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The surprising success story of the red kite
Intensive agriculture puts numerous species of the cultivated land in dire straits. Only the red kite seems to be immune to it. What are the reasons for that?
The history of the red kite in Switzerland doesn't really fit into the overall picture. The vast majority of birds in the agricultural area are under pressure. The populations of many once common and widespread species have collapsed in the course of the intensification of agriculture.
The skylark, for example, whose song was once omnipresent in our fields and meadows, has lost more than 90 percent of its original population in the canton of Zurich in recent decades. The situation is even more dramatic with the Whinchat, which has meanwhile almost completely cleared the Central Plateau. Ortolan and partridge have only been breeding in meager residual populations in recent years. According to a status report by the Swiss Ornithological Institute, the latter is now likely to have completely disappeared from Switzerland. It joins a number of agricultural birds such as the gray shrike and black-fronted shrike, which no longer find a place in this country.
It is completely different with the red kite, the largest of our birds of prey after the vultures and the golden eagle. Although it is also a typical inhabitant of the cultivated land, it has been able to steadily expand its distribution area in Switzerland over the last few decades. Since the nineties it has been advancing further and further into the larger alpine valleys. His hunting grounds now extend into the alpine zone. Around 3,000 pairs are now breeding in Switzerland. Only in the cantons of Geneva and Ticino are no evidence of breeding until now.
With the expansion into new and higher-lying areas, there was also a densification of the stocks in the lower elevations, which are unparalleled anywhere in the world. In the canton of Friborg, for example, there are over 30 pairs per 100 square kilometers. Across Europe, only Corsica has more red kites in less space. Nowhere in the world is the density of wintering red kites higher than in Switzerland.
The stocks of the red kite in Switzerland have been increasing continuously since 1990
Relative development of the breeding population of the red kite in Switzerland, in percent
Stock index/ Average
Reading example: An index value of 150 means that the population of the red kite in the corresponding year was 50 percent above the mean value for the entire time period considered.
Source: Swiss Ornithological Institute, Sempach
But how can this development be explained? Has the red kite, unlike most of the other species with which it shares its habitat, benefit from the intensification of agriculture? And why are the population densities so high in densely populated and intensively used Switzerland of all places?
A beneficiary of the intensive land use?
Adrian Aebischer holds a PhD in biology and has been working with birds of prey for 20 years. He has witnessed the success story of the species first hand in recent years, as well as the decline of numerous other species of the cultivated land. He says: "The strong increase in the red kite in Switzerland over decades was quite surprising for us." Especially in comparison to the situation abroad, where the bird is sometimes under great pressure, this has raised many questions. Together with Patrick Scherler from the Swiss Ornithological Institute in Sempach, he wrote a comprehensive work entitled “The Red Kite. A bird of prey on the rise »written about the species that was recently published by Haupt-Verlag and traces the success story of the red kite.
Aebischer says: "In order to get to the bottom of the causes for this, one must first understand what demands the red kite makes on its habitat." Most of the birds that have problems in the agricultural country are ground-nesters and insect-eaters. While the latter would have trouble finding enough food, there is hardly enough time left for ground-nesting brooders due to the ever shorter intervals between two mowing dates in a meadow.
The example of the skylark shows how harsh the conditions for such species have become. It has one of the shortest breeding and nestling times for birds: the young leave the nest just a little more than three weeks after the eggs have been laid. But even that is no longer enough in many places to bring the brood through in time for the next mowing date.
The red kite, on the other hand, builds its nest on trees. Mowing a meadow more often does not have a negative impact on its breeding success. On the contrary: he even benefits from it. For hunting, he is dependent on semi-open landscapes, which he searches for small mammals such as mice and carrion. If he mows more frequently, these food sources become more visible to him and therefore more easily accessible. “There is a lot of food in meadows and fields, especially in the first hours after mowing or plowing,” explains Aebischer. "If the next day mowing and plowing in another place, the kites will find enough forage there."
In doing so, they benefit from the small-scale landscape mosaic of Switzerland with its various agricultural cultures. "In landscapes like this, kites can always find food somewhere without having to move too far from the nest." That is also the big difference to some regions abroad, according to Aebischer. In Germany and France, for example, the cultures are much larger in many places. As a result, the kites would have to search an extensive hunting area in order to find sufficient food. This fact explains, among other things, why the population densities of the species are lower abroad than in Switzerland.
When it comes to the choice of feed, the red kite shows few demands and is extremely adaptable. If a certain food is missing, he usually has little trouble switching to another food source.
But can it be concluded from this that intensive agriculture helps the red kite? "Not really," says Aebischer. "In short: The small areas of the fields with many mowing dates or crop changes per year have benefited the kites in Switzerland." Overall, however, the intensification of agriculture is having a negative impact. In landscapes in which hedges, individual trees and prey animals are missing, there are demonstrably fewer breeding pairs of red kites than in semi-open, well-structured cultivated landscapes. "In this regard, the landscapes in Switzerland are usually even more cleared than in neighboring countries," says Aebischer. In this country, however, this is compensated for by the aforementioned small area of the fields.
In addition, both in Switzerland and in the rest of Europe, red kites and other birds of prey are proven to perish every year from poisons used in agriculture. Ultimately, it is also due to the red kite's great adaptability that the intensification of agriculture has not yet bothered him in this country.
Under pressure in neighboring countries
There are enough examples from neighboring countries that show that changes in management can, under certain circumstances, have a rapid and marked effect on the red kite population. Examples are the approval of new plant protection products, the reduction in the proportion of grassland or changes in the composition of crops, which can have an impact on the food supply.
In Europe, the red kite is one of those bird species that have experienced the greatest fluctuations in the population in recent decades. While the population in Switzerland has increased steadily over the past 50 years, the situation in the three most important countries for the species - Germany, France and Spain - was very different. Particularly in the nineties, there were massive populations there.
The reasons for this differ depending on the country. "Very often it was human-made changes in the environment that led to an increase or decrease in stocks," explains Aebischer. In East Germany, for example, the red kite came under great pressure after the fall of the Wall with the intensification of agriculture. The proportion of grassland fell sharply, and with it the populations of the European hamster - the main food of the red kite there - collapsed.
In the northeast of France, on the other hand, poisons used to control voles were responsible for the sharp decline in the number of birds of prey. The kites ate the poisoned rodents, local mass deaths were the result. As a result, the species even died out in some departments. In Spain, on the other hand, a lack of food led to the abandonment of open garbage dumps in some regions and slaughterhouse waste no longer being dumped.
Poaching and the targeted application of poisonous bait also cause local populations to decline again and again. This is particularly problematic in France, Spain, Portugal, Italy and Austria.
"Such examples show how volatile the stocks of red kites and other birds of prey are," says Aebischer. Changes in management that have a negative effect on the food base of the bird of prey, for example, can lead to local populations suddenly declining again in Switzerland.
The dangerous journey to winter quarters becomes unnecessary
Another factor that is likely to have had a positive effect on the red kite in Switzerland as well as in other regions of Central Europe in recent years is climate change. Mild winters and shorter periods with closed snow cover mean that many kites meanwhile forego the dangerous journey to the south in autumn and hibernate in Central Europe. While such overwintering was rare just a few decades ago, today over 4,000 red kites spend the cold season in Switzerland.
Aebischer suspects that this also led to more breeding pairs, since the overwinterers are no longer exposed to the dangers on the migration route. "It is interesting that the breeding populations have increased especially where the number of overwintering individuals has also increased." In addition to the Swiss plateau, this can also be observed in southern Sweden, the Czech Republic and in the French Auvergne.
The Swiss Ornithological Institute is also concerned with the surprising rise of the red kite. As part of a long-term project launched in 2015, researchers in the cantons of Friborg and Bern want to get to the bottom of the cause even more precisely. To date, more than 400 birds have been equipped with GPS transmitters to track their migratory movements. Cameras installed in nests should also document the breeding success and the factors that influence it.
"In short, the project aims to research those factors that influence population dynamics," explains Aebischer. These included breeding success, survival rate, use of space and migration. For the first time, it was possible to trace in detail how several hundred birds roam after the breeding season and what their migration routes are. Above all, the project is hoped to provide valuable insights for other countries, where the red kite still has a difficult time, according to Aebischer. "If we know the detailed reasons for the rapid increase in the red kite in Switzerland, that can provide clues as to how we can help the species elsewhere."
Literature: Aebischer, A. & Scherler, P. (2021): The red kite, a bird of prey on the upswing. Haupt-Verlag, Bern