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Titre :	The breeding population of Great Crested Grebe Podiceps cristatus in eastern Denmark 1970-2010
Auteurs :	Hans Erik Torgensen, Auteur
Type de document :	Article
Dans :	Dansk Ornitologisk Forenings Tidsskrift (109(1) 2015)
Article en page(s) :	11-23
Langues:	Anglais
Résumé :	The occurrence of Great Crested Grebe in eastern Denmark was surveyed over the period 1970-2010. The study area included potential breeding sites (lakes > 3 ha and shallow coastal lagoons) in Lolland, Falster, Mon and the southern, western and central parts of Zealand (Fig. 1). Northern Zealand was not included, but data from that area was available from the Danish Ornithological Society. Twenty lakes (Nos. 1-20 in Tab. 1) were monitored almost annually; the remaining lakes were surveyed in some years only. Population size (number of territorial pairs) in the survey lakes was based on repeated counts throughout the breeding season, and the proportion of successful breeders (pairs seen with young) was monitored in most years. The other survey lakes were visited once or twice only during the breeding season.The total population in the study area in 2005-2009 was 1600 pairs (annual average per lake; see Tab. 2). In lakes in northern Zealand (outside the study area), the population was estimated at 650 pairs (mainly based on data from 1993-1996), so that the total population in eastern Denmark amounted to 2250 pairs (Tab. 2). In general, the population seems to have increased by 35-40% since 1980, but in some lakes the number of pairs have grown even more, e.g. lakes at Maribo (Nos. 1-4) had a population increase from 138 pairs in 1980 to 531-548 pairs in 2006-2007 (Tab. 3, Fig. 2). In these four eutrophic lakes, population increase took place in a period during which environmental conditions in the lakes improved from turbid waters and no submerged vegetation to clear waters and dense submerged vegetation.This recovery began in Rogbolle So (No. 1) in 1985, Hejrede So (No. 2) in 1997, Maribo Sonderso (Nr. 3) in 1999 and in Norreso (No.4) in 2000, each being followed by rapid increase of the grebe populations (Tab. 3).Population densities (pairs per 10 ha water surface) were higher in the shallow lakes (mean depth 100 ha), but the species was found regularly in nearly all lakes of between 5 ha and 100 ha in size (Tab. 7). Sixteen per cent of the population nested in shallow and sheltered brackish waters along the coasts, with strong local aggregations (Tab. 7, Fig. 1).

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