REPRODUCTIVE SUCCESS AND FEMALE SURVIVAL AFFECT LOCAL-POPULATION DENSITY OF CANVASBACKS

Long-term data (1949-55 and 1959-80) from a canvasback (Aythya valisin eria ria) population near Minnedosa, Manitoba suggested that breeding density was limited to less than or equal to 4.5 pairs/km(2) by annual variation in reproductive success (the local-recruitment hypothesis). We attempted to test this hypothesis by experimentally increasing-rep roductive success on a 15.8-km(2) study block during 1983-90, Manipula tions included barrier fences to protect canvasback nests from mammali an predators and egg additions to replace canvasback eggs that had bee n displaced due to parasitic egg-laying by redheads (Aythya americana) . We monitored breeding pair density and duckling production on the ex perimental study block and on an adjacent 15.8-km(2) control block usi ng complete pair and brood counts (1983-90), nest and brood monitoring , (1983-90), and roadside pair and brood surveys (1983-94). We nasal-m arked pre-fledgling and adult female canvasbacks and used mark-resight ing analysis to estimate juvenile survival, adult survival, and resigh ting probabilities. Reproductive success varied substantially among ye ars, primarily in response to variable wetland conditions. The experim ental study block had higher per capita fledging success during 6 out of 8 years (P = 0.08), but survival and resighting rates did not diffe r between study blocks (P = 0.28-0.80), Indices of canvasback density were similar during 1983-84, but pair populations became higher on the experimental block during 1985-90 (P < 0.01). By 1988, canvasback den sity on the experimental block reached 8.6 pairs/km(2), which was 1.8 times the highest density recorded on the control block. Differences i n pair density persisted during the posttreatment years of 1991-94 (P < 0.003; except 1993: P = 0.72), although posttreatment duckling produ ction did not differ between the 2 study blocks (P > 0.16). Annual cha nges in pair density were correlated positively with adult survival (r = 0.69, P = 0.007) and hedged ducklings/hen (r = 0.62, P = 0.02), alt hough neither correlation remained significant when the other factor w as controlled with partial correlation (P = 0.24 and 0.63). Population simulations suggested that both factors were important in affecting p opulation changes, Management actions that enhance local breeding succ ess or annual survival should lead to larger local breeding population s.