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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Survey 1979-2014 of Little Grebe Tachybaptus ruficollis in Schleswig-Holstein using playback - methods, population size, dynamics, phenology
Auteurs :	Rolf K. Berndt, Auteur
Type de document :	Article
Dans :	Vogelwelt (Die) (136(1) 2016)
Article en page(s) :	1-29
Résumé :	This paper is based on long term monitoring data for Little Grebe collected using playback calls. Without this acoustic method statements on population status and phenology of the species would not be possible. Little Grebe respond to playback calls very well. During suitable periods nearly all individuals respond, while responsiveness is low once eggs have been laid. Also begging calls of juvenile Little Grebes support recording of the species.The Little Grebe population in Schleswig-Holstein nearly quadrupled from 300 breeding pairs in 1974 to 900-1,200 pairs in 2009. Because the population is spread over several hundred waterbodies, only a sample of breeding sites can be monitored regularly. The strong population increase is mainly a result of the creation of pond-like waterbodies, often representing suitable habitats for Little Grebe and supporting about 55% of the current population. Newly created ponds replaced fish ponds as most common breeding sites. Moreover, various additional types of waterbodies have been populated by Little Grebe in recent years, making Little Grebe the grebe species inhabiting the widest spectrum of waterbodies.The most prominent characteristics of the population investigated are strong fluctuations at single waterbodies and frequent changes of breeding sites. These dynamics make it challenging to estimate the population at federal state level. Cold spells still represent the most negative population effect, leading to declines of up to 70%. Low temperatures and freezing of waterbodies are reported for 12 out of 35 years; once a related population low lasted for five years. Also temporary precipitation deficits, normally occurring locally restricted, lead to population declines. Newly created ponds, as all shallow waterbodies, represent labile habitats and only temporarily provide suitable conditions. This aspect, as well as further impacts, can lead to massive population fluctuations at individual waterbodies. For almost half of the declines recorded within the sample analysed, the reasons are unknown. Not only spontaneous declines, but also strong population increases occur. Regardless of considerable fluctuations the Little Grebe population of Schleswig-Holstein shows a strong long-term increase, mainly as a result of the continuous creation of waterbodies representing suitable breeding sites.Observed changes in breeding phenology are surprising. A grave increase of the breeding pairs present from the first to the second half of the breeding season was detected by multiple controls using playback calls. At the same time the incubation period was extended from 11 to 14 decades during the last 40 years. Due to an increase in late broods the mean hatching date was postponed by 19 days from 4th July to 23th July. Today, late broods in July and August occur regularly, interpreted to indicate a strong increase in second and possibly third broods. Considering breeding activities, it appears likely that under suitable conditions the majority of breeding pairs attempts late, respectively second broods at several waterbodies. This considerably increased the reproductive output of Little Grebe, contributing to the observed population increase. Suitable conditions are especially characterized by stable weather and water levels. It is not explicit why the considerable changes in breeding phenology occurred. Various causes are discussed. In summary, Little Grebe appears to be a profoundly opportunistic species showing the ability to adapt quickly to changing environmental conditions.

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