Birds and butterflies are sensitive to environmental change and can indicate the health of the environment. Long-term monitoring shows significant declines in farmland birds and grassland butterflies. Between 1990 and 2019, the index of 168 common birds decreased by 8% in the 25 EU Member States with monitoring schemes. The decline in common farmland birds over the same period was much more pronounced at 27%, while the common forest bird index increased by 5%. Between 1991 and 2018 the grassland butterfly index also declined strongly, by 25%, in the 17 EU countries with monitoring data.

The status of birds and butterflies has been the subject of long-term monitoring in Europe, much of it via voluntary effort, and is a good example of how the power of citizen science can be released through effective targeting. Both species groups are sensitive to environmental change and their population numbers can reflect changes in ecosystems and other animal and plant populations. Therefore, trends in bird and butterfly populations can serve as barometers of the health of the environment and can help measure progress towards biodiversity targets.

Long-term monitoring of common birds in 25 EU Member States reveals significant population declines, particularly in farmland birds, with no signs of recovery. Between 1990 and 2019, the common bird index declined by 8%; the decline in common farmland birds was much more pronounced, at 27%; while the common forest bird index increased by 5%.  Although this indicator uses 1990 as a baseline, significant decreases had occurred before this date.

The long-term trends demonstrate a major decline in biodiversity in Europe. This has been caused primarily by the loss, fragmentation and degradation of natural and semi-natural ecosystems, mainly due to agricultural intensification, intensive forest management and land abandonment or urban sprawl. Habitat loss, fragmentation and simplification (e.g. removal of hedgerows and tree lines to make fields larger) result in loss of bird nesting sites and food sources, contributing to population decline.

Factors that can have adverse effects on the recovery of populations include climate change, intensive agricultural production, urban sprawl and increasing competition for land for production of renewable energy and biofuels.

Measures set out in, for instance, the Birds and Habitats Directives, the Water Framework Directive and the common agricultural policy (CAP) aim to help populations recover at national and European levels. However, achieving the wide and effective deployment of conservation measures remains a challenge.

In spite of year-to-year fluctuations typically seen in butterfly populations, monitoring schemes in 17 EU countries reveal that, overall, grassland butterfly numbers declined significantly between 1991 and 2018, by 25%. As with the common bird index, substantial decreases occurred before the 1991 baseline for the indicator calculation.

The main driver of the decline in grassland butterfly numbers is the intensification of farming and changes in rural land use, including the abandonment of grasslands. The loss of species-rich semi-natural grasslands has been particularly detrimental. Moreover, agricultural intensification can entail high inputs of agrochemicals, including pesticides, which can dramatically reduce insect populations, including butterflies. Urban sprawl increases light pollution (i.e. artificial light at night), which is another major driver of insect decline.