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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Differing outcomes of increased breeding failures in Western Bonelli's Warbler Phylloscopus bonelli (bonelli) and in the short-distance migrants Chiffchaff P. collybita, Robin Erithacus rubecula, and Wren Troglodytes troglodytes (2017)
Auteurs :	Wulf Gatter, Auteur ; Manfred Dallmann, Auteur
Type de document :	Article
Dans :	Vogelwelt (Die) (137(3) 2017)
Article en page(s) :	237-247
Langues:	Anglais
Résumé :	Little is known about the extent of the breeding losses of small ground-nesting bird species in woodlands. The decline in numbers of a series of trans-Saharan migrants has been ascribed to various causes, and factors associated with Global Climate Change have often been postulated. The disappearance of Western Bonelli's Warbler Phylloscopus bonelli (bonelli) from many parts of its SW-German range, as well as recent high rates of breeding failure, prompted a comparison with other woodland passerine species nesting on or near the ground in the same area.The study is based on 1,515 nest record cards of Wren, 1,063 of Robin, 487 of Chiffchaff, and 88 of Western Bonelli's Warbler. Since aroundl 990, breeding failure rates of all four species have risen compared with the previous 20 years. The worst affected species are Western Bonelli's Warbler and Chiffchaff, with critical consequences for the former.The increase in breeding losses of these woodland species corresponds over time with the rise in the populations of Red Fox Vulpes vulpes and Wild Boar Sus scrofa. These have risen over the time period studied by factors of 5 and 25-50, respectively. The increase in nest losses of all four bird species can be attributed above all to mammals. Predation pressure decreases with the height of the nest above the woodland floor. Increased predation by birds was seen only in Wren, which on average occupies higher nest sites.The three partial or short-distance migrants, with their clutch sizes of over 10 eggs/year, are apparently able to maintain their population numbers, despite increasing nest predation, while by contrast the long-distance migrant Western Bonelli's Warbler, with only 5 eggs/year, seems to be unable to compensate. The retreat of forestry from steep slopes has contributed to the decline of its dry warm habitats. Parallel to this decline in sunny places, those that remain are favoured by omnivorous and carnivorous mammal predators, whose numbers are increasing anyway, so that predation and disturbance pressure will potentially increase in the remaining favoured sunny locations. This combination of rising predation pressure and habitat fragmentation is regarded as the chief reason for the dramatic population decline and range contraction of Western Bonelli's Warbler in SW-Germany.To what extent the time expenditure involved in assessing predator densities occupies too large a fraction of the extremely narrow breeding-period window available to trans-Saharan migrants is discussed.

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