure physics can create such a wondrous and beautiful sight is just another example of the inspiring way birds adapt to the challenges of survival.