Landscape effects on waterbird densities in California rice fields: Taxonomic differences, scale-dependence, and conservation implications

Habitat use patterns are influenced not only by characteristics of the habitat itself, but also by the nature of the surrounding landscape. Conservation of waterbirds in agricultural areas, therefore, needs to consider the effects of landscape patterns on the occurrence of species in individual fields. The relationships between the densities of waterbirds using flooded rice fields in winter and characteristics of the surrounding landscape were analyzed in California's Sacramento Valley. The spatial scale at which the landscape was described was varied by calculating the amount of each habitat type within 2 km, 5 km, and 10 km of each field's boundary. Waterbird densities in flooded fields were related to landscape patterns in various ways, but the nature of the relationships differed among taxonomic groups and depended on the scale at which the landscape was characterized. Densities of geese, wading birds, and shorebirds were positively correlated with the amount of wildlife refuge or semi-natural wetland in the vicinity of a flooded field. These two variables were highly correlated with each other and their effects could not be assessed independently. The abundance of flooded rice fields in the landscape was less likely to be related to bird use of flooded fields, but was positively related to duck densities at a 5 km scale and negatively related to shorebird densities at a 10 km scale. These results suggest a number of hypotheses about the ways in which landscape scale patterns of farmland management might affect waterbird use of individual fields, with potential repercussions for both birds and farmers.