Manipulation of Farmed Wetlands Increases use by Migrating Shorebirds and Ducks

Abstract

The Drift Prairie, in central North America, has been largely converted from grasslands to croplands, but still contains thousands of wetlands used by shorebirds and waterfowl during breeding and migration periods. Consequently, many of the remaining wetlands are situated within cropland where disturbance regimes (i.e., fire, grazing, and water-level dynamics), which occurred naturally prior to agricultural development, have been highly altered by landscape fragmentation from agriculture practices. Currently, smaller wetlands within crop fields are subject to disturbances stemming from agricultural practices (i.e., manipulations), such as burning, disking, harvesting, and mowing. We evaluated vegetation structure of idled (i.e., not recently manipulated by farming practices) and manipulated agricultural wetlands to investigate whether management method or resulting vegetation structure had greater influence on occurrence probabilities and densities of dabbling ducks and shorebirds during spring. All manipulation methods reduced vegetation heights compared to idled wetlands and most manipulations reduced the proportion of vegetation cover in inundated areas. Wetland manipulations generally increased shorebird occurrence compared to idled wetlands, whereas vegetation variables better explained duck occurrence probabilities. Duck occurrence peaked in wetlands with lower vegetation coverage (32%), and duck densities decreased as vegetation coverage increased beyond 10%. While more studies are needed to understand underlying mechanisms driving these outcomes, our results indicate that including periodic disturbances that reduce dense vegetation within wetlands in agricultural fields would increase their use by migrating and breeding shorebirds and dabbling ducks.