Decline of insectivorous birds
The steep decline of insects in farmland creates problems for insectivorous birds. The use of pesticides and modern land-use practices are among the main causes for the disappearance of insects. Insectivorous birds in farmland habitats in particular are declining sharply.
About 40 % of Swiss breeding bird species feed almost exclusively on insects. A further 25 % have a mixed diet, but rely mainly on insects to feed their young. The demand for suitable insects that are easy to catch is therefore great.
Although data are scarce, it is safe to say that fewer insects exist today than a few decades ago. This loss is documented for several areas in Germany, where insect biomass has decreased by 75 % in the past 27 years. There are very few data sets from Switzerland to document the decline of insect biomass. Georg Artmann-Graf found a marked reduction in grasshoppers around Olten SO in the past 30 years. Moreover, older train drivers unanimously report having to remove a mass of dead insects from the windscreen after every run back in the 1960s, while much less frequent cleaning is necessary nowadays.
The main causes of insect decline
There are many different reasons for the drop in insect numbers: The loss of insect-rich habitats (semi-dry and dry grassland, wetlands, semi-natural waterbodies) is particularly significant. Various large insects such as grasshoppers, dragonflies and butterflies occurred in these environments. Many modern land-management practices also have a negative impact on insects: Semi-natural road and railway embankments are often mowed during the peak flowering period. Plant material for the production of silage is packed and moved shortly after cutting, including the insects trapped inside. Meadows are cut up to six times a year. Often, this is done with mower-conditioners that crush the grass immediately after cutting to promote faster drying. Mower-conditioners result in a loss of honeybees that is seven times greater (up to 90 000 dead bees/ha) than mowing without a conditioner.
The use of pesticides reduces the diversity and abundance of arthropods. Herbicides reduce the food supply of many insects. Insecticides not only decimate pests, but kill other insects as well. In addition, persistent insecticides enter the soil and sometimes the groundwater. In the 1970s, the fat-soluble insecticide DDT, which has since been banned in many places, accumulated in the food chain, leading to a dramatic worldwide decline in birds of prey. Today, persistent and water-soluble neonicotinoids are often applied as a preventive measure and have been found in waterbodies and biodiversity promotion areas in Switzerland. Pesticides are also widely used in private gardens. In the Netherlands, insectivorous birds declined more steeply in areas where surface water was more heavily contaminated with neonicotinoids. The dung and manure of livestock treated with medication to control parasites attracts significantly fewer insects, resulting in the loss of yet another source of insects. The bacterium Bacillus thuringiensis var. israelensis, used against mosquito larvae, is also deployed in Swiss nature reserves. Destroying the mosquitoes reduces the overall abundance of insects, which negatively effects the breeding success of birds.
Remaining insects are poorly accessible
Many crops and meadows are much denser than they used to be, due mainly to increased fertilisation. Sparse, low-nutrient meadows, for instance, declined by 20 % in the Engadine GR in only 20 years, while the proportion of extremely dense meadows increased considerably during the same period. Cereal fields have become denser because of new crop varieties and fertilisation. Dense vegetation in meadows and fields makes it hard for birds to prey on insects. Common Redstart and Eurasian Wryneck, for example, rely on sparse vegetation within their territory that allows them to easily catch insects. In the case of the Common Hoopoe, the accessibility of prey influences the choice of foraging sites even more strongly than prey abundance.
Hard times for farmland insectivores
For all the above reasons, it is not surprising that specialist insect feeders of farmland (e.g. larks, Tree Pipit, Red-backed Shrike, common Whitethroat, Whinchat) are in marked decline. Farmland birds whose diet contains only a small portion of insects (e.g. White Stork, Red Kite, Common Kestrel, Fieldfare, Yellowhammer) are barely affected by their disappearance. Woodland insectivores (e.g. woodpeckers, tits, Eurasian Blackcap, European Robin) and aerial feeders (e.g. Alpine Swift, European Bee-eater) even show positive overall population trends. The alarming situation of insectivores in farmland is presumably a result of heavy pesticide use, modern land-use practices and land consolidation.
Possible solutions to the problem
The situation can be improved using simple measures: Leaving at least 10 % of surface area as refuges at each cutting must become standard procedure in low-intensity and litter meadows. These uncut refuges have been proven to have a positive effect on insects. Pesticides must be severely restricted and should not be applied preventively, but only when damage has reached a certain threshold. Studies have shown that pesticides can be reduced by 42 % without loss of productivity. Information campaigns are needed to raise consumers' willingness to buy food grown with minimal use of pesticides. The majority of green spaces in settlements are artificial and over-maintained, making them unattractive for insects. Garden experts and owners should be educated about insect-friendly and natural garden design.
The decline of food for insect-eating birds is alarming, and too little is known about the extent of the problem. A monitoring scheme for insect biomass in Switzerland is therefore needed.
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