Mallards (Anas platyrhynchos) and wastewater ponds, Part I: Mallard ducks overwintering at a northern wastewater treatment pond

- Mallards overwinter on municipal wastewater ponds at high density.
- Their behavior on ponds does not indicate they are experiencing stress.
- Stable isotope analysis indicates they may be catabolizing their own protein.
- Wastewater pond use may or may not be an ecological trap.

In northern urban areas, wastewater treatment ponds (WWTPs) may provide a thermal refuge during winter (~10 °C) that is used by normally migratory mallards (Anas platyrhynchos). On the ponds, mallards may experience stress due to crowding, or through the ingestion of WWTP water, be exposed to a diverse array of synthetic chemicals, which may have adverse health effects. Photographic sampling was used to assess mallard sex ratios and behavioural patterns throughout the late winter on wastewater ponds in Edmonton, Canada. The WWTP mallard population was large (>1000 birds), but temporally variable and consistently male-dominated. Locomotion and dabbling were the primary behaviors observed; aggression was rarely observed, which suggests crowding stress was low or absent. Mallard abundance tended to be higher at lower air temperatures, suggesting that WWTP ponds acted as a thermal refuge. Stable isotope analysis of carbon and nitrogen from duck feces and potential food sources indicated that mallards were not feeding at the site, or on invertebrates or select waste grain from offsite. Rather, ducks either consumed an undetermined food source or were feeding very little. Taken together, the data suggest that winter use of northern WWTP ponds may serve as an alternative to migration, whether this strategy benefits or harms mallards likely depends on winter severity, and not on WWTP pond characteristics or water quality.