DIJK J VAN, STIENEN EWM , GERRITSEN S &' MAJOOR FA (2009) Reproduction of the Black-headed Gull Larus ridibundus in coastal and inland colonies. LIMOSA 82 (1): 13-22.

In the twentieth century the population of Black-headed Gulls Larus ridibundus in the Netherlands and the rest of Western Europe initially increased strongly. From the end of the 1980's onwards the population has declined again. To elucidate the reasons for this decline we started a seven-year research program in 1997 to investigate the reproduction of Black-headed Gulls in 17 different colo - nies in various parts of the Netherlands and across the border with Belgium (Fig. 1). A previous study indicated that a large difference in breeding success between coastal and inland Black-headed Gull colonies existed. Therefore we divided the colonies into coastal and inland colonies based on the available prey (mainly marine or terrestrial). Per colony, 25-30 nests were monitored during the breeding season. We used enclosures to monitor chick survival and growth of body mass, head+bill length and wing length. The results show that inland colonies were less successful compared to coastal colonies. Black-headed Gulls in inland colonies had smaller clutches (2.4 eggs) than in coastal colonies (2.7 eggs), and their hatching success was also lower (48% versus 73%). No difference in fledging success was found. The inland colonies showed a lower breeding success (0.4 chicks fledged per breeding pair) (Fig. 3) than colonies situated near the coast (0.9 chicks fledged per breeding pair). However, both colony types produced less than 1.0 chick per breeding pair which is the minimum necessary to maintain a stable population. Chicks in inland colonies grew faster both in mass and head+bill, and fledged earlier than their conspecifics near the coast (Figs 4,5). Our results show that not food shortage during the chick period, but predation of eggs is the main factor behind the decline of the Black-headed Gull population in the Netherlands, particularly in inland colonies. More detailed studies are needed to elucidate the mechanisms leading to these high egg losses.Black-headed GullChroicocephalus ridibundus = Larus ridibundus

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