| Summary: | Long-term surveys indicate a decline of lesser scaup (Aythya affinis : hereafter scaup) populations breeding in boreal forest of western Canada. Unfortunately, little is known about the breeding ecology and population dynamics of boreal-breeding scaup, information needed to help guide conservation efforts. Therefore, in 1999 and 2000 I estimated demographic vital rates for a scaup population near Yellowknife, Northwest Territories (YKSA). Female scaup were trapped before breeding and fitted with radio transmitters and nasal markers to help estimate vital rates. Demographic rates from YKSA were compared with those estimated on a National Wildlife Area (NW A) near Saskatoon, Saskatchewan, and elsewhere. Despite large latitudinal differences between study areas, clutch initiation dates were remarkably similar for both sites (YKSA range = 5 June- 12 July, NW A range= 7 June- 9 July)~ and best-predicted Total Clutch Volume (TCV = 492.26- 5.93 CID). Female breeding-season apparent survival rates were similar: YKSA (0.80) and NW A (0.82). Analysis of projection models indicated the YKSA population was near stability (population growth rate (λ)= 0.979) during 1989 to 1998 and in sharp decline (λ= 0. 754) in 1999 and 2000. A detailed parameterization of the model for the recent period indicated a similar decline (λ = 0.831 ). For all models, the largest changes in λ were due to changes in breeding-season and non-breeding-season survival. Nesting success was also important and, from simulation analyses, all three rates together explained about 90% of the variation in λ. Population cycles of snowshoe hare (Lepus americanus), other small mammals, and their predators have a strong influence on the population dynamics of several resident species of birds and mammals in boreal forest. This, a possible explanation for large annual fluctuations in scaup productivity, was identified from correlations between estimates of scaup productivity and indices of predator abundance (as indexed by hare density [ r = -0. 79]) and buffer prey abundance (indexed by populations of microtines [ r = 0.69]). Predation may synchronize oscillations of sympatric prey through mechanisms predicted by the shared prey (hares) and (or) alternate prey (microtine) hypotheses. Further research is needed to improve estimates of scaup demographic rates and determine the importance of extrinsic factors (e.g., predators, food, weather and harvest) that potentially limit boreal scaup populations.
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