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Aim This study investigated whether habitat fragmentation at the landscape level influences patch occupancy and abundance of the black-headed gull, Chroicocephalus ridibundus, and whether the response of the species to environmental factors is consistent across replicated landscape plots.Location Water bodies (habitat patches) in southern Poland.Methods Surveys were conducted in two landscape types (four plots in each):(1) more-fragmented landscape, in which habitat patches were small (mean size 2.2-6.2 ha) and far apart (mean distance 2.5-3.1 km); and (2) less-fragmented landscape, in which habitat patches were large (mean size 9.2-16.5 ha) and separated by short distances (mean 0.9-1.4 km). Observations were performed twice in 284 potential habitat patches during the 2007 breeding season.Results Colonies were significantly more frequent and larger in the lessfragmented landscapes than in the more-fragmented ones. Probability of patch occupancy and number of breeding birds were positively related with patch size and these relationships were especially strong in the more-fragmented landscapes. In the less-fragmented landscapes, the occurrence of black-headed gulls was negatively related to the distance to the nearest local population, but in the morefragmented landscapes such a relationship was not detected. As distance to the nearest habitat patch increased, the probability of the patch occupancy decreased in the more-fragmented landscapes. Moreover, abundance was negatively influenced by distance to the nearest habitat patch, especially strongly in morefragmented landscapes. Proximity of corridors (rivers) positively influenced the occupation of patches regardless of landscape type. The number of islets positively influenced occupancy and abundance of local populations, and this relationship was stronger in the more-fragmented landscapes.Main conclusions Our results are in agreement with predictions from metapopulation theory and are the first evidence that populations of blackheaded gulls may have a metapopulation structure. However, patch occupancy and abundance were differentially affected by explanatory variables in the morefragmented landscapes than in the less-fragmented ones. This implies that it is impossible to derive, a priori, predictions about presence/abundance patterns based on only a single landscape.

The Dunlin is one of very few wader species that moults primaries when migrating to its wintering grounds. In our study, a total of 68.2% of immatures and 26.6% of adults underwent their primary moult when passing through the southern Baltic in autumn. More than 30% of moulting birds revealed differences in moult scores of left and right wings. However, 13% of Dunlins showed differences between the left and right wing not greater than 1%, and 50% of individuals showed differences between the two wings that were lower than 3% of the total mass of all primaries. The probability of asymmetry during the primary moult increased over time and decreased with the advancement of the primary moult. Sex and age of birds had no significant effect on the occurrence of moult asymmetry. The level of asymmetry in the primary moult increased in the following days of autumn migration and with the advancement of the primary moult. The mean index of primary moult asymmetry (the absolute value of the difference in moult advancement between the left and right wing) was lower in immature Dunlins than in adults, and in females compared to males. Hence, sex (males) and age classes (adults) that spend a longer time on the breeding grounds revealed a higher index of primary moult asymmetry, most probably as a result of higher levels of physiological stress. However, the low proportion of birds showing large asymmetry suggests that this is strongly constrained by selection for aerodynamic efficiency, as asymmetry in primaries affects aerodynamic stability, takeoff costs, manoeuvrability and agility in birds with flapping flight.