Discover the fascinating science behind bird migration bottlenecks

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

Harnessing a column of rising air, the migrants ascend lots of of metres into the sky till they’ve gained ample altitude to chill out into an energy-saving glide in the direction of a distant mountain ridge. Right here they may repeat the trick, every successive rise and fall forming one other important step of their 9,000 km journey from North American breeding grounds to Argentina’s pampas grasslands. For me, nevertheless, the present is over: I can lastly exhale.

The expertise is thrilling – however so too the underlying science. Travelling huge distances prices migratory birds a lot vitality. Though New World raptors and songbirds migrate between roughly the identical locations, their methods differ radically. Songbirds energy flight via speedy wingbeats, primarily travelling at evening to keep away from each predators and overheating. Broad-winged birds of prey, nevertheless, journey by day, conserving gasoline by hovering.



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Utilizing air currents, they acquire carry earlier than cruising onwards, preserving fats shops by minimising flapping. Typically the raptors ‘slope soar’, using winds pushed upwards over mountain sides – famously so on the well-named Hawk Mountain, which straddles a 500 km lengthy ridge bracketing the US states of New Jersey and Pennsylvania. Such useful updrafts occur when the wind blows. Calm circumstances, nevertheless, demand another method. Birds circle upwards on ‘thermals’ – pockets of heat, ascending air generated when the solar differentially heats the land floor – earlier than motoring away with nary a wing flap.

These aggregations of hovering birds are often known as ‘kettles’, as if the protagonists had been steaming upwards from an imaginary container’s spout. Though the cluster seemingly behaves as a single entity, appearances are misleading. Raptors are sometimes 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 whereas scrutinising the sky for people which have already found the subsequent free journey.

Whereas songbird migration routes sometimes comply with straight strains – the crucial being to fly the shortest distance attainable – geography governs raptors’ routes, thereby chivvying them into concentrations. Well-known migratory bottlenecks usually outcome from the alternatives provided by mountain passes or gorges comparable to Organbidexka within the French Pyrenees, the place currents funnel avian travellers via a confined airspace.


Raptors congregate in the skies above Panama City © Alvaro Moises


However geography may also current obstacles to raptor migration – and none is mightier than the open sea. Water our bodies launch warmth slowly and evenly, stopping thermals from forming above them. With out aerial help, raptors should energy themselves to the subsequent landmass. This isn’t solely energetically pricey however inherently perilous: working out of vitality means drowning. Accordingly birds of prey hug coastlines to profit from land-generated thermals for so long as attainable. This may end up in gatherings of raptors in ostensibly stunning locations – witness the two.1 million birds counted over the land bridge of Panama Metropolis on 2 November 2014 – in addition to at well-known promontories both facet of sea crossings comparable to Europe’s Strait of Gibraltar.

The place sea and mountains are juxtaposed, the mom of all migratory bottlenecks could also be created. Pinched between the Sierra Madre mountains and the Gulf of Mexico, the world’s biggest raptor flyway lies within the slender coastal plain of Mexico’s Veracruz. Between September and November, some 5 million birds of prey pour south alongside this hall in a spectacle often known as the ‘River of Raptors’, together with just about your complete world inhabitants of Swainson’s Hawks.

The truth that pure physics can create such a miraculous and delightful sight is simply one other instance of the inspiring approach birds adapt to the challenges of survival.

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