1. The reed bunting Emberiza schoeniclus is one of a suite of granivorous farmland bird species that suffered a major population decline in Britain during the late 1970s and early 1980s. Extensive monitoring data indicate a large increase in the abundance of reed buntings between 1963 and 1975, followed by a decline of 58% on farmland and 66% along linear waterways during the period 1975–83. Since 1983 numbers have remained relatively stable in both habitats.
2. During the population decline, breeding numbers declined rapidly on arable and mixed farms, but remained relatively stable on pastoral farms. The decline on farmland was greater in northern Britain than in the south-east.
3. Extensive nest recording indicated that breeding performance was higher during the period of population decline (2·74 young per nesting attempt) than during the preceding period of population increase (2·65) or the recent period of population stability (2·17).
4. Minimum survival during the first year of life (estimated from mark–recapture data) declined during the 1970s and early 1980s, and increased strongly during the 1990s. The trend in first-year survival was independent of a weak positive relationship with winter temperature. Although there was evidence of a similar temporal trend in adult survival, this disappeared when winter temperature was taken into account.
5. A demographic model indicated that the timing and magnitude of the observed changes in first-year survival and adult survival were each sufficient alone to account for the observed changes in the abundance of reed buntings during the period 1969–87. A reduction in over-winter survival was probably the main demographic cause of the reed bunting population decline, although loss of breeding habitat and a recent reduction in breeding performance may also have influenced numbers.
6. During winter reed buntings feed mainly on small grass and weed seeds. The observed declines in abundance and survival rates coincide with the widespread introduction of a range of efficient herbicides and the loss of winter stubbles from British farmland. Our findings are therefore consistent with the hypothesis that the decline of the British reed bunting population was caused primarily by a reduction in food availability outside the breeding season. Changes in agricultural practices that increase the abundance of small weed and grass seed on farmland during winter are likely to allow at least a partial recovery of the British reed bunting population.