Winter habitat for dabbling ducks on southeastern Vancouver Island, British Columbia

This study is an examination of the use of coastal estuaries and nearby farmland as habitat by dabbling ducks (mallard and American wigeon) during migrating and wintering periods. Its aim was to identify aspects of British Columbia coastal habitat of importance to dabblers through an analysis of the ducks' movements among habitat types and through a description of where and on what ducks feed. Data were collected in 1979 and 1980 along a 30 km stretch of coastline on southeastern Vancouver Island. Results are presented in 3 parts. Part 1 examines the relative use of farm and coastal habitat through a series of censuses conducted weekly at 8 farm and 8 coastal sites. The strong negative correlation between counts at farm and coastal sites indicates that dabblers treat them as alternative habitats. The numbers of ducks on farms was positively correlated with the area of standing water on the fields. Farm habitat, apparently preferred during warm, wet weather, was not used when fields were dry or frozen. Part 2 is a description of feeding location on fields, at estuaries and at a shallow, nonestuarine bay. It is based on observations at selected sites at high and low tide levels. Each duck in each observation period was classified as to location and activity. Both species fed primarily in shallow water, their feeding location shifting with the tides. Both marsh and marine sections of estuaries were used extensively for feeding. The shallow bay was used especially by American wigeon at low tide in fall and early winter. The high marsh areas at estuaries were particularly attractive to mallards when flooded by exceptionally high tides. Feeding intensities were consistently high at farm sites for both species. In Part 3, 23 mallards and 40 American wigeon were shot while feeding in estuarine locations commonly used for feeding. Analysis of gullet contents revealed that both species ate a wide variety of items. Main foods were, for mallards, seeds, invertebrates and green algae and, for American wigeon, green algae, roots, seeds and green vegetation. Algae and marine Invertebrates are not usually considered to be important dabbler foods in estuaries. Major conclusions and recommendations are: 1) Both farm and coastal sites are important to dabblers, with fields being favoured as feeding locations under good flood conditions and coastal habitat being vital during dry or freezing periods. As dabblers move among sites, assessment and management of wintering dabbler habitat should be by wetland complexes rather than by individual estuaries. 2) Dabblers feed in or near shallow water. Fields that do not flood are not worth maintaining as dabbler habitat. Assessment of estuarine marshes should consider the availability of food at all points of the tidal cycle. 3) As both species feed on a wide variety of items, factors affecting shallow water flooding and thus food availability are more important than plant species composition. 4) Marine deltas and beaches are important as refuges from disturbance and as feeding grounds. They form an integral part of dabbler coastal habitat.

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