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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	[Censusing the urban Mallard Anas platyrhynchos population in Stuttgart.]
Auteurs :	Christoph Randler, Auteur
Type de document :	Article
Dans :	Vogelwelt (Die) (123(1) 2002)
Article en page(s) :	21-24
Résumé :	Population trend of urban Mallards and changes in the proportion of hybrids were studied in and around Stuttgart (9degree10'/48degree47'), SW-Germany, to assess the potential threats posed by hybridisation with other waterfowl to the indigenous population. The census covered 21 plots in winter 1993/94 and and was repeated in 1999/00. Habitat categories (a) city-areas, (b) outskirts and (c) areas outside the town were analysed separately. At all sample plots Mallard were dependent on supplementary feeding. Urban Mallards declined by 25 % from 1993/94 to 1999/00 (TRIM: p = 0.0345), hybrids increased by 6 % to 236 birds (p > 0.5). Proportionally, the increase of hybrids from 11.4 % to 15.8 % was significant (p = 0.0036; TRIM). A closer look at the different categories of habitats showed that the percentage of hybrids in city-areas increased significantly from 13.4 % to 20.2 %, (p = 0.001, n = 10; t-test). The slight increase from 10.6 % to 12.7 % in the outskirts was not significant (n.s., p = 0.525, n = 6), as was the slight increase from 8.2 % up to 10.7 % in rural habitats (n.s., p = 0.514, n = 5). TRIM did not confirm significantly different trends between habitats (p = 0.72). The opposing trends are probably due to the fact that urban Mallard populations consist of different behavioural types: a) mainly resident birds b) local residents c) wild migrants. The overall decrease may have been caused by category c) birds, while the stable population of hybrids is a result of their resident behaviour. The increase of hybrids in Stuttgart over a six-year-period is no serious cause of concern. Any definite negative impact of hybrids on the natural population has yet to be demonstrated.

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