VARIATIONS IN REPRODUCTION PARAMETERS OF BLACK-HEADED GULLS (LARUS-RIDIBUNDUS) LIVING IN DIFFERENT CONDITIONS IN THE CZECH-REPUBLIC AND SLOVAK-REPUBLIC

Significant differences in embryonic mortality and occurrence of structural developmental defects between three separated populations of the Black-headed Gull were examined. Growth retardation was not studied. Population 'C' (average 2.78 eggs per nest), living in Western Slovakia, exhibits in three-egg clutches relatively low mortality (7%) as well as a low occurrence of developmental defects (2.8%). In population 'B' (2.6 eggs per nest, living in Southern Moravia), breeding in a region that is intensively exploited for agriculture, the mortality is still very low (3%) but the proportion of malformed embryos increases to 15%. In population 'A' (2.6 eggs per nest, living in Southern Bohemia) the highest mortality was found (13%) but only 4.3% of embryos were malformed, though there was a broader spectrum of developmental defects. In this population the eggs were significantly smaller and had thinner shells. In all the populations which were examined, the mortality and occurrence of developmental defects were higher in one-egg clutches, which were more frequent in populations 'B' and 'A' than in 'C'.