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.
Soil-invertebrate feeding birds can be exposed to high doses of toxic metals trough their diet. Recently, we have shown that nestling rooks Corvus frugilegus from several rookeries in Poland have a cadmium (Cd) tissue level diagnostic for acute contamination as well as an elevated level of lead (Pb). To explain the potential pathway of bioaccumulation of 11 essential and non-essential elements, including two metals of primary concern (Cd and Pb), in target issues of these nestlings, we analyzed the relationships between the dietary characteristics of stomach content (mass of digesta, number of cereal grains, plant and animal items, and grit particles) and concentrations of metals in the liver, kidneys, lung, muscles and bones. Our analysis showed in total 17 (8 negative and 9 positive) statistically signigicant relationships between the five analyzed dietary characteristics of stomach content nad concentrations of metals in the liver, kidneys, muscles and bones. We found a significant positive relationship between the number of animal food items and Cd-level in kidneys; and a negative relationship bewteen the number of plant items and Pb-level in the liver, and between the number of grit particles and Pb-level in kidneys. Despite the limitations of our study due to the different degree of digestion of some food items, our findings suggest high bioavailability of Cd from animal food items and a low level or reduce gastrointestinal absorption of Pb from plant food (mainly cereals). We urge further research on absorption of Cd and Pb from different dietary components and application of diet analysis to explain the complex nature of bioaccumulation of anthropogenic contaminants in the internal organs and tissues of birds and other species of animals, especially in species with a mixed plant-animal diet.