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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Long-term trends of breeding success in Common Terns Sterna hirundo in the Wadden Sea
Auteurs :	Peter H. Becker, Auteur
Type de document :	Article
Dans :	Vogelwelt (Die) (119(3-5) 1998)
Article en page(s) :	223-234
Résumé :	From 1981 to 1996 the reproductive success of Common Terns was studied at 3-5 colony sites at the Jade in the German Wadden Sea (Lower Saxony). The total population as well as the biggest colony on the island Minsener Oldeoog (MO) showed no trend in the number of breeding pairs (1450-3700). Among the small colonies, the one at Wilhelmshaven (Whv) increased in numbers, whereas those on the islands Mellum (Me) and Wangerooge (Wa) were deserted during the early 1980s. Colony size at the saltmarsh of Augustgroden (Ag) decreased during the 1990s. The reproductive success fluctuated strongly between years and colonies. The highest output was found at Whv (long-term average: 1.2 fled-lings per pair and year). At MO and Ag 0.8 chicks fledged, resp., whereas the colonies on Me and Wa produced only 0.2 and 0.5 chicks, resp. At the latter sites the inter-year variation of the breeding success was also higher (coefficient of variation > 100% ) than at the other sites (51-83% ). Losses occurred mainly during the chick rearing period. At MO and Whv chick losses caused by inclement weather and food shortage predominated, in addition losses by flooding owing to high tides occurred at Ag and Wa. On Mellum predation by Herring Gulls Larus argentatus was responsible for high chick losses. Long-term estimates of the annual reproductive output of the total Jade population yielded 2.7 eggs per clutch, 87% hatching success, 35% fledging success and 0.8 fledglings per pair. 14% of eggs were lost, 57% of eggs were lost as chicks (total losses 71% of eggs laid; causes: flooding 2%, predation 21%, others 48% (weather and food shortage)). Important predators were Herring Gulls, owls Asio flammeus, A. otus and Brown Rats Rattus norvegicus. Environmental chemicals had no effects on reproduction. Compared with other marine breeding sites, Common Terns at the Jade had a relatively high reproductive success, although it was lower than at freshwater sites. The mean breeding output seems to be in the range needed to maintain the population. The level and the predictability of breeding success seem to influence fidelity to the colony site. Various aspects of the use of the Common Tern as bioindicator (monitoring breeding success, chemical contamination) and of the management of tern breeding habitats in the Wadden Sea are discussed. To measure the area-specific reproductive output at least 3-5 years of study are needed as well as the inclusion of the important colony sites.

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