
Fig, 14. Canvasback nest in hardstem bulrush in the Minnedosa 
study area. 
thus exhibited the normal tendency to nest in small wetlands 
when they first returned to the nesting ground, but after 
losing their first nests they sought sites in larger wetlands 
for their second attempts. This tendency was most pro- 
nounced in years when water levels receded rapidly and 
smaller wetlands began to dry up, but was not an important 
factor in normal years. when wetlands bordered with cat- 
tail or bulrush usually held water throughout the nesting 
season (Fig. 14). 
As the season progressed each year, canvasback hens 
tended to nest in the more permanent wetlands where water 
levels were more stable. In dry vears such as 1961, 1962, 
and 1967, rapidly receding water levels in Type 3 wetlands 
allowed increased predation. Flooding from excessive rain- 
fall usually affected Type 1 and 3 wetlands more than more 
permanent wetlands, resulting in increased nest desertion. 
There was no evidence that nests over deep water were 
more secure from raccoons than those over shallow water. 
Nests over shallow water, however, were more apt to be- 
come stranded before hatching, making them more vulnera- 
ble to crows, magpies, skunks, fox, and other upland 
predators. 
The success of nests in wooded wetlands seemed unusual 
in an area where raccoons were abundant. The estimated 
number of nests per hectare in ungrazed woodland may 
be high as a result of the small sample recorded on the beat- 
out census. Most raccoons in the area seemed to live in aban- 
doned farm buildings, granaries, and heavy marsh cover 
(Cowan 1973). 
There are several possible reasons for the low use of 
whitetop areas, in contrast to the abundance of this vege- 
tation type. When weather and water levels permitted, 
most whitetop was harvested for hay or burned each fall 
because it generally occurred in wetlands that dried up by 
July or August. Generally, most whitetop was green and 
Fig. 15. Canvasback nests stranded by drought in the Minnedosa 
study area. 
too short for use as nest cover for the first canvasback nest- 
ing attempt. Lower nest success in this cover type was 
mainly due to predation on nests that were stranded after 
the shallow whitetop wetlands dried up. Nests in willow 
clumps also seemed vulnerable, probably because of their 
conspicuousness. Nests in willows were particularly prone 
to predation by raccoons and crows, The apparent low use 
of the “mixed” cover type was due to the fact that although 
many wetlands were classified as mixed, the nests were 
usually in a single cover type. 
Factors affecting waterfowl! nest success are numerous. 
Canvasback nest destruction or desertion is usually caused 
by predators, parasitism by redheads, weather, or agricul- 
tural operations. Prolonged cold weather may inhibit pro- 
duction of predator food and cause increased predation on 
early nesting ducks. Dry weather may encourage burning 
of pond edges, which destroys both nest cover and nests. 
Long periods of rainy weather may save some nests by pre- 
venting farmers from burning nest cover. Extended drought 
during the nesting season may delay or reduce initial nest- 
ing efforts and seriously curtail renesting attempts. Drought 
may also cause a rapid lowering of water levels, stranding 
nests on dry ground and increasing their vulnerability to 
predation (Fig. 15). 
Freezing weather often occurred after early nesting had 
begun. Most wetlands iced over on 22 May 1963 and some 
canvasback hens deserted nests where incubation had not 
begun. In 1965, some canvasbacks abandoned their nests 
during 24-27 May. From 24 to 26 May, 2 days of rain were 
followed by | day of snow and high winds; and on 27 May 
the temperature dropped to nearly —6° C, nests were 
ringed with snow, and some wetlands froze over. 
On 30 April 1966, 45,7 cm of snow and a temperature 
of — 17° C halted migration, and most canvasbacks moved 
to the larger lakes. All wetlands and small lakes froze over. 
