Mute swan (Cygnus olor) winter distribution and numerical trends over a 16-year period (1987/1988–2002/2003) in France

Abstract

The numbers of mute swans (Cygnus olor) at 98 wetland locations in France were monitored monthly during the winter (December-February) for 16 years by a national network of observers as part of a broader national wildfowl monitoring scheme. Log-linear Poisson regressions with TRIM software were used to estimate missing counts and produce national numbers and indices. These corrected indices were in turn used to calculate an average annual rate of change and the associated confidence interval that subsequently enabled the computation of a cumulated (global) rate of change and associated confidence interval. The latter were interpreted to classify the numerical trends over the whole period. For the six wetlands with the largest numbers of mute swans, average numbers of swans were also compared between wetlands and months. General Linear Models were then used to test simultaneously for (1) differences between years and between wetlands for each month and (2) between years and between months separately for each of the six wetlands. The mute swan average annual rate of change was >7% for each month. The global rate of variation corresponded to a ''strong increase'' after TRIM analyses. This result strongly supports the necessity of maintaining both national and international monitoring schemes in order to be able to quickly detect major increases in swan numbers at specific sites, particularly where increasing numbers may result in a conflict with farmers or in inter-specific competition with other waterbirds and/or represent a possible sanitary/public health hazard due to the potential for swans to carry avian influenza viruses.

Ecology of avian influenza viruses in a changing world

“…Their increases are likely in part due to food subsidies provided by nitrogen and calcium rich croplands that are important in reproduction. 61,62 In many countries there has also been a significant shift towards agricultural intensification. An example of this is the decrease in the agricultural practice of "summerfallow," where land was traditionally tilled but left un-cropped for one year to allow for moisture accumulation, nitrogen release, and weed-control benefits.…”

A social–ecological approach to landscape epidemiology: geographic variation and avian influenza

“…In Western Europe, broad-scale landscape modification and intensive farming have had significant effects on waterfowl. For example, the expansion of the mute swan (Cygnus olor Gmelin, 1789) population is in part due to food subsidies provided by nitrogen and calcium rich croplands that are important in reproduction (Fouque et al 2007). In the Camargue, France, division of the marshes and large inputs of freshwater have resulted in lower water salinity and longer flood periods, leading to losses of biological diversity and higher plant biomass favouring use by wintering waterfowl (Tamisier and Grillas 1994).…”

Breeding mute swan habitat selection when accounting for detectability: a plastic behaviour consistent with rapidly expanding populations

“…Swans colonized a vast array of habitats over the last decades, suggesting a highly plastic behaviour (Wieloch 1991;Fouque et al 2007). This is further supported by the present results, which demonstrate that swan presence is not restricted to the richer sites during the pre-laying and laying periods.…”

“…Mute swans (Cygnus olor) are now present in a wide variety of aquatic habitats (see Kirby et al 1994;Fouque et al 2007), including, e.g. lakes, urban ponds or fluvial canals during the breeding season.…”