12 
PERCENT OF NESTS 
25-29 5-9 
JULY 
26-30 
APRIL 
6-10 16-20 26-30 5-9 
MAY 
15-19 
JUNE 
Fig. 8. Chronology of canvasback nesting in the Minnedosa study 
area, 1961-72. 
Nesting Chronology 
Chronology of nesting was determined for all nests where 
it was possible to backdate the laying of the first egg. There 
was often an observable difference in success between early 
and late nesting attempts. As the season progressed, the cur- 
rent year’s growth of vegetation attained sufficient height 
and density to afford additional cover for late-nesting or 
renesting canvasback hens. Although nesting attempts 
began as early as 26 April, the first attempts peaked be- 
tween 6 and 20 May (Fig. 8). First nesting attempts peaked 
from 6 to 15 May during 10 of the 12 years; the peak was 
10 days later in 1961 and 1962, which were both drought 
years. 
In 1962 there was a gradual buildup to the first nesting 
peak during 16-20 May, followed by a tapering off until 
a second peak occurred 5-9 June (Fig. 9). The 1969 data 
indicated a rapid buildup of initial nesting attempts to a 
peak during 6-10 May, and few first attempts after 16-20 
May. Early nesting success was comparatively high that 
year. Cold weather, flooding, and predation by raccoons 
caused poor early nest success in 1965, but a strong second 
nesting effort peaked during 5-9 June. 
Nest Success 
During the 12-year period, 2,511 canvasback nests were 
found, of which 841 (45%) were successful (Table 15). 
Nesting success differed among the various units of the study 
area from 1963 to 1972; nests south of the intensive study 
unit were less successful than those northwest of it (Table 
16). 
Some overlap occurred between first and second nesting 
attempts but the period when first attempts had practically 
ended and renesting had started was usually discernible 
when the data were arranged by 5-day periods (Fig. 9). 
Most first nesting attempts ended and substantial renest- 
ing usually began between late May and early June. The 

25 
20 
1969 

PERCENT OF NESTS 
25 
20 
1962 
26-30 1-5 6-10 11-15 16-2021-2526-3031-4 5-9 10-14 15-19 20-24 25-29 
APRIL MAY JUNE 
Fig. 9. Variations in nesting chronology of canvasbacks in the 
Minnedosa study area. 
renesting effort was apparently not closely related to 
phenology of the previous month nor to chronology of the 
first nest initiation, but appeared to be more closely related 
to predation. Peak dates of the renesting effort did not vary 
more than 10 days among the 12 years of the study. 
Several sources of information were used to adjust data 
to reflect the actual number of hens that renested. In addi- 
tion to these data, breeding pair and brood data from beat- 
out counts were used to determine the percentage of hens 
that renested. Data from 1966 (Fig. 8 and Table 15) are 
presented as examples of adjusted nesting data: First nesting 
(19 April-26 May), 115 nests; success from 115 nests, 47% ; 
renesting (27 May—29 June), 58 nests; success from 58 nests, 
55 % . 
Breeding pair and brood data from a 7.8-km? block 
within the study area south of Minnedosa were used to 
calculate the percentage of canvasback hens that renested. 
Inasmuch as 32 breeding pairs were found on this area in 
1966, and nesting success was 47 % , the 32 hens should have 
produced 15 broods on the first nesting attempt. The re- 
maining hens might have renested, but in late July the 
