Two study sites were selected for intensive investigations of the effects of military overflight activity on Dall’s sheep.  One (Cirque Lake) is located in Yukon-Charley Rivers National Preserve and is overlain by airspace mitigated for Dall’s sheep.  Military aircraft flying over this area are restricted to flying 5,000 feet above ground level from May 10 - June 15 (lambing season).  Because the floor of this military airspace is 100 feet above ground level at other times of the year, sheep in this area may be exposed to a significant change in flying activity during the 1 month mitigation period.  The 2^nd site (West Point) is located approximately 35 miles to the west of the 1^st site and has no associated mitigation measure for Dall’s sheep. 

There were no statistical differences in the pregnancy rates, lamb to ewe ratios, or yearling to ewe ratios between Cirque Lake (mitigated) and West Point (non-mitigated).  There was a statistical difference in the mean body weights of captured ewes at the 2 study sites with ewes at West Point heavier than those at Cirque Lake.  In general, there was greater variation in pregnancy rates, lamb to ewe ratio, yearling to ewe ratio, and body weights among years than there was between study sites.  Survival rates of adult ewes were similar between the 2 study sites.

Field crews observed Dall’s sheep behavior, overflight activity and collected information on sound levels during 4 field sessions per year during 2000 and 2001.  Military overflight activity was extremely variable among years, seasons, and between study areas.  There was no difference in the number or frequency of overflights observed between the mitigated study site and the non-mitigated site.  There was no difference in the number of observed overflight before and after Cope Thunder excercises in comparison to during Cope Thunder excercises.  Loud and low overflight events were rare at both study sites.  The number of overflights we observed over Dall’s sheep on a given day did not influence Dall’s sheep behavior.  Significant differences occurred in the feeding efficiency of Dall’s sheep when behavior was examined in the 10 minutes before, during and 10 minutes after an overflight event.  In models that included all seasons, Dall’s sheep ewe feeding efficiency was higher after the overflight event in comparison to before the overflight event.  In models that just examined lambing season and early summer (May, June and July), sheep feeding efficiency was higher before the overflight event in comparison to during the event.  In models that considered the proximity and sound level of overflight events, a higher proportion of sheep were active with increasing sound levels.   In models that just examined lambing season and early summer, more sheep were active with closer military overflights.  In all models, factors other than military overflights, explained greater proportions of the variation in sheep behavior.  In particular, the date sequence (time of year of the field observation) nested within year and study site explained greater proportions of the variability that we observed in Dall’s sheep behavior than did military overflight activity.

We observed no significant effect of the number of military aircraft sorties launched on 2-week within-home-range scale: 1) total distance moved; 2) home range size; or 3) habitat use (proportional use of landcover classes and aspect classes, average elevation, slope, terrain ruggedness, and relative vegetation biomass.  However, there was substantial variance in total distance moved, home range size and habitat use between study areas, among years within study areas, and among 2-week sequential periods within years within study areas.  Once this natural variance was accounted for, the number of military aircraft sorties added no further explanatory power to the assessment of variance in sheep behavior and habitat use.

We conclude that the levels of military activity which we observed in this study during Major Flying Exercises (~60 sorties/day) generally did not cause significant effects on sheep behavior and habitat use at the home range scale when compared to the background level of military sorties (~ 20 sorties/day).  When statistical differences in feeding efficiency and activity of groups of sheep were observed, military aircraft activity typically accounted for a small proportion of the total variance in sheep behavior, the direction of these effects were variable for sheep, and there were no overt indications that military overflights affected sheep populations during 1999-2002.  Few of the overflights that we observed were low or loud and the mitigation measures in place at the Cirque Lake study site provided no detectable reduction in sheep response to military overflights.  We emphasize, however, that 1) the studied sheep population has had opportunity to acclimate to military overflights for over 20 years, 2) we made no comparisons to areas that were free of military overflights and thus cannot draw conclusions regarding the effect of the background level of military overflights on sheep behavior and populations, and 3) if the nature, intensity, or frequency of military flights in interior Alaska changes substantially compared to the situation described here, then sheep may respond differently than documented in this study.