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We used extensive atlas and census data to assess trends in the distribution and population levels of birds on lowland farmland in Britain between the late 1960s and early 1990s. Many species of farmland birds have become less widespread or have declined in numbers, or both, but few have become more widespread or have increased. Of the 28 species classified as farmland birds the distributions of 24 contracted between 1970 and 1990. Of the 18 farmland species for which it was possible to assess population change, 15 were less abundant in 1990 than in 1970. Seven of the species were estimated to have undergone population decreases of at least 50%. Farmland species showing the largest population declines tended also to show substantial range contractions. Farmland species underwent an appreciably larger contraction of distribution than species associated with any other habitat. Furthermore, farmland species tended to decrease in abundance, whereas woodland species tended to increase. Population declines among farmland birds became evident in the mid-to late 1970s, a period when several fundamental changes were taking place in British agricultural practices. These included a great reduction in the spring sowing of cereals, a simplification of crop rotations, increased use of chemical pesticides and inorganic fertilizers, and more-intensive grassland management. We suggest that the declines of farmland bird species have been caused or aggravated by this pervasive intensification of agriculture. Existing research on declining farmland birds, however, indicates that there is no single mechanism underlying the population changes. We identify priorities for research, focusing mainly on relationships between bird populations and agricultural practices, but we also recognize a need for a better understanding of the role of predation.Declinaciones poblacionales y contracciones en el firea de distribuci6n de aves que habitan tierras bajas agricolas en Gran Bretafia Population Declines and Range Contractions of British Farmland BirdsFuller et al.fundamentales en las pr~cticas agrfcolas britdtntcas. Estos cambios incluyeron una gran reducci6n en la siembra primaveral de cereales, una simpliflcaci6n en la rotact6n de las cosechas, un aumento en el uso de pestictdas quimicos y ferttlizantes inorg~ntcos y un manejo mds intensivo de las pasturas. Sugerimos que la declinaci6n de las especies de aves de tlerras agrtcolas, ha sido causada o aumentada por esta tntenstflcact6n de la agrtcultura. Sin embargo, las investigaciones existentes sobre las aves de tierras agrfcolas en decli-naci6n indtcan que no existe un mecantsmo anico responsable estos cambtos poblacionales. Identiflcamos prioridades para la investtgaci6n, enfatizando prtnctpalmente las relactones entre poblactones de aves y pr~cticas agrfcolas, pero tambi6n reconocemos que es necesario una mejor comprensi6n del papel que juega de la predaci6n.

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