Factors Determining Number Fluctuations and Variation of the Breeding Success of an Urban Population of the Black-headed Gull Larus ridibundus (N-Poland)

In this paper a study conducted on a breeding colony of the Black-headed Gull Larus ridibundus between 1998-2004 is presented. The colony is located on an island on a small (4.6 ha area) artificial lake situated in the most abundantly visited recreational area of Bydgoszcz (53°07'N, 18°03'E). The island has an area of 0.9 ha and in some parts there are 25-30 year old Pinus silvestris; the banks are overgrown by Phragmites communis, Typha latifolia and Salix sp., and the remaining part of the island is covered by grasses and perennial pants. During seven breeding seasons remarkable fluctuations in colony size were noted (from 1877 pairs in 2000 to 998 pairs in 2003). Most, i.e. 71% of females started breeding between the 15^th and 25^th of April. In subsequent seasons they laid 2.2-2.8 eggs (mean 2.5 eggs). Egg losses during incubation varied between 9.7% and 17.6% (mean 13.7%). Mortality of chicks during the first three weeks of life was at an average level of 15.2% and varied in subsequent seasons from 5.6% to 29.4%. In consequence, breeding success of the studied population was at a level of 72.6%, and each pair raised on average 1.81 offspring. The collected data on numbers, phenology, dynamics of broods and selected parameters of reproduction (i.e. size of broods, egg losses and chick mortality) indicate that in the studied population – despite its specific location and several years of existence – there are no changes that would give evidence to the progressing process of synurbisation.