A communal hibernaculum situated in Southwestern Bulgaria and its adjacent area (4 km in diameter) was studied. Seven snake species used this hibernating den, Malpoloninsignitus, Dolichophiscaspius, Platycepsnajadum, Elaphequatuorlineata, Telescopusfallax, Viperaammodytes and Xerotyphlopsvermicularis. The emergence of the snakes was related to the rise of daily temperatures. Malpoloninsignitus emerged from hibernation earlier in the spring than the other species, and most individuals were the first to leave the hibernaculum. There was a temporal segregation among the basking area in early spring, between the latter species and D.caspius, which were the two most abundant species. These two species also used the same area of the hibernaculum, in contrast to P.najadum and E.quatuorlineata, which used a different area. After spring dispersal, M.insignitus and D.caspius used the same microhabitats during the rest of the active period. The earlier emergence of M.insignitus (and earlier departure from the site) probably reduces the competition for basking sites. The early departure of the species may give advantage in the competition for optimal microhabitats with D.caspius.

The seasonal and circadian activity patterns of Vipera ammodytes were studied in five sites in western Bulgaria between 2014 and 2017. Vipera ammodytes was active from the end of February to the first half of November. In spring, adult males emerged a few weeks earlier than females and immature vipers. In autumn, the different sex/age groups started their hibernation approximately at the same time. We registered differences in the seasonal and circadian activities between the different sex/age groups. Adult males and subadults were mostly encountered during spring and autumn, and juveniles mainly in autumn, while the encounter rate of adult females was steady throughout the activity period. Females’ circadian activity varied according to their reproductive status. The observed patterns might be a result of the complex effects of many factors, such as climatic conditions, differences in the reproductive behaviour and the cost of reproduction between sexes, as well as the trade-off between precise thermoregulation, predation risk and foraging needs.

Different snake species may be active at various times of the day. In Europe, most snake species are predominantly diurnal, but about a third are partially or principally crepuscular and/or nocturnal. Here, we report the first instance of multiple in situ observations of the crepuscular and nocturnal activity of V. ammodytes in Bulgaria. Overall, usually as a result of general herpetological surveys and chance observations between 2001–2020, we recorded crepuscular/nocturnal activity of twenty-seven individuals from both sexes and all age classes (juvenile to adult), observed from April to September, following warm (maximum daily Tair = 20–28 °C) and hot (Tair > 28 °C) days. These records represent less than 1.5% of all our personal viper observations. However, given that our observations occurred throughout the country, within five different climatic zones, we hypothesise that nocturnal activity of V. ammodytes is more common than previously reported.

In a recently published paper (Popgeorgiev et al. 2019), we compiled for the first time published and unpublished data on localities of the herpetofaunal species observed up to ca. 10 km south of the Bulgarian Danube River. Overall, we identified 687 published records belonging to 62 cells of the 10×10 km MGRS grid. Another 1918 records with real coordinates of our unpublished data from the past ca. 13 years belong to 1269 cells of the 1×1 MGRS grid. As a result, 34 native species – 15 amphibians (4 salamanders and 11 frogs) and 19 reptiles (1 turtle, 2 tortoises, 8 lizards and 8 snakes), and one invasive turtle, have been recorded; further species findings are unlikely. This study further identifies areas that are under-sampled and species such as Pelophylax lessonae that are cryptic. Our data are especially relevant concerning the high economic incentives to change the hydrology of the Danube, which will likely negatively impact the biodiversity along the river and its surrounding.