Mg. Anderson et al., REPRODUCTIVE SUCCESS AND FEMALE SURVIVAL AFFECT LOCAL-POPULATION DENSITY OF CANVASBACKS, The Journal of wildlife management, 61(4), 1997, pp. 1174-1191
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.