Swiss Breeding Bird Survey (1999–2025)

The Swiss Common Breeding Bird Survey tracks annual species counts across 267 fixed survey routes distributed throughout Switzerland, spanning elevations from 250 m to over 3,000 m — a range that captures everything from lowland agricultural plains to alpine tundra. Collected over 27 years (1999–2025) and published via Zenodo, the dataset records 7,113 route-year observations covering the number of bird species detected, visit frequency, route length (median ~5 km), and precise GPS coordinates. The most striking pattern is a clear elevation penalty on biodiversity: median elevation is moderately negatively correlated with both survey visits (r²=0.48) and species richness (r²=0.40), meaning higher-altitude routes are not only harder to survey but consistently host fewer species. Survey effort itself matters enormously — routes with more visits record significantly more species (r²=0.54), with counts ranging from just 3 to 64 species per route, underscoring that detectability is as important as true abundance. This long-running dataset is invaluable for tracking how climate change and land-use shifts are pushing (or squeezing) bird communities up and down the Swiss altitudinal gradient.

• Bird diversity is rising across Switzerland: average recorded species per site increased 14% from 31.4 (1999) to 35.9 (2025)
• The gain is elevation-dependent — alpine sites above 2000m saw a 31% increase vs. only 7% at low elevations, consistent with uphill range shifts
• Mid-elevation sites (1000–1499m) host the richest communities at 39.7 species on average, edging out lowland sites at 37.9
• Species diversity drops sharply above the treeline: alpine sites average just 15.6 species, less than half the mid-elevation average
• The 267 monitoring routes have been surveyed consistently for up to 27 years, making this one of Europe's longest-running bird biodiversity datasets