Long-distance migrants in difficulty

Common Redstart © Beat Rüegger

Overall, numbers of long-distance migrants are gradually declining, while those of short-distance migrants and residents appear to be increasing. This trend is not confined to Switzerland. Compared to short-distance migrants and residents, long-distance migrants are more specialised, more severely affected by habitat changes in breeding and wintering grounds, and thus more vulnerable.

Our native breeding birds can be divided into two groups based on their migration behaviour. Residents and short-distance migrants spend the winter in their breeding grounds or relocate to the Mediterranean region. In many species, only part of the population migrates, while the rest winters in the breeding grounds. The European Robin is an example of such a partial migrant. The second group includes species that mostly winter in sub-Saharan Africa. Only few of our native birds migrate to Asia (mainly India), like the Common Rosefinch.

The overall population trend of long-distance migrants in Switzerland has been negative since 1990, while residents and short-distance migrants are increasing in number. Other European countries report similar findings. Long-term declines are especially evident among long-distance migrants that winter in open habitats and breed in similarly open country in Europe.

Distribution change since 1993–1996 of residents and short-distance migrants (85 species, above) and long-distance migrants (30 species, below). The map combines the distribution change maps of the relevant species. For residents and short-distance migrants, the resulting picture is far from uniform, but reflects a positive overall development. In contrast, the trend for long-distance migrants is almost entirely negative at lower and medium altitudes.

Long-distance migrants are particularly vulnerable

It became apparent around 1960 that long-distance migrants were in difficulty. A first wave of decline occurred between then and 1970, in some cases continuing into the early 1980s. The decline concerned species wintering in the Sahel zone, which was affected by drought at the time. A second phase began in the 1980s and involved species that winter in the tropics and the forest zone of Guinea in West Africa, such as Willow Warbler and Wood Warbler. But still today, the declines predominantly affect species that winter in dry, open habitats in Africa.

Long-distance migrants tend to be specialists. They have evolved to spend short periods of time in the breeding grounds, occupying habitats and searching for food that is available only briefly (mostly insects in dense vegetation or in open terrain that is uninhabitable in winter). Residents and short-distance migrants, in contrast, need to be generalists (e.g. corvids, finches or sparrows) that can cope with the constant seasonal change of food sources and habitats. Moreover, long-distance migrants travel between several completely different locations, spending 4–5 months in the breeding grounds, two months on spring and autumn migration, and 5–6 months in the wintering sites. Certain species move considerable distances within the wintering range, while others stay in one location, returning to the same site year after year. Long-distance migrants are thus vulnerable in several respects: As specialists, habitat changes at one of the sites frequented in the course of the year can quickly put them under pressure. Moreover, they need to be in certain places at certain times, in keeping with their tight annual schedule. Finally, many species face a high risk of mortality during migration.

Swiss Bird Index SBI® for residents and short-distance migrants (green) and long-distance migrants (blue). While the first group has done well (with some exceptions), long-distance migrants have declined overall.

Changes in breeding grounds play a decisive role in the decline

Compared to residents and short-distance migrants, the group of long-distance migrants includes more species that occupy open habitats such as farmland, but also wetlands. There have been significant changes in these types of habitat both in the breeding and in the wintering grounds, so it is not possible to attribute the decline to either one or the other.

In the breeding grounds, open habitats in particular have deteriorated: farming practices have intensified, causing the decline of farmland breeding birds; wetlands have decreased in size and often suffer from a lack of water, causing the local extinction of species. The fact that long-distance migrants have above all disappeared from the Swiss lowlands, where the impact of human activity is especially strong, is an indication that the decline is largely «home-made».

For each altitude level, the graph shows the average number of species of long-distance migrants per kilometre square in 1993–1996 (light blue) and 2013–2016 (dark blue). Species richness declined in areas below 1500 m while staying constant above that altitude, suggesting that many causes of decline can be found in the breeding grounds.

Changes in the wintering grounds are also detrimental

Migrating birds face huge problems if stopover sites are unavailable. In particular, stopover sites at the edge of the Sahara are essential for migrants that replenish their fat reserves there (in northern Africa during autumn migration, in the Sahel during spring migration).

In the 1970s, the Sahel zone suffered from massive drought, leading to the decline of several species that winter in this region (e.g. Common Redstart, European Pied Flycatcher, Collared Sand Martin and commonr Whitethroat). Rainfall has increased again in the Sahel since the 1990s, without however reaching the amounts of earlier years. The area also suffers from deforestation. Landscapes are changing rapidly in the more southern latitudes of Africa as well, as forests are cleared, trees in the savannah are thinned out, and water use increases. Other negative factors include hunting of birds in the Mediterranean region and North Africa as well as climate change, which tends to cause droughts and irregular rainfall along the migration routes and in Africa.

Long-distance migrants have evolved remarkable adaptations and accomplish extraordinary feats of flight. Barn Swallow, Common Nightingale and Eurasian Golden Oriole manage to take advantage of peak insect abundance during summer in Europe while spending the rest of the year thousands of kilometres away. We must make every effort to offer them the best possible conditions in our country.

keine Übersetzung benötigt: Lukas Jenni & Hans Schmid

Recommended citation of the Atlas online:
Knaus, P., S. Antoniazza, S. Wechsler, J. Guélat, M. Kéry, N. Strebel & T. Sattler (2018): Swiss Breeding Bird Atlas 2013–2016. Distribution and population trends of birds in Switzerland and Liechtenstein. Swiss Ornithological Institute, Sempach.

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