BTO First Atlas of Breeding Birds in Britain and Ireland: 1968-1972.

The dataset was used to produce the first BTO/IWC Breeding Atlas 1968-72 (Sharrock, J.T.R. (1976) The atlas of breeding birds in Britain and Ireland. T. & A.D. Poyser).
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Volunteer (mostly) observers compiled as complete a list as possible of breeding bird species in each 10-km square and noted whether the species was Possibly, Probably or Confirmed as breeding -- a series of standard categories (and codes) were used to specify these, categories which have since, with mostly minor modifications, become a worldwide standard. To make the set directly comparable to the Second Atlas these were converted into S -- Seen (for Possibly Breeding) or B -- Breeding (for Probably and Confirmed Breeding).