The Yellow-legged Gull Larus cachinnans was first recorded in Poland in the 1980s. We analysed the probable factors responsible for its successful colonization of new areas. We also expected that such a large species should affect populations of other colonial waterbirds. We studied the breeding and feeding ecology in the largest inland colony of the Yellowlegged Gull in Poland, located in a sedimentation basin near Tarnów (southeastern Poland). The first breeding pair was recorded in 1992 and the population reached 177 pairs in 2001. The population growth rate in this colony, of about 58% per year, fits an exponential model. Nine localities with breeding pairs have been found recently in southern Poland and we now estimate the total population size to be 200-250 pairs. The large clutch size, and high hatching and breeding success in the Tarnów colony suggest that food was plentiful. Food items were frequently found at the nests. Fish, mainly Carp Cyprinus carpio , were the predominant food items delivered to chicks; however, there was more refuse brought to nests during the incubation stage. Immigration probably caused the growth of the colony studied, although our calculations have shown that natal productivity alone is sufficient to maintain this population. The study showed that the growing population of Yellow-legged Gull might cause considerable reduction in the population sizes of some of the native waterbird species.Many hypotheses attempt to explain the range size and distribution of species . It is believed that populations in the centre of the range occupy habitat of better quality and have a higher intrinsic rate of growth than those at the range margin (Lawton 1993). However, as populations of some species have increased rapidly, others have become vulnerable to extinction, and this is often associated with boundary range changes. Finding the factors that affect these changes is important in understanding the processes occurring in populations at a species' geographical boundary; this may be crucial for predicting the effects of population growth in invasive species, and may be helpful for conservation of endangered species. Yet few studies have addressed these issues.There are essentially two possible explanations for range-size changes. First, range size may increase when high productivity occurs in central source populations, which causes intensive immigration into sink habitats located mainly at the range edge (Pulliam 1988). In this situation, range size relates to the source populations' productivity at the range centre, and range size should extend during periods of higher productivity. However, when the productivity of source populations drops, this should cause a reduction in range size because sink populations at the range boundary are unable to sustain themselves alone. Alternatively, sink habitats at the range edge may become source habitats. This is possible when individuals living in the sink habitats adapt to local conditions, or the habitat quality changes. In such cases, range size s...
The feeding behaviour of the Caspian Gull Larus cachinnans was analysed in southern Poland in 2001. During the pre-breeding period, most birds foraged on a refuse dump and some foraged in a river valley. During incubation, similar numbers of birds foraged on fishponds, gravel pits and the refuse dump. During the chick-rearing period, fishponds were the most important foraging grounds. The foraging success of three main foraging tactics was analysed: digging on refuse, fishing and kleptoparasitism. We found that digging success was higher in juveniles than in immature or adult birds. However, older birds moved and ate more items per unit of time than juveniles, which indicates that older birds improved their energy gain simply by a higher speed of searching. The opposite was found for fishing success. As juvenile birds made fewer attempts than immature or adult birds, fishing success was higher in adults. Adults and immature birds interrupted more attacks than juveniles, which indicates that older birds were better able to assess the probability of fish catching than juveniles. Kleptoparasitism was observed almost exclusively on the refuse dump during the pre-breeding period. Young birds kleptoparasitised more frequently than adults, but they had a lower rate of success. However, the lower success in young birds was due to victim choice, rather than differences in flight skills. Young birds kleptoparasitised Black-headed Gulls Larus ridibundus and Jackdaws Corvus monedula more frequently than adults, but none of the attacks towards these species was successful. Generally, Caspian Gulls kleptoparasitised conspecifics more often than expected from species frequency. Only attacks towards conspecifics yielded any success.
Results are presented of research, the aim of which has been to determine the selected physicochemical qualities, including electric quantities, of apple purées and pulpy juices obtained from them. The characteristics of the electric properties have covered the measurements of impedance (Z) and effective resistance (R), admittance (Y) and conductance (G), as well as equivalent parallel capacitance (Cp) and equivalent series capacitance (Cs). Changes have been determined in the purée properties relating to the technology of production as well as the function of diluting and the storage period. Apple purées, depending on the production technology, are characterized by the diversified, characteristic ranges of the resistivity and conductivity values. As the degree of the purée dilution rises, conductivity of juices falls. These relationships remain true throughout the purée storage period. In the conditions of the experiment, the measured values of resistivity and conductivity of pulpy juices clearly determine the content of apple purées in juices.
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