Sagebrush Soundscapes and the Effects of Gas-Field Sounds on Greater Sage-Grouse

Greater Sage-Grouse (Centrocercus urophasianus) use elaborate acoustic and visual displays to attract and select mates, and females and chicks depend on acoustic communication during brood rearing. A potential threat to the grouse is sounds associated with human activity. During April, 2013–2020, we collected 17,825 hours of acoustic data in three different acoustic situations in the sagebrush of Wyoming: rural, undeveloped areas (6), at Greater Sage-Grouse leks in a natural-gas field (20), and near active machinery in that gas field (17). The average existing sound levels in undeveloped sagebrush areas were LAeq = 26 dB and LA50 = 20 dB, and the average background sound level was LA90 = 14 dB. These values are lower than previously reported, due in part to our use of more sensitive equipment as well as addressing the influence of the instruments’ electronic self-noise. LAeq and LA50 at leks in the gas field ranged from 25.5 to 33.7 dB and 20.5 to 31.3 dB, respectively, depending on the distance, number, and type of nearby activities. Sound levels at leks were correlated with trends in the number of grouse using the lek: the higher the sound level, the greater the likelihood of a decline. Thresholds above which declines occurred were LAeq = 31 dB and LA50 = 26 dB. Leks with LAeq > 31 dB and LA50 >26 dB, 100% and 91%, respectively, had declining trends. Our findings suggest that the current policy of limiting sound levels at leks to LA50 < 10 dB (or LAeq < 15 dB) over the background sound level is appropriate, if an accurate background level is used.

Rates of Ovulation and Reproductive Success Estimated from Hunter-Harvested Greater Sage-Grouse in Colorado

Numerous studies provide estimates of nesting propensity rates (proportion of females attempting to nest at least once in a given year) for greater sage-grouse Centrocercus urophasianus. However, females may initiate nests without being detected during the course of normal research, leading to negatively biased estimates. We evaluated nesting propensity rates (rate of females laying ≥1 egg/y) by examining ovaries from 941 female sage-grouse collected at hunter-check stations in North Park, Colorado, during 1975–1984. Mean rate estimates of nesting propensity were lower for yearlings (0.926, 95% CI = 0.895–0.948) than adults (0.964, 95% CI = 0.945–0.978). We did not attempt to estimate laying rates (number of eggs laid per year) because they were likely unreliable. Nesting success—estimated as the probability of females producing a successful clutch in a given year based on primary feather replacement from hunter-harvested wings—was lower for yearlings (0.398, 95% CI = 0.370–0.427) than adults (0.571, 95% CI = 0.546–0.596). There were more chicks per female produced when nesting propensity rates were high, indicating nesting propensity rates correlate with the number of juveniles in the autumn population. Both nesting propensity rates and nesting success were positively related to precipitation during the lekking and brood-rearing seasons, respectively. Nesting propensity rates were positively related to spring abundance (as measured from annual lek counts), but nesting success was unrelated to spring abundance. A range-wide estimate of an unadjusted, apparent nesting propensity rate available from a previous study was approximately 7% lower than the North Park population. Postovulatory follicles provide a direct source of information on nesting propensity rates estimated from hunter-harvested sage-grouse. These estimated rates may prove useful to gain insights into annual variation of hunted populations' reproductive efforts.