Dr. Scott Petrie, Long Point Waterfowl Executive Director
Dr. Robert Bailey, University of Western Ontario
Dreissenid mussels (zebra and quagga mussels) were introduced to the Great Lakes and represented a major change to the benthic community. As a result, most molluscivorous waterfowl using the Great Lakes now incorporate dreissenid mussels into their diets. This novel and abundant food source, combined with a general long-term moderation of winter temperatures, has likely resulted in the substantial increase in numbers of diving ducks over-wintering on the Lower Great Lakes. Increased numbers and feeding activities of diving ducks on the Lower Great Lakes may be responsible for localized reductions of mussels; a situation that has already been well documented in Europe. However, changes in resource availability and effects on wintering diving duck communities have not been adequately studied or documented in North America.
Diving duck feeding activities, ice cover, or other factors can reduce and limit accessibility of macroinvertebrate prey during winter. Reduced prey availability during winter may lead to increased niche breadth and dietary convergence among ecologically similar species, which may also increase potential for interspecific competition. Alternatively, competition for food or other limited resources may be reduced by temporal, spatial, or dietary segregation of waterfowl. Substantial depletion of prey, however, may preclude resource partitioning and cause noticeable changes in body condition and shifts in habitat use. Thus, food availability may influence community structure of diving ducks during winter. Unfortunately, we know very little about the ecology of diving ducks during winter on the Lower Great Lakes. By documenting the dynamics of resource overlap among species of diving ducks throughout winter we can begin to make inferences regarding niche overlap and competition during this critical period in their annual cycle.
Study hypothesis and objectives
We hypothesized that resource limitation during winter and resulting niche overlap may shape community structure, thereby regulating distributions and populations of some diving duck species on the Lower Great Lakes.
Thus, our main objectives were to document seasonal resource availability, use and partitioning, and their effects on body condition of diving ducks wintering in the vicinity of Prince Edward Bay, Lake Ontario. We anticipated that depletion of food resources will cause changes in diet, habitat use, and behaviour of diving ducks during winter and that those changes also will affect body condition of diving ducks throughout winter. Differential changes in resource use and body condition among species may suggest differential competitive performance for limited resources during winter. To address these broader ecological questions, we will compare resource use and determine the extent of resource overlap among Bufflehead (Bucephala albeola), Common Goldeneye (Buecephala clangula), Greater Scaup (Aythya marila), Long-tailed Ducks (Clangula hyemalis), and White-winged Scoter (Melanitta fusca).
Data were collected from mid-December through late-April during winters 2002/2003 and 2003/2004. Resource use by each diving duck species will be determined from waterfowl inventories, assessment of habitat use, behavioral observations, and dietary studies. To establish baseline resource abundance and availability data, we will develop a food availability index by collecting benthic food samples during fall, winter, and spring. Resource partitioning will be determined from comparisons of dietary overlap among species and spatial segregation of flocks.
Benthic samples and food availability
Benthic samples were collected at various depths from four locations during fall (late-October) and spring (late-April). Analyses of 2002/2003 data showed numbers of invertebrates differed by location and collection depth, but abundance did not differ between fall and spring. Thus, food availability did not decline throughout the wintering period. Observational evidence also corroborated results from benthic sampling. For example, we observed that winds and water currents regenerated food supplies along ice banks that formed during winter (note the photograph showing wind-rows of mussels, scuds, chironomids, and snails). Large multi-species flocks often fed nearshore within the ice-water interface zone where waves and currents deposited food. Because of this, during winter 2003-2004 we sampled the level of food availability at the ice-water interface and found that food was in fact highly abundant. This provided further evidence that winter food availability during 2002/2003 was not likely a factor limiting the number of diving ducks using this part of Lake Ontario.
Waterfowl inventories, habitat use, and behaviour
Waterfowl inventory surveys and behavioural observations (scan samples) of diving duck flocks were conducted weekly to determine foraging pressure, activity budgets, foraging depth, and flock composition/behaviour of White-winged Scoter, Long-tailed Duck, Greater Scaup, Bufflehead, and Common Goldeneye. It was soon found that, White-winged Scoter and Greater Scaup were largely absent from our study area during both years. Therefore, we narrowed our study scope to include Long-tailed Ducks, Buffleheads, and Common Goldeneye. Each of these three species increased their foraging time from December – March, but Bufflehead spent much more time feeding than did Long-tailed Duck and Common Goldeneye. While foraging behaviour differed among species, large, mixed-species flocks fed regularly along ice bank where wind and currents deposited large amounts of food. It also was notable that even within these large multi-species flocks very little aggressive behaviour (< 0.10% of time) was observed; if food was limiting, much higher levels of intra- or interspecific aggression might be expected.
Waterfowl collections and energetic condition
Diving ducks (n = 750) were collected (shot by drifting into flocks, jump-shooting, pass-shooting, and shot over decoys) throughout the study to determine changes in foraging habits and body condition; liver samples also were obtained for contaminant analyses as part of a larger Long Point Waterfowl study. During winter 2002/2003 temperatures were below normal and ice coverage was extensive and abdominal fat reserves of all species decreased substantially throughout the season. During winter 2003/2004 temperatures were near normal and ice cover was less extensive. However, fat reserves of birds collected during this time are currently being determined, so comparison of body condition between the two study periods is not yet available. Food habits and dietary overlap information also are currently being determined for the latter study period.
In most years, Lake Ontario remains relatively ice-free whereas lakes Erie and St. Clair freeze by late January. Therefore, Lake Ontario is an important wintering area for tens of thousands of diving ducks. While ducks that stage on the Lower Great Lakes may remain for 2-4 weeks, those that use Lake Ontario during winter may remain for as long as 4 months. Other Long Point Waterfowl studies indicate that contaminants, such as selenium, could be a potential issue for staging diving ducks that feed for a shorter period than wintering birds. Therefore, understanding what affects distributions and limits the numbers of diving ducks wintering on the Lower Great Lakes is important. Our preliminary results suggested that food was readily available and that birds lost weight despite increasing their foraging time. Given those findings, it seems probable that air temperature, thus increased thermoregulatory costs, and the extent of nearshore ice coverage are two factors that may contribute to limit the numbers and spatial distributions of diving ducks wintering on Lake Ontario.