Following range expansion and colonization, hybridization between Herring and Caspian Gulls, Larus argentatus and L. cachinnans, takes place in central and eastern Europe. To examine how hybrid zone is affected by the abundance dynamics of these species and their reproductive performance, we studied a mixed colony at Włocławek Reservoir, central Poland, for over 7 years, from 2002 to 2009, and included data from the species monitoring from 1990 to 2001. To evaluate the species abundance dynamics and possible mechanisms of reproductive isolation, breeders (n = 226 individual birds) were trapped on nests and colour-ringed; breeding performance was studied in detail for 202 breeding pairs with both mates known. Between 2002 and 2009 the proportion of Caspian Gulls among breeders had strongly increased (from 14% to 42%), whereas the proportion of Herring Gulls had declined (from 70% to 35%). The frequency of hybrids varied a little with no clear trend (mean 20%, range 15-28%). The colony size during that time was approximately stable, with 125-135 breeding pairs. 32 individuals originating from outside the zone, ringed as nestlings in the core range of either species, were recorded as breeders at the study site, documenting dispersal of parental species into the zone. The immigration of the two parental species showed contrasting temporal patterns in the two compared decades, 1990-1999 vs. 2000-2009. The immigration of Herring Gulls as measured by the reencounter probability declined nearly three times, while approximately twofold increase was seen in Caspian Gulls. Birds tended to choose phenotypically similar mates, so that there were fewer heterospecific pairs than expected under random mating. Numbers of homospecific, heterospecific and mixed pairs were similar during 7 years. On average, males of Caspian Gulls were significantly heavier than males of Herring Gulls. Caspian Gull pairs bred on average 7 days earlier than pairs of Herring Gulls. No differences in clutch size, clutch volume or hatching success among pairs of different composition were found, indicating weak postzygotic isolation. Current abundance of species in the hybrid zone is changing dynamically and is primarily driven by the strength of immigration from outside the zone.
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