. The Canadian field-naturalist. Natural history. 2001 Schmutz, Houston, and Barry: Decline Among Swainson's Hawks 265 100-1 90- 80- 70 >, 6(H Q. V4_ 50" O â M £ 40- V CD °- 30 20- 10- 0J Richardson's ground squirrel Other species. 1970 1975 1980 1985 1990 1995 Figure 5. The proportion of Richardson's Ground Squirrels in relation to other vertebrate prey in Saskatchewan. The shaded region represents means (50% and 32%) and standard deviation (17 and 11) for two periods of moderate and low ground squirrel use, respectively. maintained, produced strikingly different results from the mo
. The Canadian field-naturalist. Natural history. 2001 Schmutz, Houston, and Barry: Decline Among Swainson's Hawks 265 100-1 90- 80- 70 >, 6(H Q. V4_ 50" O â M £ 40- V CD °- 30 20- 10- 0J Richardson's ground squirrel Other species. 1970 1975 1980 1985 1990 1995 Figure 5. The proportion of Richardson's Ground Squirrels in relation to other vertebrate prey in Saskatchewan. The shaded region represents means (50% and 32%) and standard deviation (17 and 11) for two periods of moderate and low ground squirrel use, respectively. maintained, produced strikingly different results from the more open grassland areas mentioned above. Near Saskatoon, from 1964 through 1987, Swainson's Hawk brood size averaged young per successful nest (154 young banded in 67 nests). Brood size dropped slightly to (96 young in 52 nests) from 1988 to 1992, then returned to (29 young in 13 nests) from 1993 to 1996. Ground Squirrel abundance Alberta (Hanna): A direct measure of ground squirrel abundance using mark-recapture was avail- able only for two years. On six 4-ha plots and sur- rounding areas, 1497 ground squirrels were individu- ally marked. Densities on these selected well-drained study plots ranged from to squirrels per ha in 1975, and to in 1976 (Schmutz et al. 1979). The shift in the 1990s away from ground squirrel prey (Figure 4) was independently supported by a second index of ground squirrel abundance, , counts of used burrows on study plots (Schmutz et al. 1979). In June and July 1975, 23 1-km transects, on which used burrows were counted within 1 meter of a slowly driven motorcycle, yielded on average burrows/km. On 25 July 1996, a repeat single 1-km transect on each of the six plots yielded no used burrows, and no ground squirrels; although diminished and locally variable in numbers, ground squirrels were still present sparingly elsewhere on the study area during the 1990s. Jones (1993) also documented the extreme paucity of Ric
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