Context This study examined how human-induced landscape changes affected bird diversity in mountain villages with mixed forests and cultivated fields. Objectives We focused on the bird species composition (beta diversity) to determine whether species homogenization varied with forest cover differences. This study developed it as a novel metric potentially quantifying homogenization level comparison in species functional groups. Methods Bird surveys were conducted at 27 sites with forest cover scattered from 0 to 100%. Bird species were divided into forest birds and nonforest birds. Beta diversity was partitioned into turnover and nestedness-resultant components, and their contribution ratio to Sørensen beta was used to examine the effect of forest cover and pairwise cover difference. Results Our results indicated that forest birds exhibited low turnover and low nestedness, whereas nonforest birds exhibited nestedness distribution characteristics. The nestedness-resultant contribution ratio of nonforest birds was higher, driven by forest cover difference (slope = 0.0080, Pseudo R 2  = 0.35 ** ) than that of forest birds (slope = 0.0018, Pseudo R 2  = 0.02 ** ). Thus, we quantify the result of forest cover decreased lead to the dominated by nestedness species in nonforest birds. Conclusions These findings highlight the distinct effects of environmental changes on these two bird functional groups. Although the Sørensen beta diversity increases with forest cover decrease, most are contributed by nestedness-resultant nonforest birds and lead to homogenization. Thus, it is recommended that conservation plans should separately address forest and nonforest bird species to avoid the risk of underestimating species homogenization due to anthropogenic land-use and habitat degradation.