. The Canadian field-naturalist. 242 The Canadian Field-Naturalist Vol. 103 groups of scoters with females. The proportion of females that were paired increased in May from 44% on 9 May (N = 41 females) to 86% on 14 May (N = 139), and 96% by 31 May (N = 139). The mean number of scoter pairs on Redberry Lake between 1 and 20 June was relatively stable among years of the study (1977—396; 1978—389; 1979— 419; 1980—423) and followed a similar seasonal trend each year (Figure 1). Sex ratios (58-65% male before egg-laying began) were similar to those reported on Redberry Lake and Jessie Lake, Albert
. The Canadian field-naturalist. 242 The Canadian Field-Naturalist Vol. 103 groups of scoters with females. The proportion of females that were paired increased in May from 44% on 9 May (N = 41 females) to 86% on 14 May (N = 139), and 96% by 31 May (N = 139). The mean number of scoter pairs on Redberry Lake between 1 and 20 June was relatively stable among years of the study (1977—396; 1978—389; 1979— 419; 1980—423) and followed a similar seasonal trend each year (Figure 1). Sex ratios (58-65% male before egg-laying began) were similar to those reported on Redberry Lake and Jessie Lake, Alberta, in 1976 (Brown and Brown 1981). Numbers of pairs declined rapidly after mid- June, as females began incubation (Table 1). The small mid-June decline in total numbers resulted from females beginning incubation and not being counted. After 17 June, the number and percentage of birds in groups and flocks increased as some males and females ended reproductive attempts and joined groups or flocks. Counts of pairs should be made between 1 and 16 June; this is 7 to 14 days before most females began incubation. Birds in aggregations soon left the lake, presumably to molt elsewhere. Males departed earlier and in greater numbers than females. The departure of scoters from the lake caused a decline in total numbers after 24 June (Table 1). At the end of July, only 24% of the peak adult population (1159 birds) remained and 92% of these were females (Table 1). Nest Sites and Nest Success. Although we counted about 400 scoter pairs on Redberry Lake each year, we found only 89 to 132 nests on the islands. Many females must have nested on the mainland or not nested at all. Of the nests located in the most frequently used cover types of Redshoot Gooseberry and Common Reed (Table 2), 90% were completely concealed. In contrast, 71% of the nests in other habitats were partially visible from above. Nests in habitats that provided complete or at least 80% cover had significantly greater r
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