Methodology
How the bird data is processed
Waarzitdievogel.nl turns a large GBIF download into monthly totals by bird species and Dutch birding area. This page explains which records are selected, how locations are assigned and which limitations apply.
1. Source selection
The input is a full GBIF download of bird records for the Netherlands. Processing includes only records with country code NL, class Aves, status PRESENT, valid coordinates, year, month and a GBIF species key. Each download has its own DOI, which is cited alongside data on the site.
2. What a count means
The displayed count is the sum of individualCount from selected GBIF records. If a source record has no positive individualCount, the importer uses 1. It is not a count made by Waarzitdievogel.nl and not automatically a number of unique individual birds. One record may represent one bird, several birds or a source registration without a supplied number.
3. Duplicate records and source differences
The import groups records sharing species, coordinates, year and month, but does not independently determine whether separate GBIF records describe the same biological observation. Duplicate submissions, shared datasets, different survey methods and later source corrections can affect totals. The figures are best used for broad historical patterns, not exact population counts.
4. Assignment to areas
A georeferenced record is assigned to every area whose stored polygon covers that point. Areas without a usable polygon use a radius around the location point: 15 kilometres by default, unless a location has its own radius. Areas may overlap. The same source record can therefore count in more than one area, and area totals should not be added without correcting for overlap.
5. Aggregation and publication
After spatial matching, values are summed by location, species, calendar year and month. Public tables store these monthly aggregates, not every individual source record. A bird-location page exists from 5 counts across at least 3 calendar months. Search indexing uses a stricter threshold of at least 100 counts and 6 months; weaker pages remain accessible but are marked noindex.
6. Observer and location bias
More records do not automatically mean greater biological abundance. Large or popular sites often receive more visits. Conspicuous, rare or easily identified birds are reported differently from inconspicuous species. Access, season, survey projects and source organisations also affect coverage. Interpret patterns with knowledge of area, effort and source context.
7. Updates and quality control
The update cycle periodically requests a new full GBIF download. A processed run replaces public monthly aggregates only after checks on species, spatial matches and minimum row counts. Each successful run stores its processing date, DOI and totals. Pages show the latest processing date and source DOI where available.
How to use the figures responsibly
- Use monthly charts for broad seasonal comparisons, not daily predictions.
- Read counts as aggregated GBIF record values, not population estimates.
- Do not add totals from overlapping areas.
- Check a recent-observation source before planning a visit.
- Report unexpected values with the URL, species, location and reason for concern.