Box 1: Major changes in arable farming since 1950 and their relevance to farmland bird populations in lowland Britain (based on Fuller 2000)

Increasing mechanization
The use of machinery allows farming activities such as cultivation, chemical application, harvesting and hedgerow management to be undertaken extremely rapidly on a large scale. The capacity for changing the face of the countryside is greater than ever. Use of combines and more efficient harvesting have resulted in less food for seed-eating birds. Ground-nesting birds may be at greater risk of losing nests and chicks.

Fertilizer and pesticide usage
Nitrogen inputs increased greatly on tilled land during the 1970s and 1980s and there was increasing reliance on inorganic compounds but less use of farmyard manure (FYM). This has resulted in faster growing crops, which are less suitable for ground-nesting birds such as Skylarks and Lapwings. The increasing reliance on synthetic fertilizers has reduced the need for mixed farming with fertility-building leys and livestock to produce FYM. Mixed farming offers a wider range of food resources, feeding sites and nesting sites for farmland birds than pure arable or grassland. Since the 1950s farming has become dependent on a large number of herbicides, insecticides, molluscicides and rodenticides. Many of these chemicals act in some way to reduce potential foods of farmland birds including seeds and invertebrates. Furthermore, the development of herbicides that could control grass weeds was a major factor enabling the switch from spring to autumn sowing (see below). The use of pesticides to control disease has also contributed to the demise of traditional rotations and, hence, of mixed farming.

Reduction of spring sowing
Since approximately 1970 there has been a decline in spring sowing of cereals. The general move towards late summer and autumn sowing has resulted in far less overwinter stubble with an associated reduction in winter-feeding opportunities for seed-eating birds, notably buntings, sparrows, finches and Skylarks. Cereal crops now grow throughout much of the winter period and consequently by the spring are often too tall and dense for ground-nesting birds.

Simplification of rotations and reduction in mixed farming
There has been a trend of specialization in farming with arable dominating in the east of lowland Britain and grassland in the west. Within arable systems, grass leys have become scarce and rotations are much simplified with cereals often being grown for several successive years. The implication for birds is that the countryside has become more uniform in terms of the foods and nest sites that it offers. Skylarks and Lapwings are examples of breeding species that benefit from a diversity of field types.

New crops and the introduction of set-aside
Two major features of positive significance to birds are the increasing prominence of oilseed rape in the countryside and the large-scale introduction of set-aside during the 1990s. Rape fields have proved attractive to several nesting birds and to Linnets, which feed on the seeds. Rotational set-aside has acted to some extent as a substitute for the loss of traditional stubbles as it has provided food resources for many seed-eating birds in winter as well as being used heavily by nesting Skylarks and many feeding birds during the summer (Henderson & Evans 2000).