At first glance, the numbers recorded between 1980 and 2012 appear to be quite balanced. 68 of the 158 bird species that inhabit the area around Lake Constance became more populous, while 67 species declined; each of these figures approximates to 43 percent of all the bird species in the region. The total number of species has even increased slightly: although eight species have died out, 17 have either returned to the region or settled there for the first time. These include the white stork, the peregrine falcon and the eagle owl, all of which have benefitted particularly from the protective measures put in place.
This seeming contradiction is due to the fact that the most common species are disappearing particularly rapidly. Six of the ten most common bird species around Lake Constance have declined dramatically in number, while two have remained the same and only two have increased. The population of house sparrows, for example, has declined by 50 percent since 1980, at which time it was still the most common species. "These are really shocking figures - particularly when you consider that the bird population started declining decades before the first count in 1980," explains Hans-Günther Bauer from the Max Planck Institute of Animal Behavior. Viewed over a lengthier period, the fall in numbers may therefore be even greater.
Agricultural landscape hostile to birds
It is particularly noticeable how differently the various habitats have been affected. The study indicates that bird populations around Lake Constance are dwindling particularly rapidly in countryside, which is intensively used by humans. This applies above all to modern farmland: 71 percent of the species that inhabit fields and meadows have declined in numbers, in some cases drastically. The partridge, for example, which was once a common inhabitant of the region's farmland, has completely died out around Lake Constance. The great grey shrike, the meadow pipit and the little owl have also disappeared from the area.
One of the main reasons for this decline is the scarcity of food. According to the ornithologists, 75 percent of the bird species that eat flying insects and 57 percent of those that eat terrestrial invertebrates have decreased in number around Lake Constance. "This confirms what we have long suspected: the human extermination of insects is having a massive impact on our birds," says Bauer. In addition, today's efficient harvesting methods leave hardly any seeds behind for granivorous species. Moreover, the early, frequent mowing of large areas of grassland, the agricultural practice of monoculture, the early ripening of winter grains, the implementation of drainage measures and the shortage of fallow land are destroying the habitats of many species that live in the open countryside.
However, the birds are disappearing not only from the fields and meadows but also from the towns and villages around Lake Constance. "The increasing need for order and decreasing tolerance of dirt and noise are making life more and more difficult for local birds. It appears that successful breeding is becoming increasingly rare since the birds are being forced to nest amid tower blocks, ornamental trees and immaculate kitchen gardens," says Bauer. Even species that can survive virtually anywhere, such as blackbirds (down 28 percent), chaffinches and robins (each down 24 percent) are suffering greatly due to the deteriorating conditions in settled areas.
Winners and losers in the woods and on the water
In contrast, the woodland birds around Lake Constance appear to be doing comparatively well. 48 percent of the forest-dwelling species are increasing in number, while only 35 percent are dwindling. One example is the spotted woodpecker, whose numbers have grown by 84 percent. Like other woodpeckers, it seems to have benefited from the larger quantities of timber in the forest. Furthermore, more of the species that inhabit the wetlands around Lake Constance have increased than decreased. The winners here include the mute swan.
Nevertheless, the numbers of many forest-dwelling species are also declining. The wood warbler population, for example, has fallen by 98 percent, firecrest numbers by 61 percent. This is how the intensive use of timber around Lake Constance and the shorter felling intervals are making themselves felt. Trees containing nests are being felled even in protected areas, and breeding seasons are largely being ignored. Older trees are often felled for traffic safety reasons; new paths are laid in the forests and wet areas are drained.
All in all, the last population count in 2010-2012 documents the same developments and causes as those that preceded it. However, the situation has clearly worsened in some cases. There is hardly any indication that things have changed for the better since then. "The living conditions for birds around Lake Constance have in fact deteriorated further over the last seven years. This means that their numbers have presumably fallen still further in this time," says Bauer.
More food and living space for birds
With its diverse structure and location in the foothills of the Alps, the Lake Constance region actually provides excellent living conditions for birds. However, the changes it has undergone over the last few decades are typical of densely populated regions with intensive farming and forestry. "This means that the rapid decline in the populations of many species that we have observed around Lake Constance is sure to be happening in other regions as well," says Bauer.
The study is one of only a few long-term investigations of breeding bird populations ever conducted in Germany. In order to collect the most recent data, which dates from between 2010 and 2012, 90 volunteers joined the scientists and counted all the birds in an area of approximately 1,100 square kilometres surrounding Lake Constance. The ornithologists first recorded the bird population between 1980 and 1981 and have repeated the count every ten years ever since. The next count will take place between 2020 and 2022.
- - Measures that would benefit the bird populations include:
- The scientists are calling for agricultural and forestry policy to be reconsidered in order to counteract the rapid loss of biodiversity.
- Drastically restricting the use of insecticides and herbicides in forestry and agriculture, in public spaces and in private gardens
- Significantly reducing the use of fertilisers
- Converting at least ten percent of agricultural land to ecological conservation areas
- Leaving some areas of arable land and grassland uncultivated in winter and during the breeding season
- Late mowing outside the grassland birds' breeding season, maintenance of flower strips and fallow areas for seed production
- At least five percent of woodland should be left completely unused
- Creating natural gardens using indigenous plants
Hans-Günther Bauer, Georg Heine, Daniel Schmitz, Gernot Segelbacher & Stefan Werner
Starke Bestandsveränderungen der Brutvogelwelt des Bodenseegebietes - Ergebnisse aus vier flächendeckenden Brutvogelkartierungen in drei Jahrzehnten.
Vogelwelt 139: 3 - 29 (2019)