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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	How densely can Common Terns Sterna hirundo breed? Extreme situations of rafts
Auteurs :	Stefan R. Sudmann, Auteur
Type de document :	Article
Dans :	Vogelwelt (Die) (119(3-5) 1998)
Article en page(s) :	181-192
Résumé :	The breeding success of two Common Tern colonies in the Lower Rhine area, Germany, declined with increasing nest density during the period 1991 to 1997. On the rafts serving as artificial nesting platforms, the highest nest density was 2.13 clutches per m2 (51 clutches on 24 m2). Chicks were frequently carried away from the rafts by adults and dropped into the water, where they drowned or were predated by Common Gulls Larus canus. The frequency of such losses correlated positively with nest density on the rafts. Up to 60% of chicks from first clutches were lost in this way. Intraspecific aggression was the most important cause of chick mortality. In a year with high nest density, the breeding success of replacement clutches was better than that of first broods, because prey availability improved during the course of the season. In 1996 and 1997, small young fish were only available from early June onwards. Prior to June terns obtained much food by piracy (intraspecific kleptoparasitism). In this early period, 20% of the attempts to feed a chick were unsuccessful due to piracy, in contrast to only 5% in the later (mostly replacement) broods, when prey availability had improved. Thus intraspecific aggression among adults was intense when food was scarce, and in conjunction with the high nest density on artificial rafts left many chicks as victims. Common Terns nesting on rafts need a minimum territory size of 1,25 m2 per breeding pair. Fledgling success can be greatly improved by providing shelters where the chicks can hide. Such conditions approximate those found at natural breeding sites.

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