Feeding behaviour and impact of ducks on ripening barley crops grown in Otago, New Zealand.
Field feeding by mallard ducks was not random; fodder barley and stubble fields were preferred and standing oat crops were avoided. Ducks were attracted to barley crops by the presence of spilled grain and the field margins sustained the highest level of duck damage. Crop damage was measured along transects, using an empirical weight-length table, and compared with the feeding pressure exerted by the number of birds present as grain losses caused by disease, insects, sparrows and greenfinches were difficult to distinguish from duck damage. Estimates of feeding pressure suggested that duck damage was overestimated using transect measurements. Grain consumption by ducks was comparable to the amount of grain spilled during harvesting. The number of ducks which fed on ripening barley was influenced by the proximity of water, moisture content of the grain, and the number of days the crop was available. Plant height, grain moisture content, amount of spilled grain, density and area of the barley crop were directly associated with levels of duck damage.