Mitrevichin, E., and L. Sakelarieva. “Contribution to the Study of the European Ground Squirrel’s Population (Spermophilus Citellus Linnaeus, 1766) in Rila National Park, Bulgaria”. Studia Ecologiae Et Bioethicae, vol. 18, no. 4, Dec. 2020, pp. 75-83, doi:10.21697/seb.2020.18.4.07.