26 Apr 2021

Discover the fascinating science behind bird migration bottlenecks

One of the wonders of the natural world, migration “bottlenecks” occur where vast clouds of large soaring birds – especially raptors – concentrate at narrow land passages. Get a bird’s eye view on the science behind the world’s largest bird migration congregations.

Aggregations of birds soaring upwards on thermals are known as "kettles" © Christian Gelpke
Aggregations of birds soaring upwards on thermals are known as "kettles" © Christian Gelpke
By James Lowen

After successive damp days in central Colombia’s Andes, the November morning has delivered rain-free warmth. And how the Swainson’s Hawks Buteo swainsoni are responding. Flanking a mountain ridge swathed in tropical forest, hundreds of broad-winged, spread-tailed forms are swirling upwards. Their simultaneous release of pent-up migratory urges manifests itself as a fairground helter-skelter in reverse.

Harnessing a column of rising air, the migrants ascend hundreds of metres into the sky until they have gained sufficient altitude to relax into an energy-saving glide towards a distant mountain ridge. Here they will repeat the trick, each successive rise and fall forming another vital step in their 9,000 km journey from North American breeding grounds to Argentina’s pampas grasslands. For me, however, the show is over: I can finally exhale.

The experience is thrilling – but so too the underlying science. Travelling vast distances costs migratory birds much energy. Although New World raptors and songbirds migrate between roughly the same places, their strategies differ radically. Songbirds power flight through rapid wingbeats, mainly travelling at night to avoid both predators and overheating. Broad-winged birds of prey, however, journey by day, conserving fuel by soaring.


Raptors emerge from the mist as they glide higher and higher © Lars Soerink


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Using air currents, they gain lift before cruising onwards, preserving fat stores by minimising flapping. Sometimes the raptors ‘slope soar’, riding winds pushed upwards over mountain sides – famously so at the well-named Hawk Mountain, which straddles a 500 km long ridge bracketing the US states of New Jersey and Pennsylvania. Such helpful updrafts happen when the wind blows. Calm conditions, however, demand an alternative approach. Birds circle upwards on ‘thermals’ – pockets of warm, ascending air generated when the sun differentially heats the land surface – before motoring away with nary a wing flap.

These aggregations of soaring birds are known as ‘kettles’, as if the protagonists were steaming upwards from an imaginary container’s spout. Although the cluster seemingly behaves as a single entity, appearances are deceptive. Raptors are typically solitary creatures for which co-operation makes little ecological sense. The spectacle derives from coincidence, not co-ordination: birds independently following the migratory path of least resistance while scrutinising the sky for individuals that have already discovered the next free ride.

Whereas songbird migration routes typically follow straight lines – the imperative being to fly the shortest distance possible – geography governs raptors’ routes, thereby chivvying them into concentrations. Famous migratory bottlenecks often result from the opportunities offered by mountain passes or gorges such as Organbidexka in the French Pyrenees, where currents funnel avian travellers through a confined airspace.


Raptors congregate in the skies above Panama City © Alvaro Moises


But geography can also present barriers to raptor migration – and none is mightier than the open sea. Water bodies release heat slowly and evenly, preventing thermals from forming above them. Without aerial assistance, raptors must power themselves to the next landmass. This is not only energetically costly but inherently perilous: running out of energy means drowning. Accordingly birds of prey hug coastlines to benefit from land-generated thermals for as long as possible. This can result in gatherings of raptors in ostensibly surprising places – witness the 2.1 million birds counted over the land bridge of Panama City on 2 November 2014 – as well as at famous promontories either side of sea crossings such as Europe’s Strait of Gibraltar.

Where sea and mountains are juxtaposed, the mother of all migratory bottlenecks may be created. Pinched between the Sierra Madre mountains and the Gulf of Mexico, the world’s greatest raptor flyway lies in the narrow coastal plain of Mexico’s Veracruz. Between September and November, some five million birds of prey pour south along this corridor in a spectacle known as the ‘River of Raptors’, including pretty much the entire global population of Swainson’s Hawks.

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