Maximizing the detection probabilities of dusky grouse for population monitoring

Despite its status as a game species in the western United States, rigorous monitoring of dusky grouse (Dendragapus obscurus) populations is limited. Obtaining an adequate number of observations for effective population monitoring of dusky grouse is challenging due to difficult-to-reach montane habitats, cryptic behaviors, and limited personnel, time, and funds at conservation agencies. While habitat accessibility and survey effort constraints may be outside a manager’s control, identifying the most effective protocols to better detect a target species is possible and will improve monitoring programs. Our objectives were to 1) evaluate field-based protocols for maximizing dusky grouse detections and 2) identify survey conditions and timing that maximize the probability of detecting dusky grouse in western Montana. We conducted replicated point-count surveys with and without electronic playback of grouse calls in spring and summer 2019, and spring surveys solely with electronic playback in 2020–2021. We used N-mixture models to compare the probability of detecting dusky grouse in spring versus summer sampling and evaluate the effects of electronic playback of grouse calls, route type (off-trail, trail, and road), weather, background noise, day, and time on the probability of detection. Spring sampling yielded substantially more dusky grouse detections than summer sampling. The use of electronic playback of female sooty grouse calls in the spring increased detection probability, but dusky grouse chick distress calls during the summer did not improve detection rates. The probability of detecting a dusky grouse was similar across the different route types (off-trail, trail, and road), and maximized on clear days, with little wind and background noise, with surveys occurring 9–162 minutes post-sunrise during 3–23 May. By examining the effects of different factors on dusky grouse probability of detection, our results provide necessary information for the development of effective protocols for field surveys for population monitoring of dusky grouse.

Methods
Data consists of point-count surveys conducted in western Montana, off-trail in spring and summer 2019, and on roads and trails in springs 2020 and 2021.  During the 2019 season, we surveyed each point three times on three different mornings within a 2-week period, and during the 2020 and 2021 seasons, 4 point-counts were conducted within the same morning.

In 2019, surveys consisted of two 4-minute consecutive point-counts, the first without electronic playback, and the second with electronic playback played used a portable speaker. During the spring, we used electronic playback of a female sooty grouse call (cantus, whinny, and cackle) and in the summer we used a dusky grouse chick distress call. Playback recordings consisted of 30 seconds of playback, then 30 seconds of silence, repeated for the entire 4-minute period.

In 2020 and 2021, surveys were only conducted using electronic playback of a female dusky grouse call (cantus and cackle).

During all years of the study, all dusky grouse observations (visual and auditory) were recorded, with observers trained to not record the same individual twice within a single survey. Point-count surveys were conducted between dawn and 11am on days without precipitation and minimum (< 19 km/hr) wind speed. We recorded survey conditions, which included day (relative to the first day of the survey period, which was 10 April), minutes since sunrise, background noise level, temperature (C), precipitation (fog, rain, snow, or none), wind speed (km/hr), and cloud cover (0-15%, 16-50%, 51-80%, 81-100%).