Long-term changes in agricultural practices and wildfowling in an internationally important wetland, and their effects on the guild of wintering ducks

1. The Marais Poitevin, one of the principal wintering and passage sites fo r ducks and waders in the East Atlantic flyway, underwent agricultural inte nsification in the 1980s. The numbers of ducks declined in the period 1979- 96 in the main roost, Bale de l'Aiguillon; this was a site-specific phenome non for which local factors were responsible. We describe here long-term ch anges in some key characteristics of the wetlands, paying particular attent ion to the principal factors affecting the quality of waterbird habitat: th e area of semi-natural grasslands, water management and wildfowling. 2. The area of grasslands declined by 50% between 1970 and 1995, principall y as a result of conversion to arable farmland. The pattern was observed ov er the whole area and was slightly more marked close to the day-roost of th e ducks. The rate of change was greatest in the 1980s. Water levels, and th erefore the flooded area, were reduced over the study period during the mon ths of May to November. The numbers of wildfowlers declined from the 1970s, but the number of artificial wildfowling ponds increased, especially after 1980. The number of birds killed did not decline and may have increased. T he quality of the ducks' habitat therefore declined by all three measures. 3. Four species, mallard, shoveler, teal and gadwall, showed an increase in other sites in north and west France either over the whole period or over the last part (1984-96), while declining (shoveler and mallard) or remainin g stable in the Bale de l'Aiguillon. Pintail remained stable in the other s ites, while showing a long-term decline (over 30 years) in the Bale. Wigeon did not use the Bale in large numbers in the 1996 cold spell as they did u ntil the 1980s. 4. The uncoupling of the trends for these species in the Bale from their tr ends in the other sites may reflect the loss of grassland feeding habitat i n the Marais Poitevin. However, for mallard, pintail and shoveler the decli nes in the Bale were much stronger than the 50% loss of grasslands so other factors, such as the reduction in autumn flooding and/or shooting, are lik ely to have played a part. 5. Since the three measures of long-term trends in duck habitat co-varied i t was not possible to separate their effects statistically. The hypotheses arising from this study need to be tested: this should be done using compar ative and experimental approaches. Suggestions are made for the management of water levels and shooting in the feeding habitat close to the Bale de l' Aiguillon.