Warning Signs from the Warmth: Japanese Breeding Birds Signal Emerging Crisis
Four decades of bird survey data reveal diverging fates for Japan’s avifauna - with native species retreating from warming areas while non-natives expand unchecked
Range expansions hide abundance declines
Japanese breeding birds have broadly expanded their geographical ranges over the past 40 years, but this apparent success story masks a more troubling trend: many species are declining in number even as their distribution grows. A new national study analysing data from over 2,300 transects between the 1970s and 2010s reveals that this divergence is especially stark for open-land birds, waterbirds, and raptors. While range sizes have often recovered since declines in the 1970s–90s, actual abundance has continued to fall - suggesting that restored distributions do not necessarily indicate recovery.
Species such as the Yellow-breasted Bunting and Brown Shrike showed persistent range contractions. And although forest species saw some gains in certain ecological groups like ground-nesters and stem-probers, no overall abundance rise was observed, challenging assumptions that forest maturation would universally benefit woodland birds.
Non-native birds thrive in the heat
Non-native species are rapidly increasing both their abundance and range size in Japan - with some, like the Red-billed Leiothrix and Chinese Hwamei, tripling their occupied range in just two decades. These birds are disproportionately present in warmer regions and are better suited to Japan’s rising temperatures, raising concerns about biotic attrition of native communities. In one Shikoku forest site, Red-billed Leiothrix made up 75% of all birds captured in 2021.
Birds abandoning the hottest parts of their ranges
An analysis of long-term average temperatures within species’ distributions suggests that many native birds are retreating from the warmest parts of their ranges. Despite overall warming, maximum temperatures experienced by species have actually declined over the last 20 years. This implies that birds are shifting distributions northward or upslope where possible - but in much of Japan, geographic limits restrict such escape routes. As a result, birds adapted to cooler climates may face continued range attrition and possible local extinction.
Declines of open-land and waterbirds defy expectations
Grassland and farmland birds have long been recognised as vulnerable, but their continuing decline - even as some range sizes have recovered - is a cause for renewed concern. The reduction in traditional agricultural landscapes, abandonment of farmland, and changes in pesticide use may explain some of these trends. Waterbird declines, meanwhile, appear to be driven by light-weight species, and possibly linked to changing agrochemical regimes and predation risks.
Conservation in a warming, urbanised landscape
The authors warn that warm regions of Japan - already heavily modified by centuries of human activity - now face compounded threats. There are few intact forests or wetlands left, and nature reserves are sparse in the south. Yet these areas are also where non-native species are spreading fastest, and where native bird communities are losing ground. To respond effectively, conservationists must focus on managed landscapes such as abandoned farmland, urban woodlands, and traditional agricultural systems. Collaborative efforts across the East Asian–Australasian flyway will also be key for migratory species under pressure on both breeding and wintering grounds.
As climate change accelerates and human-altered habitats dominate much of Japan’s landscape, this study highlights a sobering truth: measuring only range size can mislead conservation efforts. In the heat of the crisis, it is the birds themselves - or their absence - that must guide where action is urgently needed.
June 2025
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