. The Canadian field-naturalist. 404 The Canadian Field-Naturalist Vol. 97 • First nests o Renests K Second nests 8- 7- 6- 5- 4- 3 O 3H 2- 1 -. -I— 10 20 "I— 10 20 May June Clutch initiation Figure 2. Variation in clutch size in relation to date of laying. July from 10 clutches was 23-24 days, but in 2 nests hatch- ing did not begin until the first eggs had been incu- bated for 28 and 34 days. Attentive periods of males and females from 8 pairs ranged from 17 to 102 min (mean ± min; n = 33). During egg-laying incubation was shared almost equally (55% by females, 45% by males), b
. The Canadian field-naturalist. 404 The Canadian Field-Naturalist Vol. 97 • First nests o Renests K Second nests 8- 7- 6- 5- 4- 3 O 3H 2- 1 -. -I— 10 20 "I— 10 20 May June Clutch initiation Figure 2. Variation in clutch size in relation to date of laying. July from 10 clutches was 23-24 days, but in 2 nests hatch- ing did not begin until the first eggs had been incu- bated for 28 and 34 days. Attentive periods of males and females from 8 pairs ranged from 17 to 102 min (mean ± min; n = 33). During egg-laying incubation was shared almost equally (55% by females, 45% by males), but after egg-laying females spent considerably more time on the nest (63%) than did males (37%). Hatching During both years, first eggs in 47 nests hatched between 10 June and 2 August (Figure 3). Clutches hatching after 5 July were either replacement or second clutches. Generally, the interval between hatching of consecutive eggs reflected the laying inter- val; most chicks hatched at 1-2 day intervals. Hatch- ing periods for the three most frequent clutches were 7-8 days for clutches of five eggs, 8-9 days for clutches of six, and 9-10 days for clutches of seven. Hatching success was low in both years ( of 637 eggs). Predators destroyed 38% of eggs laid, waves generated by high winds washed out 12%, and adults deserted 2%. Eggs that disappeared from nests (18%) were not attributed to a specific cause, but predators were likely responsible for most of those losses as well. The appearance of eggshell remains (see Rearden 1951) indicated that Raccoons (Procyon lotor) were responsible for most of the predation. Most destroyed eggs were characterized by an opening in the large end of the shell which suggested that the end had been bitten off. The Raccoon's activity as a predator on over-water nests in the Minnedosa district is well doc- umented (Kiel et al. 1972; Cowan 1973). Canvasbacks {Aythya valisineria) nesting on the study area in 1974 and 1975 also suffered high
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