. The Canadian field-naturalist. Natural history. 266 The Canadian Field-Naturalist Vol. 115 ,— 1975. "Clutch" size Brood size -. *A 1 l ' 1995 1985 1990 Figure 6. Number of young at banding (Brood) and eggs in late hatching nests ("Clutch") of Swainson's Hawks for the Hanna study area. Numerals represent number of nests involved. Shaded regions represent the 95% confidence interval around the respective means. The mean brood size and standard error for peri- ods prior to 1990, after 1990 and for all years were ± , ± and ± ,
. The Canadian field-naturalist. Natural history. 266 The Canadian Field-Naturalist Vol. 115 ,— 1975. "Clutch" size Brood size -. *A 1 l ' 1995 1985 1990 Figure 6. Number of young at banding (Brood) and eggs in late hatching nests ("Clutch") of Swainson's Hawks for the Hanna study area. Numerals represent number of nests involved. Shaded regions represent the 95% confidence interval around the respective means. The mean brood size and standard error for peri- ods prior to 1990, after 1990 and for all years were ± , ± and ± , respectively. 1995, travels of > 300 km each day by CSH recorded, sequentially, 3, 9, 1, 29, 11, and 10 ground squirrels in June, and 2, 10, 3, 1, 1, 14, 2, 0 and 0 ground squirrels in July. From 1992 through 1996, few Richardson's Ground Squirrels were encountered as prey in hawk nests, none at all in 1995. Discussion Dispersal Our study areas included grassland and cropland interspersed with permanent or semi-permanent wet- lands in mixed prairie. Both the Hanna, Alberta, and Saskatchewan study areas lie about 200 km south of the northern edge of a large region of western North America occupied by Swainson's Hawks. Some colour-marked breeders at Hanna were recruited locally, but others moved onto the Hanna area from localities outside it, including five from the Kindersley area of Saskatchewan (Houston and Schmutz 1995a). Natal dispersal of Swainson's Hawks across the Canadian prairies, with no moun- tain ranges or large bodies of water to serve as a bar- rier, was apparently sufficient that no subdivisions could be detected using DNA fingerprints from Swainson's Hawks in 85 nests from Manitoba to Alberta (Portman 1997). It appears, therefore, that our hawks were part of an interacting metapopula- tion semi-isolated only through the species' charac- teristic natal and breeding site fidelity. We suspect that the Saskatchewan and Alberta (Hanna) study areas were a sourc
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