Gaining information on habitat requirements is the most important activity in predicting occurrence of a particular species of fauna and flora. This thesis is focused on estimating population densities and habitat requirements of a small passerine bird Phylloscopus collybita in the mature beech-oak forest in the area of the upland Abovska pahorkatina. The objective of the thesis is to describe the population abundances and densities, territory dispersal and detect floristic and structural vegetation variables indicating territory placement. I established two census plots in size totalling I 57,75 ha. Territory mapping method (spot-mapping) was applied to census breeding populations and detect territory dispersal. In total, I carried out 11 mapping visits to each census plot from April until June 2018. I detected 14 territories. The mean population density was 1.8 pairs/ 10 ha. Vegetation variables was estimated by the 11.3 radius circular plot method developed for studies of bird-habitat relationships (Noon 1981). In total, I estimated 48 floristic and structural variables describing the given habitat. Bird habitat requirements were estimated by comparison of vegetation characteristics of territories and randomly distributed plots. I described vegetation variables in 14 territories (7 per plot) and 40 randomly distributed circular plots (20 per plot). Habitat requirements were statistically tested by the logistic regression by comparing vegetation variables in circular plots placed within territories with the randomly distributed circular plots. From total set of 48 variables, I selected 10 least correlated variables. Logistic regression detected only one negatively associated variable (horizontal canopy cover 1-2 m), however ecological meaning of the analyses remains unclear. I also tested differences between 10 classes of horizontal canopy covers between territories and the randomly placed plots by Mann-Whitney test. This test showed strong significant differences in all classes of horizontal canopy cover. It may indicated that the species selects territories in denser ground vegetation cover. This founding is confirmed in the previous researches of this species. Simple linear regression analysis of published meta-data yielded strong significant negative relationships between population densities and elevation and indicated the highest population densities of species in lowlands. Results of the thesis can be in conservation biology of this species, modelling habitat requirements and forestry management.