We have just published the observation data used for the in 2004 published "Atlas van de Vlaamse broedvogels 2000-2002":
Vermeersch G., Anselin A., Devos K., Herremans M., Stevens J., Gabriëls J. & Van Der Krieken B. (2004) Atlas van de Vlaamse broedvogels 2000-2002. Mededelingen van het Instituut voor Natuurbehoud 23, Brussel, 496p. ISBN 90-403-0215-4
The dataset contains bird observations gathered by volunteers using four sampling methods and was used to assess the distribution - and where possible relative abundance - of all breeding bird species in Flanders at the time.
As always, the dataset is documented with extensive metadata, largely based on Bird Census News 2004 1/2: 35-47. The dataset contains over 330.000 observations for 197 taxa and is available through our (recently upgraded) GBIF Integrated Publishing Toolkit and GBIF. Special thanks to editors Glenn Vermeersch (INBO) and Anny Anselin (INBO), data liaison officer Dimitri Brosens (Belgian Biodiversity Platform), and the many volunteer observers to make this possible.
Broedvogels - Atlas of the breeding birds in Flanders 2000-2002 is a species occurrence dataset published by the Research Institute for Nature and Forest (INBO). The dataset contains more than 330,000 bird observations, collected during three breeding seasons (2000, 2001, 2002) using a standardized field methodology using a 5km x 5km square grid system covering all of Flanders, Belgium (645 squares) as well as up to eight 1km x 1km sample squares within each larger square. Test data from 1999 and additional data to increase coverage from 2003 are also included. The dataset includes information on 197 taxa, of which 183 species (13 exotic) and 3 subspecies are considered breeding in Flanders. The data are released in bulk as open data. See the dataset metadata or the paper Bird Census News 2004 1/2: 35-47 for contact information, scope and methodology. Issues with the data or dataset can be reported on GitHub. [from the metadata]