Farmland bird declines - An explanation
Research into the decline of some farmland bird species have shown that there is no single reason, but that different species reacted to different factors.
The explanation for farmland bird declines
Research has shown that often declines in farmland bird species are associated with more than one factor impacting on a species simultaneously.
The main factors driving declines since 1970 are:
- Loss of mixed farming causing a loss of habitat diversity, leading to less diversity of plants and animals and fewer opportunities for birds to forage in different habitats throughout the year
- Increased use and efficacy of pesticides leading to the loss of insect food and weed seeds
- Changes in crops grown, such as the loss of spring crops which provide nesting habitat for lapwings and better habitat for other ground-nesting birds such as skylark, yellow wagtail and corn bunting. Also loss of winter fallow habitats, such as winter stubbles, which are important for winter foraging. Large areas of single crops can have dramatic impacts on the abundance of less mobile invertebrates, particularly when insecticides are used
- Changes in grassland management have led to a reduction in the abundance of seeds and insects, and the quality of grassland as a nesting habitat. These include increased stocking rates, more efficient grazing systems, increased nutrient inputs, reseeding and the switch from hay to silage
- Increased field sizes have caused the loss of nesting habitat for some species, but more importantly, the loss of insect-rich foraging habitat and weedy field margins
- Field drainage has not only removed wetland habitats, but also damp areas with more soil invertebrates which provided food for birds such as tree sparrows, reed buntings and yellow wagtails
- Predation has had an additive effect to the impacts of pesticide use on grey partridges and the loss of nesting habitat on some local populations of lapwings when populations are already at low levels. Many predators, such as foxes and crows, have increased since 1970. Population trends of common birds of prey have been largely stable since the mid-1990s, apart from the buzzard, which has increased substantially. Sparrowhawk numbers climbed in arable England in the 1980s and the increase in buzzard numbers started in the 1990s, both after the sharpest decline of the farmland birds occurred
- Weather has an impact on all bird species: the harsh winters experienced in the late 1970s and early 1980s hit resident species hard, but these were interspersed with milder winters when most species recovered. This could have had an additive impact on birds that were also struggling for other reasons. Much of the annual variation in population trends is related to weather, with stable species declining in years with poor weather (such as cold winters or dry summers) but bounce back in better years. To date, weather-related factors have not been found to cause any long-term declines
- Migration: There are a number of declining bird species which make an annual migration across the Sahara, including two declining farmland birds - the turtle dove and the yellow wagtail. The decline of turtle doves has been linked to the shortage of summer seed sources on farmland, but it is also likely that factors affecting them on their migration routes or wintering grounds are exacerbating this decline.
Frequently asked questions
Why are farmland birds still declining now more farmers are in agri-environment schemes than ever before?
The growing number of farmers in agri-environment schemes is a very positive trend for the enhancement of the farmed environment.
The main reason why this has not led to an increase in farmland bird numbers is that options which boost in-field insect and seed food, which are the main causes of the declines, have not been taken up in sufficient quantity to make a difference. There is also the possibility that there will be a time-lag between the high uptake of Entry Level Stewardship in England and the benefits of this scheme being realised.
Was set-aside good for birds, and if so, why did it not halt the decline of farmland birds?
Set-aside held higher densities of birds, particularly the seed-eating farmland birds, in winter and summer than other farmland habitats. In late winter and early spring, rotational set-aside provided critical foraging habitat when other seed sources were in short supply after other stubbles had been cultivated for spring crops.
In summer, both rotational and non-rotational set-aside were used to forage for insects. Set-aside supported higher breeding densities of skylarks and higher breeding productivity (despite higher predation rates) than arable or improved grassland fields.
The Farmland Bird Indicator continued to decline after the introduction of compulsory set-aside in 1992, but at a slower rate. We do not know how populations of farmland birds would have changed if it had not been introduced. Set-aside management was not ideal for farmland wildlife:
- The spraying of the green cover on rotational set-aside after 15 April reduced the summer benefits, especially if this occurred before mid-May
- The period of the annual cut of non-rotational set-aside overlapped with the tail-end of the nesting season, which led to losses of late nesting attempts for some ground-nesting species.
If set-aside land had been managed specifically for wildlife, then it is highly likely that it would have halted the decline in farmland birds.