Many breeding birds move to higher ground

Swiss breeding birds are distributed along an altitudinal gradient of more than 3000 m. Using atlas data from 1993–1996 and 2013–2016, we examined whether a shift in altitudinal distribution had taken place. The sample included 71 common species for which we have density change maps; of these, 40 are woodland birds.

The centre of distribution of the studied species (their average altitudinal distribution) has shifted upwards by 24 m in the past 20 years. However, there are significant differences between the species: the range of the Hooded Crow, for example, has shifted downwards by 166 m, while the Willow Warbler has gained 205 m. Overall, altitudinal distribution increased for almost two thirds of all species between the two atlas periods. The difference between upward and downward shifts is particularly apparent in larger movements: only four species have shifted downward by at least 50 m, while 22 species have moved upward along the altitudinal gradient by 50 m or more.

Among the 47 species that shifted their range upwards, 20 show a similar pattern: their populations have decreased at lower altitudes while increasing in the upper ranges of their distribution, independent of their ecological requirements and their average altitudinal distribution. The remaining 27 species either show only increases at higher altitudes or only losses in lower areas. Only four species show losses at high altitudes and gains in the lowlands.

The upward shift between the two atlas periods is particularly pronounced in species whose populations are concentrated at high altitude. The ten species with the highest altitudinal distribution in 1993–1996 experienced an average upward shift of 51 m; the ten species with the lowest distribution decreased in altitude by 8 m. In addition to the species included in this analysis, Black Grouse and Rock Ptarmigan also show an upward range shift.

Finally, the ten species that are either included on the Red List of threatened breeding birds or classified as Near Threatened (NT) experienced a marked upward range shift of 84 m. In this group, the change presumably does not reflect a spread to higher altitudes but, rather, losses at lower elevations (e.g. Common Redstart, Garden Warbler). In contrast, the altitudinal distribution of species with increasing populations (e.g. Eurasian Blackbird, Common Woodpigeon) remained largely unchanged or even shifted downwards somewhat.

There is little doubt that climate change is influencing the upward shift in the distribution of Swiss breeding birds. Climate warming is twice as great in the Alps as it is in the valleys, which could explain the above-average upward range shift of mountain birds compared to lowland species.

Other factors also need to be considered and their effects analysed in more detail. First and foremost are human activities, especially agricultural intensification and the spread of settlements. Both these trends are much more pronounced on the Central Plateau than in the mountains. Most bird species in decline in Switzerland occur at low altitudes and are therefore particularly exposed to human activities. That birds like the Eurasian Skylark and others appear to be moving to higher altitude is actually a result of population collapses in the lower reaches of the range. Changes in mountain farming also influence the distribution of breeding birds. These include intensification and the abandonment of marginal land as well as the spread of forests. However, every species needs to be considered separately, as the interplay of various factors affects each one in a different way.

The upward shift of several species and the stable or even increasing populations of other mountain species suggest that the Alps may serve as a refuge in the future, when even more pronounced environmental changes are expected to occur. This is an important aspect to consider when planning large-scale conservation programmes or development projects (tourism, agriculture) in Alpine areas.

Scientists have only recently begun to study altitudinal range shifts. More research is needed in this area. Current trends do not bode well for birdlife. On the one hand, human activities such as intensive agriculture, leisure activities and the construction of roads and tourism infrastructure are expected to continue at much the same pace. On the other hand, the area of suitable habitat for species like the Rock Ptarmigan inevitably shrinks with increasing altitude. Finally, habitats respond to climate warming with a certain time lag, especially forests. But how the resulting ecological imbalances will affect the species occupying these habitats is impossible to predict. The Alps therefore play a central role in conservation, because of their rich and fragile biodiversity, but also because of their future role as a refuge.

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