| Popis: |
Current ecological theory states that food and danger considerations underlie patch selection. Foraging sites for the ~ 100,000 ducks wintering on the Fraser River delta, British Columbia, Canada, are embedded in a matrix of suburban and rural land use types. I investigated foraging by American wigeon (Anas Americana), mallard ( A. platyrhynchos), northern pintail (A. acuta) and green-winged teal (A. carolinensis) to test the hypothesis that features adjacent to foraging sites such as buildings or roads cast a 'shadow of danger' that reduces patch use and thus habitat carrying capacity. I measured patch use with winter-long dropping counts on transects across fields adjacent to residential areas, greenhouses, roads and berry fields. Usage was highest adjacent to greenhouses, lowest adjacent to residential areas, and intermediate adjacent to berry fields and roads. Seasonal usage of a field was steady once begun, began soonest adjacent to greenhouses, and latest adjacent to residential areas. The distribution pattern of droppings across fields showed that ducks avoided residential areas, and foraged close to greenhouses. They showed no strong distribution pattern at berry fields and roads. The measured level of activity (wildlife, people, traffic, noises, lights, etc.) was highest at residential areas and roads, and lowest at greenhouses. Patch use and seasonal usage was lower in fields bordering land uses with higher activity levels. Previous studies on wintering ducks on the Fraser River delta widely report that upland foraging is largely nocturnal, that diurnal use is restricted to roosting on flooded fields, and that crop type strongly influences field usage. In contrast, I found that nocturnal and diurnal foraging were similar, though fields were visited more often at night. Landscape-scale selection of fields was best explained (AIC) by models including field-level measures of danger and greenhouse proximity: neither available energy nor standing water were included in the most informative models. These findings are consistent with the hypothesis that danger from various land use types strongly influences the foraging distribution of wintering ducks. |
