Focus

      New species still arriving in our cities

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      Yellow-legged Gulls nesting on flat roofs are a recent phenomenon in Switzerland. © Jürg Hostettler

      The built-up areas of Switzerland continue to expand, crowding out species that inhabit open spaces. But some species benefit, because they can adapt to conditions in towns and cities and occupy them as new habitat. This colonisation of urban areas is still in progress – the most recent examples are Common Woodpigeon and Yellow-legged Gull.

      The Eurasian Blackbird’s melodious song is a familiar sound in settlements. In the early 19th century, however, the Blackbird was only found in woodlands. The species began to settle in towns from 1820, initially in Germany, and then in other countries. Moving from western Europe towards the east, this process is still ongoing in the eastern-most reaches of the Blackbird’s range. Other species such as Black Redstart, Common Swift and Alpine Swift have also colonised settlements, using buildings as «artificial cliffs» for nesting. Today, the majority of their Swiss populations occur in settlements.

      Colonisation of urban areas: a process of adaptation and claiming new territory

      Species that want to survive in settlements face a host of challenges. Besides distinct habitat features, a different species composition (e.g. new competitors or predators, many non-native plants) and a range of hazards (e.g. traffic, glass windows), the direct and indirect disturbance caused by the permanent presence of humans is a particular challenge. Birds also need to adapt to noise and artificial light. The colonisation of towns can occur independently in several locations (e.g. Eurasian Magpie) or be initiated by a few «urban specialists» that spread with their descendants from one town to the next. In the case of the Eurasian Blackbird, for example, there seems to be a specific type of «urban Blackbird». Such urban colonisation processes are still ongoing today.

      The «urban Woodpigeon»

      In 1993–1996, the Common Woodpigeon only occurred very locally in settlements in Switzerland, with the exception of Geneva and Basel. The further colonisation of urban areas started only at the beginning of the 21st century. To determine the trend in settlements, we compared 85 kilometre squares (1 × 1 km) that lie predominantly in urban areas and were surveyed in both 1993–1996 and 2013–2016. The Woodpigeon population has tripled in these kilometre squares. Thus, the population increase in settlements is much greater than in other habitats (mainly woodland), where numbers «merely» doubled in the same period. So far, there is no indication that the colonisation of cities has reached capacity – the populations in urban areas will presumably continue to grow.

      Comparison of Common Woodpigeon territory numbers in 85 kilometre squares with a large settlement area that were surveyed in both 1993–1996 and 2013–2016. In the 1990s, the seven squares in the Geneva region supported 27 % of all territories in settlements; 20 years later, they made up only 13 %.

      Compared with the rest of Europe, the Woodpigeon was late to colonise settlements in Switzerland. The species began to occupy cities such as Paris and London in the first half of the 19th century. In Germany, colonisation of urban areas was hesitant at first, until the process began to accelerate in the 1960s. Many cities in Sweden and Finland followed up until the 1990s. In many places, the presence of the Woodpigeon extends to the city centres, sometimes reaching remarkable densities of up to 25 pairs/10 ha. The species has lost its fear of humans and behaves much like the feral pigeon. This behavioural adaptation greatly facilitates the colonisation of settlements. A shorter escape distance than in other habitats has been observed in several other species too.

      The colonisation of towns by the Woodpigeon is aided by mild winters, the ample food supply, lower predation pressure and the general increase in numbers. In many parts of Germany, the species’ population density is greater in urban areas than in the surrounding woodlands.

      Mild winters have resulted in an increase in winter observations in Switzerland ever since the 1990s. Since winter 2003/2004, there has been a sharp increase in records – many of them from settlements. Other species, such as the Blackbird, have probably also wintered in settlements prior to breeding there. Settlements at our latitude offer advantages for wintering birds because they are somewhat warmer than the surroundings and food is provided by humans directly or indirectly (e.g. feeding, rubbish, berries, snow removal).

      Yellow-legged Gulls nest on flat roofs

      Several other species have gained a foothold in settlements. The colonisation of towns by the Yellow-legged Gull, for example, is also associated with a marked population increase. In 2016, about ten times as many Yellow-legged Gull pairs bred in Switzerland as in 1993–1996. While roof nesting was first recorded back in 1994 near Versoix GE, the number of breeding records in settlements, mostly on flat roofs, increased mainly between 2013 and 2016, almost tripling in that period. In 2016, 104 broods were counted on buildings. Compared to natural breeding sites, predation pressure is probably lower on flat roofs. On the downside, roof-nesting birds have to cope with higher temperatures. But the benefits clearly outweigh the disadvantages, as roof-nesting birds are prepared to fly large distances to their feeding grounds near water or on farmland.

      Roof-nesting Yellow-legged Gull colonies (yellow symbols) are often observed near traditional colonies. So far at least, roof nesting rarely takes place away from the natural breeding sites.

      Which will be the next «urban species»?

      Some species have inhabited European cities for centuries. In Switzerland, the process has been slower. In Germany, for example, many more species are known to inhabit settlements, such as Dunnock, Song Thrush, Eurasian Jay, Crested Tit, Common Chiffchaff and Northern Wren, and even raptors like Eurasian Sparrowhawk, Northern Goshawk, Eurasian Eagle-owl and Northern Long-eared Owl. So far, these species are absent from gardens and parks in Switzerland, or at most breed occasionally. The reasons are not known, though the larger size of parks and gardens in Germany may be an advantage. In any case, it will be interesting to see which species shows up next in our green spaces. To make sure this happens, we need to preserve large trees and remaining near-natural habitats despite increasing urban density, and manage gardens and parks in a way that encourages biodiversity.

      Text: Thomas Sattler


      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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      Species concerned
      Subject
      Settlements
      New breeding species
      Species on the rise
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