As a result, 51 species of invertebrates, of which 12 species are listed in the category Extinct (EX) and 39 species in the category CR have been included. Having said this, in comparison with the first edition of the Red Data Book, which included a total of 157 animal species and subspecies, there is a significant increase in threatened species. However, this is mainly due to changed methodology and the enlarged scope of taxonomic assessment. Birds remain the best studied, being regularly monitored by the Bulgarian Society for the Protection of Birds.