. The Canadian field-naturalist. Natural history. 258 The Canadian Field-Naturalist Vol. 115. ? 41 km2 Study Plots Native Vegetation Remaining 40 km Less than 10% 10% to 50% ] More than 50% Figure 1. Percent of native vegetation in the two study areas. Boundaries of the completely searched Hanna study area are within a larger study perimeter. Of the 78-83 randomly selected study plots used, 30 lay within the region shown. The Saskatchewan area was searched opportunistically. Alberta: The town of Hanna (51° 38'N, 112° 33'W) is near the northwest corner of the study area, which encompassed 480 k
. The Canadian field-naturalist. Natural history. 258 The Canadian Field-Naturalist Vol. 115. ? 41 km2 Study Plots Native Vegetation Remaining 40 km Less than 10% 10% to 50% ] More than 50% Figure 1. Percent of native vegetation in the two study areas. Boundaries of the completely searched Hanna study area are within a larger study perimeter. Of the 78-83 randomly selected study plots used, 30 lay within the region shown. The Saskatchewan area was searched opportunistically. Alberta: The town of Hanna (51° 38'N, 112° 33'W) is near the northwest corner of the study area, which encompassed 480 km2 in all years except 1975 (southern 335-km2 portion) and 1983 (southern 326-km2 portion). This ranching area, with approxi- mately 85% mixed grass remaining, was monitored intensively from 1975 through 1978 and from 1983 through 1996. During summers (May-August) in the mixed grass ecoregion of Alberta, the mean tempera- ture overall was °C (range °C). Average summer rainfall was cm ( = 25% & 75% percentiles; Strong and Leggat 1992). Here, at the southern edge of the southward-advancing aspen parkland, expansion of trees has been noticeable since the mid-1970s. Some Swainson's Hawks nest- ed in P. tremuloides that grew during this study in previously treeless sites. Saskatchewan: Kindersley (51° 28'N, 109° 09'W) is the largest town in the study area, and Alsask (51° 21'N, 109° 59' W), is at its western margin (Figure 1). This area was sampled mainly from 1972 through 1996. Near Kindersley, the land has been extensively altered for cereal crop production (< 10% native habitat), whereas Alsask was intermediate (10-50% native). On the large pastures near Kindersley, trees remaining in deserted farmstead shelterbelts contin- ue to die out. Mean July temperatures were °C and annual rainfall was cm (Padbury and Acton 1994). Methods In Alberta, a complete search for nests was carried out each year on the Hanna study area, and thrice o
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