WINTER MANAGEMENT OF CALIFORNIAN RICE FIELDS FOR WATERBIRDS

1. Recent legislation designed to reduce air pollution has restricted Californian rice-farmers from burning rice stubble after harvest. Inte ntional flooding of fields during winter to speed straw decomposition is becoming increasingly common as growers seek alternatives to burnin g residual straw. The potential for flooded fields to act as a surroga te for destroyed wetland habitat may be an additional benefit in a reg ion that hosts a large proportion of North America's wintering waterbi rds. We investigated thr degree to which waterbirds use flooded fields and whether the method of flooding affects their use. Specifically, w e tested whether waterbird use (a) was greater in intentionally floode d fields than in unflooded fields, (b) differed among flooded fields r eceiving different straw manipulations and (c) varied with water depth . 2. Intentionally flooded rice fields received significantly greater use by 24 of 31 species studied. Only great blue herons Ardea herodias and sandhill cranes Grus canadensis were significantly more common in unflooded fields. Geese densities did not differ between flooded and unflooded fields. 3. We found no differences in the densities of most species in flooded fields that received different straw manipulations to improve decomposition rates. Exceptions included several small shor ebirds which occurred at highest densities in fields where straw was i ncorporated into the soil. 4. Species differed in their use of differe nt water depths. For 14 species we tested whether preferred depths, su ggested in the literature, received disproportionately higher use. Mos t of these species were more likely to be encountered within the sugge sted depth ranges. Depth, however, was a poor predictor of bird densit y. Depths of 15-20 cm resulted in frequent use by the greatest number of species. 5. We conclude that flooding rice fields increased suitabl e habitat for most, but not all, species studied. Different straw mani pulation methods had little effect on most species. Water depth, howev er, was important in determining species occurrence. During the first half of the winter, water depths were greater than the median depths u sed by most species.