The paucity of the avifauna of the Eurasian highlands (only 75 species breed regularly above the timber line) is determined by severe natural conditions and scarcity of food sources.

• 欧亚地区高地鸟类区系的贫乏性（仅有75种繁殖于林线之上）受严酷的自然条件和食物资源短缺所决定。