Land cover change for agriculture is thought to be a major threat to global biodiversity. However, its ecological impact has rarely been quantified in the Northern Hemisphere, as broad-scale conversion to farmland mainly occurred until the 1400s-1700s in the region, limiting the availability of sufficient data. The Ishikari Lowland in Hokkaido, Japan offers an excellent opportunity to address this issue, as hunter–gatherer lifestyles dominated in this region until the mid-19th century and land cover maps are available for the period of land cover changes, i.e., 1850-2016. Using these maps and a hierarchical community model of relationships between breeding bird abundance and land cover types, we estimated that broad-scale land cover change over a 166 year period was associated with more than 70% decline in both potential species-richness and abundance of avian communities. We estimated that the abundance of wetland and forest species declined by >88%, whereas that of bare-ground species increased by >50%. Our results suggest that broad-scale land cover change for agriculture has led to drastic reductions in wetland and forest species and promoted changes in community composition in large parts of the Northern Hemisphere. This study provides potential baseline information that could inform future conservation policies.