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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Black-necked Grebes Podiceps nigricollis at lake Alfsee (NW Germany): breeding and resting birds at a shallow lake during mass occurrence of Western Waterweed Elodea nuttallii
Auteurs :	Bernd-Olaf Flore, Auteur
Type de document :	Article
Dans :	Vogelwelt (Die) (132(4) 2011)
Article en page(s) :	197-206
Résumé :	At the artificial lake Alfsee (SW Lower Saxony, Germany; protected by EC Bird Directive) waterbirds are counted three times per month throughout the year. Numbers of resting Black-necked Grebes have increased since 1990, especially in some years (1996, 2005). Peak numbers occurred during migration from March to early May and late July to early October. The year 2011 was exceptional, as from June onwards numbers rose to 295 Black-necked Grebes on 30th July (moulting place), the highest number of this species ever recorded in Lower Saxony.In 2011, the species started to breed for the first time ever at lake Alfsee, without any breeding occurrence of gulls. Up to 52 occupied nests were present at the same time, performing the greatest colony in Lower Saxony ever. Nests were built mainly from mid June until early July on massive growing Western Waterweed Elodea nuttallii in the middle of the 210 ha lake, nearly 300-350 m off the waterline. The colony covered an area of 2-3 ha. By observation from distance, in total 91 breeding attempts were documented, of which 89 failed. Only two of the very early broods resulted in four fledged juveniles, i. e. breeding success was 0,08 juv./pair referring to the minimum breeding stock of 52 pairs. Losses were caused mainly by predation by a female Marsh Harrier. That bird flew into the colony on three of four evenings with special controls (3rd, 7th, 10th and July, but not 8th July). The Marsh Harrier visited up to three nests for feeding within 20 minutes each day. The Black-necked Grebes could not defend their broods against the raptor and escaped by diving immediately before the Marsh Harrier landed on the nest. The latest replacement clutch was registered at 20th July.The colony was potentially endangered due to surfing and sailing, but those activities mostly ceased early June because of the growing waterweed. To protect the grebe nests against pedal boating, the southern part of the lake (70 ha) was closed with a line of buoys. A replacement clutch 50-70 m nearby man made structures was destroyed by a swimmer (man or dog). After a professional firework in the night of 30th July (10:54-11:12 h p.m., sunset 9:23 h p.m.) directly beneath the northern part of lake Alfsee about 23 % of all Black-necked Grebes were missing the day after (including 11 of the former 15 first-year birds, which had arrived from elsewhere).

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