The paper deals with the population changes of birds inhabiting an extensive area (15.8 km2) dominated by arable land, situated on the Silesia Lowland in south-western Poland. The number of territories was established with the use of the mapping method in the years 1977-1979 and 2001. Some changes in habitat and indices of agricultural production occurred between the two study periods. The 2001 study revealed a clear rise of the species diversity and the area was colonised with 18 new bird species. In the group of agricultural birds the only visible decrease was recorded for the species nesting in or using open cultivated areas, where the total abundance dropped. Within this group the most affected were: Perdix perdix, Vanellus vanellus and Alauda arvensis. The abundance of species inhabiting non-cropped, treeless habitats within open farmland went up. The total abundance of twelve species nesting in hedgerows and woodlots also rose. The severe reduction of the bird populations inhabiting open farmland is most probably associated with much larger environmental changes linked with intensification of agriculture, compared to the small transformations of the non-cropped or marginal habitats (sometimes becoming even larger and more diverse), where bird abundance went up.