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.


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.

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