Methodology

Build 2026-08-31, records to 2026-08-16. Classes computed from records 2010 onwards.

Sources and purpose

The Atlas aggregates bird records published through GBIF and the NBN Atlas (see data sources). It is a free, non-commercial resource. Records under non-commercial licences are used on that basis; every dataset can be withdrawn from a build on request from its provider.

Grid and precision

Maps use the 10 km squares (hectads) of the British National Grid, and the Irish Grid in Northern Ireland. A record is placed in a square only if its stated precision is 10 km or better. Records generalised to 50 km or coarser feed the national seasonality curve but never a square. Channel Islands and offshore records outside 12 nautical miles are excluded.

Visits and reporting rate

A visit is one observer at one place on one date. The reporting rate for a species in a square and month is the share of qualifying visits that recorded it. Only visits that could have recorded any species count in the denominator: complete checklists (five or more species), breeding-season transects in April to June, and waterbird counts for waterbird species. Single-species casual records, ringing and photographs add presence but never absence.

Recent visits carry more weight: each visit is weighted by 0.5 to the power of its age in years divided by five, so a record from ten years ago counts a quarter as much as one from this year. Squares last recorded more than five years ago are shown in the historic style.

Classes

Classes are relative to each species' own best squares: Very high is the top quarter of occupied square-months, High the next quarter, and so on. The legend on every map prints the absolute share of visits behind each class so common and scarce species can be compared. Surveyed, not recorded needs at least 20 qualifying visits with no record; fewer visits show as insufficient records rather than absence. Squares with fewer than 15 visits are drawn lighter. In months when a species is nationally almost absent, a square needs records from at least two visits in two different years to show at all; otherwise it appears as a small dot.

Sensitive species

For species on Schedule 1 of the Wildlife and Countryside Act 1981, the national sensitive-species lists and the Rare Breeding Birds Panel list, breeding-season months are not mapped, classes are collapsed to presence, and square-level counts and years are withheld. Records of such species from any source are never shown finer than 10 km.

Verification

For rarities, scarce breeders and a list of confusion pairs outside their core range, a square needs a verified record or two independent observers before it renders. Records-only pages state where records are unverified. Errors can be reported from every species page.

Known limits

• eBird records reach GBIF annually, so eBird coverage lags by up to a year.
• Coverage is uneven: uplands and parts of Wales and Scotland have few visits in many months. Use the coverage layer before reading an empty square as absence.
• Squares are drawn in British National Grid coordinates, so the map is not a slippy web map and Northern Ireland squares appear slightly rotated.

Privacy and corrections

Observer names in source records are hashed before processing and never published. Corrections and takedown requests:contact@example.com.