Why Do Birds Cross Continents?
Birds migrate across continents because their trips track seasonal food peaks that shift across latitudes; tracking individual birds reveals that arrival and stopovers line up with those resource pulses.
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How could a scientist use tracking data to tell why a bird migrates across continents?
A single Arctic tern leaves Greenland in August and is found months later in Antarctica; what evidence would reveal why it made that trip?
The tempting shortcut is to say birds flee cold weather, but tracked routes often skip warm regions and connect to food peaks, so we need evidence to tell the two apart.
An interactive month-by-month map comparing a tracked tern's location with seasonal peaks in food abundance.
The rule for investigation: treat migration as a movement from one seasonal food peak to the next, and test it by matching a tracked bird's schedule to maps of resources.
- Navigation mechanisms (magnetoreception, celestial cues)
- Genetic basis of migratory restlessness
- Climate change impacts on migration timing
- 01A bird that leaves the Arctic in autumnslideSlot 1Hook
Meet the Arctic tern. Every year it leaves the Arctic and appears months later on the other side of the planet. How could we investigate why it makes that journey?
- Arctic terns migrate between polar regions
- The journey spans continents and oceans
- We can test possible drivers with evidence
PhenomenonEvery autumn, millions of Arctic terns leave the Arctic and appear months later near Antarctica.
QuestionHow could we test what drives such a cross-continent journey?
- 02The cold-weather shortcutslideSlot 2Tension
The most tempting explanation for migration is cold: birds fly south to escape winter. But if that were the whole story, tracked birds should stop at the nearest warm place. They often don't.
- The common intuition is that birds migrate to avoid cold
- That prediction says destinations should be the warmest nearby places
- Tracked routes can test whether this is true
PredictionIf birds migrate mainly to escape cold weather, then a bird's wintering destination should line up with the warmest available place.
Tempting intuitionBirds fly south for the winter because northern winters are too cold.
- 03Following a tern with a geolocatorinteractiveSlot 3Reveal
Move through months of a tracked Arctic tern's journey. Compare its route and stopovers with maps of seasonal food abundance — not just temperature — and watch what the bird is actually following.
- Light-level geolocators record where a bird is each day
- The tern's arrival and stopovers match seasonal food peaks
- The route makes sense as a chain of resource pulses
EvidenceLight-level geolocators show that individual Arctic terns travel about 70,000 km per year, and their stopovers align with local peaks in insects and fish.
ConclusionThe migration pattern is driven by seasonal food availability across latitudes, not just cold avoidance.
Mechanism- 1Day length triggers departure when days shorten in autumn
- 2The bird follows a route where seasonal food peaks are still available as it moves between continents
- 3Stopovers and breeding timing are timed to the peak food windows along that route
- 04How to investigate a migrationslideSlot 4Takeaway
To test why a bird migrates, compare its track's timing with a map of seasonal resources. If arrival matches the food bloom, you have evidence for resource tracking.
- Don't stop at temperature
- Match the bird's schedule to seasonal food
- Use tracking data as your test
TransferSuppose a small songbird leaves Africa in March and arrives in Europe in May; to test its reason, compare its arrival with the spring insect bloom, not just with average temperature.
Expected inferenceWhen arrival matches peak food rather than warmth, the migration is likely tracking seasonal resources.
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