Distribution of the grass snake (Natrix natrix) and dice snake (N. tessellata) in Bulgaria

We summarise the distribution of the two species of the genus Natrix occurring in Bulgaria, based on records from 147 peer-reviewed publications, grey literature, and data repositories, combined with unpublished data. This is the first extensive mapping for N. natrix; records fall in 560 cells of the 10-km MGRS/UTM grid, of which 102 cells (18.2%) were with published information we could not confirm with new data, 175 (31.2%) were with published and confirmed, and 283 (50.5%) were with new localities. For N. tessellata we increased the number of cells with records by 64% compared to the 2011 mapping, by identifying 445 cells with localities: 162 cells (36.4%) were previously published and unconfirmed, 152 (34.1%) were published and confirmed, and 131 (29.4%) were with new data. Gross climatic conditions for records with exact locations were assigned following the Köppen-Geiger classification; the distribution for both species does not seem to be highly correlated to climate as they were found in 9 of the 12 Köppen-Geiger classes present, only missing from the 3 classes that are limited to high elevations in Bulgaria and account for less than 1% of the area. The vertical distribution of the observations supports our knowledge that the species are most numerous at lower elevations (92.4% of records were <1000 m above sea level for N. natrix and 92.6% were <500 m for N. tessellata). Higher elevations and some lowlands remain relatively understudied and future sampling will likely reveal new localities for both species.