How bird numbers are falling due to insect decline

Anyone older than 30 might remember the windshields full of dead insects after a drive on the motorway in the summer. Though unpleasant, it was always a sign of the abundance of insects in our surroundings. In the past decades though, a decline was first slowly and then strongly noticeable. From a recent landmark long-term study in Germany, it was derived that in the past 27 years the total volume of flying insects had declined by an astonishing 76%, as cited in the scientific journal PLOS ONE in October 2017. Following the devastating numbers, a first meta-study on the insect decline worldwide was conducted in 2020 and, after analysing 166 studies from 1500 countries, it concluded that annually about 0,92% of all insects vanish.

Of course, any loss in species derives from diverse causes and is usually highly different between various species of the same order, just as they are diverse in their habitats and dependencies. Nevertheless, larger trends can be detected. Concerning insects three important factors play a role: loss of habitat, loss of food and harm through direct human intervention such as pesticides and other pest control measures. The degradation of habitat and food is largely driven by the application of monocultures. Monocultures flower only during a certain period, thus providing food for a limited amount of time. With a lack of food in closer proximity - making it impossible for many insects to reach them - insect numbers and diversity decline. Furthermore, monocultures oftentimes require a large amount of intervention, such as the use of pesticides, that not only diminishes the numbers of pest insects but often also of less harmful or even beneficial insects.

In urban landscapes the largest problem for insects is the lack of plants overall or a missing presence of native plants they can feed on. Especially important are plants flowering at different times of the year within closer proximity of each other. Many insect species are dependent on either a large diversity in plants or are highly specialised. Though many cities try to maintain green spaces, it is often not possible for a large diversity of insects to thrive. Furthermore, air and light pollution are making an already tough life even harder for insects. Light pollution interferes with the behaviour of night-dwelling insects while air pollution is disturbing the scent signals flowers send off to their respective pollinators. This leads to a combination of insects finding less food and flowers attracting less pollinators - resulting in the <a href="https://projects.research-and-innovation.ec.europa.eu/en/horizon-magazine/what-urban-nature-really-means-insect-biodiversity" ta

As insects are an important source of food for many animals, a decline at the scale we are witnessing, will and does impact on other species. Just as insect numbers, the amount and diversity of birds has taken a big hit in the past 25 years. The numbers of insect-eating birds have declined on average by 13% in Europe, according to a study published in the journal Conservation Biology. As in insects this is not based on a single cause - as it impacted mostly birds living on meadows, grasslands and pastures, it can be concluded that they as well suffer from other debilitating effects our current agriculture regimes have on other species, such as loss of habitat, loss of breeding space and loss of food sources. While many European countries have successfully managed to protect large waterfowl or birds of prey, the abundance of smaller birds is declining at high rates, impacting numbers of common species such as starlings, house-sparrows, coldcrests or the common chaffinches.

Some, mainly smaller, birds can depend on insects in two ways: One is the direct feeding on insects by adult birds, the other is use of insects as food for their offspring. While adult birds will not depend on insects and often forage also on seeds, many species feed their young solely on invertebrates such as for example many finches and tits. Mostly those birds depend on lepidopteran larvae, but with butterflies (Lepidoptera) being one of the worst hit order of insects, the negative effects trickle down to the dependent birds. With less insects around, finding food for their young and themselves becomes ever harder for many birds. Some species that would scavenge for crickets on short grassland now cannot find this kind of habitat to feed from. But it is not only small birds that directly depend on insects as a food source that is leading to a decline in birds.

The pressures on bird populations in cities are manyfold - as in rural surroundings one cannot limit the causes to just one problem. Nevertheless, apart from the loss of habitat and nesting space in modern cities, the lack of plants and plant diversity has largely made it hard for birds to find insects to feed on and to use as food for their young. The impact that low-quality food has on nestlings in urban birds has been shown in a recent study on great tits. They found that the urban insect population would need to increase 2.5-fold to provide enough food for great tits to prosper as well as their relatives in forests do, with forests being one of the few habitats where bird decline is less evident. As soon as the great tit chicks were fed with mealworm larvae that were supplied to the parent birds by the study team, their chance of survival was as high as that of their rural cousins.

In a nutshell it can be stated that the degradation of insect habitat and food sources has not only led to a dramatic decline in insect numbers, but also to a loss of birds both in cities and farmland. But the situation is not devoid of hope. Many measures can be taken to increase the quality of habitat for insects and thus birds. The European Union is favouring and subsidising farmers that apply biodiversity-friendly measures on their fields. A big part is the reduced use of pesticides, the use of favourable insects and arthropods for pest control and most importantly the planting of rows of wildflowers or hedgerows to increase the food sources for insects.
In cities measures are being taken to increase the abundance of different wildflowers that provide pollen and nectar for many insects throughout the year. This can be in parks, via roadside plantings, in gardens of public buildings, such as schools or universities or in cooperation with companies on their rooftops and balconies.

Even ordinary citizens can play a big role in the protection and restoring of habitat for insects and therefore birds as well. It starts as simple as the choice of plants for the balcony or in the garden. Over-managed gardens with exotic plants are usually not a good food source. Insects need plants that are native and diverse species that flower at different times. Therefore, a more natural, local approach can be useful, using local wildflowers and leaving some parts un-mowed at least at times, such as during the no-mow-may activities. Furthermore, as the biodiversity research always depends on data gathered, everybody can help by participating in citizen science activities, such as the big bird counts or insect conservation projects conducted by universities of NGOs.

Businesses and local entities can help to morph zones with low biodiversity and insect-adverse environments into zones of diverse vegetation with a steadily increasing availability of habitat for insects and other wildlife. An example is the creation of Biodiversity Oasis’ with 3BEE-technology. While nectariferous trees are planted, insect shelters are installed and bee hives are placed for a biomonitoring purpose, these zones can play a vital role in reviving lost or mismanaged urban landscapes. Through 3Bee-technology it becomes possible to measure and visualise the improvements of these areas into hubs of biodiversity. Also, the observation technology makes it possible to react to the demands of each zone individually and to observe the positive changes.