. The Canadian field-naturalist. Natural history. 264 The Canadian Field-Naturalist Vol. 115 -i â «-» > â L. (1) Q. tfi £ ⢠CD 4-» O.'l- >> 0 Grams Richardson's ground squirrels Voles. 75 65 55 (/> 45 > t_ < « i_ Ql <4_ ?5 o V) E TO i_ 15C9 I I I // I I I I I I I I I I II 1975 1985 1990 1995 Figure 4. Prey items in nests of Swainson's Hawks on the Hanna study area. Shaded regions span the "moder- ate" and low ground squirrel years (see text), and represent 95% confidence intervals around the respec- tive means. Brood size departed
. The Canadian field-naturalist. Natural history. 264 The Canadian Field-Naturalist Vol. 115 -i â «-» > â L. (1) Q. tfi £ ⢠CD 4-» O.'l- >> 0 Grams Richardson's ground squirrels Voles. 75 65 55 (/> 45 > t_ < « i_ Ql <4_ ?5 o V) E TO i_ 15C9 I I I // I I I I I I I I I I II 1975 1985 1990 1995 Figure 4. Prey items in nests of Swainson's Hawks on the Hanna study area. Shaded regions span the "moder- ate" and low ground squirrel years (see text), and represent 95% confidence intervals around the respec- tive means. Brood size departed little from a aver- age during the first 12 years, 1975-1978 and 1983-1990 (Figure 6). During the following six years, average brood size declined by 31% to only The number of nests with 1 (n = 158), 2 (n = 215) and 3-4 young (n = 172) was significantly dif- ferent during 1975-1990, compared to 1991-1996 (101, 32, 10, respectively; G = , = 2, P < ). The proportion of nests with young raised to near fledging was 78% from 1975-1990, and 58% from 1991-1996 (G = , P (except 1984) through 1987. During four years, 1972, 1976, 1978, and 1979, nests with three young were more common than those with two young. The percentage of nests with three young was 28% through 1987 and then dropped to 11% from 1988 through 1996, while the percentage with one young increased from 29% to 52% in the same periods (Houston and Schmutz 1995b). Brood sizes declined fairly steadily (by 29%) from young per successful nest in the 1970s to young in the 1990s (Table 4; Spearman rho = , P < ; Figure 3). The least successful year was 1993, when the minimum nest failure rate between mid-June and late July was an all-time high of 61%, and the brood size was an all-time low of ( for the only eight success- ful nests near Alsask). Of 45 successful nests in 1993, adult hawks failed to appear during the band- ing visit at seven;
Size: 1949px × 1281px
Photo credit: © Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, booksubjectnaturalhistory, booky
