The reserve lies east of the main Shahrud to Shahpasand highway, c.70 km north-east of Shahrud, spanning the divide of the eastern Alborz Mountains from the upper limits of the humid Caspian forest in the north to the semi-arid Artemisia steppes of the Central Plateau in the south. Much of the central portion of the reserve consists of high grass plains and tragacanthic steppe between rocky hill ranges. Several springs support luxuriant herbaceous vegetation and some shrubbery. Juniperus forest occurs on some of the higher, south-facing slopes.
See box for key species. Spanning Caspian forest, montane steppe and near-desertic steppe, the reserve has a very diverse breeding bird fauna including eight species of lark. Other notable species include Cursorius cursor, Melanocorypha bimaculata, Phoenicurus erythronotus, Sylvia hortensis, S. nana, Rhodopechys mongolica, Emberiza leucocephalos, E. buchanani and E. bruniceps. Phylloscopus trochiloides nitidus is fairly common as a passage migrant. At least 114 species have been recorded in the reserve.
Non-bird biodiversity: Mammals: Canis lupus (V), Ursus arctos (rare), Caracal caracal (rare), Panthera pardus (rare), Gazella subgutturosa (rare), Capra hircus aegagrus (rare) and Ovis ammon (rare). During the 1970s Acinonyx jubatus (V) became fairly common, more frequently observed here than anywhere else in Iran. An attempted re-introduction of Equus hemionus (V) took place in 1973-1974.
BirdLife International (2019) Important Bird Areas factsheet: Khosh-Yeilagh. Downloaded from http://www.birdlife.org on 23/10/2019.