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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Change of feeding ecology of Whooper Swan, Bewick\'s Swan and Mute Swan Cygnus cygnus, C. bewickii, C. olor during the period 1965-2010 at the Dannenberger Elbbogen (N Germany).
Auteurs :	Wilhelm Meier-Peithmann, Auteur
Type de document :	Article
Dans :	Vogelwelt (Die) (132(2) 2011)
Article en page(s) :	57-79
Résumé :	Since the 1980s swans switched from (semi) aquatic to terrestrial food in their Central European staging and wintering areas. This paper describes the course of this change with circumstances, scale and consequences by the example of the Dannenberger Elbogen (River Elbe), one of the most important staging and wintering sites in Germany.At least from the mid 1970s onwards all three swan species occasionally and to a small amount took winter cereals close to their roosts, drinking waters and flooded foraging waters. The extension of oilseed rape cultivation and the introduction of \'low\' and \'double low\' rape together with losses of aquatic food since the mid 1980s caused a general habitat switch from non-diked floodplains to diked areas (within a few years in Mute and Whooper Swan, over about 15 years in Bewick\'s Swan). From 1965-1985 to 1990-2010, the proportion of birds found in diked areas increased strongly in Whooper Swan (from 0.6% to 69.8%), Bewick\'s Swan (from 1.7% to 67.2%) and Mute Swan (from 1.2% to 79.9%).The polder around the oxbow Taube Elbe held 5-20% of those foraging swans staging in the western part of the Dannenberg Marsh in the 1960s, followed by 90% around 1980, but below 10% in the early 1990s. This increase followed by a decrease is thought to be connected with the development of food plants due to increased nutrient contents in the water. At first, Whooper and Bewick\'s Swans benefited from well-developing plants in swamp communities, but later on overfertilization caused significant losses of Reed Sweetgrass, the preferred food of Whooper Swans. At the same time more terrestrial food became available, mainly rape. While Bewick\'s Swans showed strong avoidance of the Taube Elbe polder for the last ten years, Mute Swan stocks in that part of the study area recovered to former numbers due to their feeding on submerged plants.For the whole non-diked area, regression lines from 1985-2010 indicate decreasing proportions of utilization by 70% (Whooper Swan), 60% (Bewick\'s Swan) and 30% (Mute Swan). In the same period, the proportion of Mute Swans feeding in pools from aggregate extraction increased from 1% to 6%, because eutrophication improved the supply of aquatic vegetation. The highly significant increase of foraging on rape fields with up to 90% of Whooper and Bewick\'s Swans and 80% of Mute Swans was interupted in winters with strong flooding. More recently, the decrease of rape field foraging can be explained with the novel utilization of maize stubble fields: straightaway 24% (2008/09) and 34% (2009/10) of Whooper Swans and 70% (both winters) of Bewick\'s Swans were seen in this habitat, though it allows feeding only in a small time frame in late autumn and early winter.In the period 1995-2009, the number of bird days/10 ha remained constant in the Whooper Swan (440-480), but decreased from 300 to 120 in the Mute Swan (due to switching to aquatic food) and from 110 to 15 in the Bewick\'s Swan (due to switching to maize). in the small fields left of River Elbe measuring only 4.6 ha in average the number of bird days/10 ha was twice as high as in the in average 47.8 ha large fields on the other side of the river.All swans used to return from rape fields to non-diked areas or oxbows as soon as swamps and wet grassland were flooded and thus could serve as foraging habitat. The median of the water level in River Elbe, at which half of the Whooper Swans showed such a habitat switch, increased by 1.75 m from 1988/89 to 2009/10. Instead of remaining in suboptimal non-diked habitats such as wet grassland in case Reed Sweetgrass and Sharp Sedge swamps became unaccessible, more and more Whooper Swans switched to rape during the years, and finally about 20% remained there despite of favourable water levels. Such behaviour was also observed in the other two swan species, but took place on a broader amplitude of water level. Only in the Mute Swan the triggering water level did not move upwards.The change in feeding ecology in combination with a series of mild winters had impact on the annual cycle, the distribution and the population size of the swans. The early and continuous supply and accessibility of rape as food has smoothed the pattern of occurrence from showing distinct and species-specific peaks to a more balanced picture throughout the wintering season without large differences between the swan species. After switching to terrestrial food resources especially Whooper Swans increasingly opened up staging and wintering areas between the river systems of Elbe, Weser and Ems, which had been the only sites used in earlier decades, and now can be found far from lakes or coastal habitats. As a consequence, the totals of Whooper and Bewick\'s Swans staging in the Elbe floodplains near Dannenberg have decreased slightly, despite a general two- or threefold increase in Germany.Future studies should focus on the consequences of further increasing cultivation of maize (serving as swan food only during the period of stubble fields) for annual cycles, population sizes, and distribution on both smaller and larger scales in the three species.

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