Inventorying mammal assemblages is vital for their conservation and management, especially when they include rare or endangered species. However, obtaining a correct estimation of the species diversity in a particular area can be challenging due to uncertainties regarding study design and duration. In this paper, we present the biodiversity estimates derived from three unrelated camera trap studies in Osogovo Mt., Bulgaria. They have different duration and positioning schemes of the camera trap locations: Study 1 -grid based, 34 days; Study 2 -random points based, 138 days; Study 3 -locations based on expert opinion, 1437 days. Utilising EstimateS, we compare a number of estimators (Shannon diversity index, Coleman rarefaction curve, ACE (Abundance-based Coverage Estimator), ICE (Incidence-based Coverage Estimator), Chao 1, Chao 2 and Jackknife estimators) to the number of present and confirmed and/or potentially present mammals (excluding bats) in the mountains. A total of 17 mammal species were registered in the three studies, which represents around 76% of the permanently present mammals in the mountain that inhabit its forested area and can be detected by a camera trap. The results point to some guidelines that can aid future camera trap research in temperate forested areas. A grid-based design works best for very short study periods (e.g. 10 days), while the opportunistic expert-based positioning scheme provides good results for longer studies (approx. a month). However, the grid-based design needs to be further tested for longer periods. Generally, the random points approach does not yield satisfactory results. In agreement with other studies, analysis based on the Jackknife procedure (Jack 2) appears to result in the best estimate of species richness. When performing camera trap studies, special care should be taken to minimise the number of unidentifiable photos and to take into account «trap-shy» individuals. The results from this study emphasise the need for careful preliminary planning of camera trap studies depending on aims, duration and target species.

The gray wolf and the domestic dog are closely related species that can interbreed and produce fertile offspring. In settings where unrestrained dogs are present in the wild, hybridization can happen naturally. However, the behavior of the resulting hybrids and their ecological impact is largely understudied. In September–November 2018, a putative gray wolf was repeatedly camera-trapped in a group of 10 presumably feral dogs in a remote mountainous area (the Osogovo Mountain) along the border between Bulgaria and North Macedonia. The most feasible explanation for this individual’s atypical behavior is that it is of hybrid origin (assumption based on phenotype). To the best of our knowledge, this is the first documented observation of such a kind. A discussion of its recruitment and position in the group is presented, setting the basis for further investigation of the complex interaction between wolves, dogs and hybrids in the wild.

The red deer is a main game species in Bulgaria, as well as in large parts of Europe. However, its behaviour has not been studied in depth on a local scale, especially on the Balkans. This study presents the first GPS telemetry data for red deer in Bulgaria –in the Central and Eastern Balkan Mountains, the Rhodopi Mountains and the lowlands of North-eastern Bulgaria. 18 individuals (2 subadult stags, 12 adult stags and 4 adult hinds) were tracked, accumulating a total of 1159 (for males) and 1464 (for females) GPS fixes. In spite of the pronounced individual differences, all stags were more mobile (average step length/12 h = 857 m) and had larger home ranges (average HR = 7393 ha) than hinds (average HR = 2085 ha, average step length/12 h = 448 m). Three of the individuals (1 hind and 2 stags) dispersed, forming two distinct home ranges. The results from this study outline the need for detailed studies on the behavior of the species, as a vital part of its management and conservation.

The raccoon dog, which lives especially near water and is rare in areas with low humidity, has been spreading throughout Europe since its introduction to Western Russia in the beginning of the 20th century. Official accounts of its distribution in Europe are often inaccurate due to scarce data. A literature search was conducted to identify records of the raccoon dog in the Balkans. More than 60 records were identified, including ones unlisted by the cited source from central and western Bulgaria, southern Serbia, Macedonia, Bosnia, Herzegovina, and Greece. The raccoon dog can be found on the Balkans either along the Danube (which is a major corridor for its invasion) or along its tributaries, which might represent secondary invasion pathways.