Annual and seasonal fawn mortality rates (Table 5.5) were 

 equally high during periods of low (1975-1977) and high (1983- 

 1985) coyote populations (Fig. 5.8). Fawn mortality rates 

 were not linearly related to coyote population level, as 

 determined by the siren answer survey for annual or seasonal 

 periods (all r < 0.33, P > 0.05). The small variation in the 

 number of adult coyotes among years also made it unlikely that 

 fawn mortality should vary with numbers of coyotes. 



Although there was a tendency for fawn mortality to be 

 lower when alternate prey populations were highest (Fig. 

 5.13), the relationship was not statistically significant. 

 Fawn survival to autumn and winter was highest during 

 microtine irruptions and lowest during the periods between 

 microtine irruptions (Fig. 5.13). Use of average numbers of 

 microtines during summer as an index may have obscured the 

 relationship. Trapping in mid-summer and early autumn and 

 general observations during 1978 indicated that microtine 

 populations did not reach high levels until late summer, after 

 almost all mortality of fawns that summer had occurred (Hamlin 

 et al . 1984). Although more microtines than usual were 

 present during 1982, highest numbers occurred from autumn 1982 

 through early summer 1983. Thus, most mortality of the 1983 

 cohort occurred after microtine populations had declined. 

 Similarly, white-tailed jackrabbit populations peaked during 

 summer 1983 and had "crashed" by November. 



Population levels of microtine rodents were probably 

 influenced by vegetation production. All 3 irruptions were 

 concurrent with much greater than average production of 

 grasses and f orbs . A certain vegetation cover threshold may 

 need to be met (Birney et al . 1976) before microtine 

 population growth can take place. 



Alternate prey populations, especially microtine rodents, 

 influenced the degree of predation by coyotes on mule deer 

 fawns. Annual fawn survival was lowest for the 1976, 1977, 

 1983, 1984, and 1985 cohorts (Fig. 5.13). All prey 

 populations were at lows during those years, except 1983 when 

 survival to early winter was relatively good for the 1983 fawn 

 cohort, but overwinter survival was poor. Severe declines in 

 both microtine rodent and jackrabbit populations had begun by 

 autumn 1983. Consequently, much of the mortality of the 1983 

 fawn cohort occurred after alternate prey populations had 

 declined. 



Fawn survival increased substantially for the 1978 and 

 1979 cohorts, over that for 1976 and 1977 cohorts and 

 coincided with both excellent forage conditions and a 

 microtine rodent irruption. Populations of other alternate 

 prey species were relatively low during 1978 and 1979. 



144 



