1.5K Citations
Censuses cover all observed bird species. Annual counts have been done in the winter time since 1957. Methodology is explained in link:
http://www.luomus.fi/seurannat/methods.htm
Data quality: From year 2019, 390 documents are incorrectly missing MY.taxonCensus: aves -key/value, which tell that all observed birds have been recorded during the census.