Dr. Scott Petrie, Long Point Waterfowl Executive Director
Dr. Dave Ankney, University of Western Ontario
Over the past several decades, Tundra Swan (Cygnus columbianus columbianus) populations have increased considerably in North America. Coincident with their continent-wide increase, Tundra Swan use also has increased at several important waterfowl staging areas, including Long Point, Ontario. Tundra Swans are large, herbivorous birds that can consume large quantities (£ 5 kg/bird/day) of aquatic vegetation. It is well established that feeding activities of large numbers of swans and other large herbivorous waterfowl can dramatically reduce the amount and availability of aquatic plants at staging and wintering areas. Tundra Swans and ducks apparently show some overlap in food choice, especially in their use of carbohydrate-rich plant tubers during the staging period. Thus, there is some concern that feeding activities, abundance, or behaviour of Tundra Swans may influence abundance, distribution, and feeding behaviour of other waterfowl at staging areas.
In response to this concern, Long Point Waterfowl and the University of Western Ontario initiated and completed a three-year (1998-2000) fall and spring staging ecology study specifically addressing if:
1) Tundra Swans (and geese) substantially add to seasonal reductions in plant biomass attributable to ducks and biotic or abiotic factors
2) abundance, proximity, and foraging activity of swans influence abundance, distribution, and activities of other staging waterfowl
3) swan proximity and foraging activity affect foraging activities and feeding methods of American Black Ducks (Anas rubripes) and Mallards (Anas platyrhynchos) at Long Point, Lake Erie.
Other aspects of our research involved documenting diurnal and nocturnal feeding activities and broad-scale habitat use of Tundra Swans in aquatic and terrestrial habitats during fall and spring. We also were interested in the intra- and inter-group behavioural dynamics of staging Tundra Swans.
Exclosure experiments conducted during fall in aquatic habitats showed there were large seasonal declines in both above and below-ground biomass of aquatic vegetation (mainly Chara spp, Vallisneria americana, and Potamogeton pectinatus) that were attributable to herbivory and removal by wave action or plant senescence.
However, there was no evidence that Tundra Swans and/or Canada Geese (Branta canadensis) contributed more to that seasonal decline than did other herbivores (ducks, aquatic mammals, fish, etc.) and other biotic (plant senescence) or abiotic (wave action) factors. Further, behavioural observations at exclosure locations, mostly sites within the Long Point and Big Creek National Wildlife Areas, showed that Tundra Swans likely did not greatly reduce food availability to other waterfowl because they spent much less time feeding than did most other waterfowl and often fed on tubers that generally were too large for most other waterfowl to consume (e.g., Nuphar spp and Sagittaria spp). We also found that densities of individual waterfowl species and Tundra Swans were not negatively correlated in either large (e.g., Turkey Point Marsh, Long Point Company Marsh, Big Creek Marsh) or small (e.g., individual pond within the larger marsh complexes) wetland areas. To the contrary, some duck species showed positive correlations with increasing swan densities at large scales, whereas most pond-level waterfowl distributions showed that ducks tended not to avoid swans.
Despite the relatively considerable amount of resource defense that was apparent among swans, very little overt aggression was directed toward foraging ducks. Abundance, proximity, and foraging activities of swans within ponds did not substantially reduce time waterfowl spent feeding. In fact, some species followed foraging swans and copied their feeding locations within ponds and several ducks, including American Black Duck, Mallard and Ring-necked Duck (Aythya collaris), showed
increased feeding activity in response to swan feeding activities. Specifically, we found that foraging Black Ducks and Mallards also increased their feeding rates and decreased their vigilance when swans were feeding on ponds; these two species also altered their feeding methods when foraging with active and inactive swans.
Given our results, we concluded that Tundra Swans generally did not greatly add to reductions in abundances of important tuber producing plants, mainly Vallisneria americana and Potamogeton pectinatus, during fall. We also suggested that Tundra Swans did not have substantial negative effects on abundance, distribution, or feeding activities of other waterfowl staging at Long Point, Ontario. Rather, most of our findings suggested that Tundra Swans had neutral or positive effects on waterfowl distributions and feeding activities, especially over the short-term and at small spatial scales. Although, future studies are needed to determine if ducks actually benefit from swan feeding activities both in the short- and long-term. Longer-term studies will be necessary to understand better the complex interactions that can occur between waterfowl and their aquatic food-plants at staging and wintering areas.