About this map

Bird Migration Map shows where migratory birds are recorded through the year, on a 3D globe. It currently covers 10 species.

What the glow means

For each species and month we count openly licensed GBIF records in 1.41° grid cells. Because bird watchers are concentrated in Europe and North America, a raw count mostly maps people. So by default we divide each cell’s count by the number of records of all birds in that cell and month (plus 300, so a single record in an almost empty cell does not light up). Each month is then scaled on its own, so a quiet winter month is as visible as the busy breeding season. Cells with fewer than 3 records are hidden. You can switch the adjustment off with the “Adjust for observer effort” chip.

Limits

• Records are sightings, not a census: they show where a species is seen and reported, not how many birds there are.
• Africa, Asia, South America and the oceans have far fewer records. The effort adjustment helps, but sparse cells are noisy, and some winter ranges show only a few dots.
• Values are monthly; movements within a month are smoothed by blending neighbouring months while the year plays.
• Tracks are individual birds from published studies, so they show real routes but not every population.
• UK arrival dates are worked out from weekly UK records and are given only when the pattern is clear.

The globe

The globe moves only when you drag or flick it, search, or press Spin: it never turns by itself. On slower devices it switches to a lighter mode automatically; add ?hq=1 to the address to keep full quality.