What exactly does a drumming survey tell us…or not tell us?
There’s something special about that feeling you get in the grouse woods in springtime – that moment when you pause and can’t quite hear anything, but actually feel something thumping in your chest (besides your heart). Finally it becomes clear that it’s actually a grouse drumming from a hundred yards away. Drumming is done by male grouse as they beat their wings from an elevated log, stump, rock, or other platform. Contrary to what many people believe, they’re not hitting their wings against their breast – it’s actually caused by the vacuum of air left behind from their rapid wingbeats. Scientists use this behavior to conduct drumming surveys each spring.
Most hunters keep tabs on drumming surveys when they come out each summer, and get excited (or dismayed) about the results. The surveys have been used reliably for several decades. But in recent history, there seems to be somewhat of a disconnect between spring drumming counts and fall harvest numbers/observations. So what is their purpose and what can we still learn from them?
Drumming Survey Background
To conduct drumming surveys, biologists typically go out from late March to mid-May depending on which state they are in. The exact survey dates are determined from multiple and very site-specific factors, including wind speed thresholds, optimal temperatures, and precipitation/weather considerations. Surveyors stop at roadside points along preplanned routes (or transects) to listen for grouse drumming – for example, Minnesota routes have 10 listening stops that are approximately 1 mile apart. Drumming surveys use the same routes year after year to assure that an index is reliable and unbiased. Many people wonder why they use the same spots repeatedly instead of going where the grouse are. However, biologists can’t survey great habitat areas alone because it would inflate the estimates. Also, constantly switching locations wouldn’t reveal any meaningful trends.
After arriving at a survey point, the surveyor typically waits for a calm-down period to let the birds relax and return to their normal activities (Dougherty 2008). Then the surveyor listens for 4 to 5 minutes (again, highly specific methods that vary by state) and records the number of grouse drums heard. Back at the office, they will calculate the average drums per stop from these observations. For perspective, an average of 2 drums per stop generally corresponds to a peak in the approximately 10-year grouse cycle.
What Does a Drumming Survey Tell Us?
So what does all of that actually tell us about ruffed grouse? Drumming surveys essentially help us estimate the density of drumming male grouse on a repeated route over time. But it’s critical to remember that a drumming survey is an index – it is not a census. An index only indicates the relative size of a population and can reveal population trends (i.e., whether a population is increasing, decreasing, etc.), but it does not provide actual estimates of the total number of birds (e.g., non-drumming males, hens, chicks, etc.). Biologists use these indirect drumming surveys because it’s just not feasible or possible to go count every individual bird in the forest.
To that point, probably the most useful aspect of a drumming survey is the broad trends that it can reveal. When tracked every year and combined with hunter harvest statistics in the fall, spring drumming surveys provide a general trend of the grouse population to track their 10-year cycle, especially when surveys are timed during the drumming peak (Jones et al. 2005). Ted Dick, the Minnesota Forest Game Bird Coordinator for the Department of Natural Resources, noted that “for the first 50 years of the drumming survey, the fall hunter harvest correlated more closely with the rise and fall of the drumming surveys in their ten-year swings. Now over the past 20 years, the high correlation seems to be decreasing.”
What Do These Surveys Not Tell Us?
This is the part that can cause confusion and frustration for hunters. After seeing high spring drumming counts, one would expect autumn to be a great hunting season. But drumming surveys do not inform us about how many hens and chicks are around in the spring or how many birds survive throughout the summer. Nesting success and brood survival is much more important for fall bird numbers, and drumming counts cannot inform that.
The 2017 Minnesota grouse season is a great example of this. The spring drumming counts were very high that year with an average of 2.1 drums per stop statewide, which was 57% higher than the 2016 counts and certainly at the crest of their cycle (MDNR 2017). But when the hunting season arrived, grouse hunters were hard pressed to flush birds. Biologists were puzzled and began looking for answers. Very likely, the chicks and juvenile birds didn’t survive well over the summer to make it to the fall hunting season. We asked Ted for input on this observation. He mentioned, “We have said for years that we would like to develop a better gauge of summer brood survival and our DNR Forest Wildlife Research Unit is in the early stages of evaluating methods.”
Other Factors Affecting Grouse Survival
It certainly seems that there are other factors in play today that we need to learn more about. Ted also noted, “Adult grouse seem to survive in sufficient numbers that the drumming surveys still exhibit some of their cyclical nature, but then fall harvests seem to be depressed recently – has some environmental factor changed, thus lowering survival?”
If drumming surveys only reveal broad trends as an index, what other issues could be affecting grouse survival that we might be able to track or influence?
As usual, habitat is likely number one on the list. Without adequate nesting and brood cover (e.g., dense young forests), predators can negatively affect nest success and brood survival (Dougherty 2008). Ted commented, “We seem to have an abundance of appropriate habitat on state-managed lands, but what about other ownerships?” In other words, there’s a lot of privately owned land that isn’t managed for similar objectives as public lands.
How many average landowners are working specifically to improve grouse nesting habitat on their properties? What if more private landowners participated in forestry projects (e.g., timber harvests, planting fruiting shrubs, etc.) on their own parcels? Some have theorized that the increase and enhancement of habitat on the landscape level could help improve the survival of grouse throughout the year. Given the importance of quality habitat, it definitely seems reasonable to me.
Typically when the fall harvest survey data or hunter opinions disagree with expectations from the spring drumming counts, many blame the spring and summer weather. Cold and/or rainy conditions during the brood-rearing season can have negative effects on some bird species. Young birds likely don’t have the ability to regulate their body temperatures well when they are small, so cold or wet weather could be lethal. A predicted effect of climate change is an increase in precipitation, so it’s likely that this potential threat could continue for many bird species. Habitat improvements may provide additional places for chicks to escape the weather, but that’s not a guarantee.
Additionally, ruffed grouse have evolved to survive harsh winters through an interesting adaptation called snow roosting. Essentially, they bury themselves in a deep snowdrift to insulate their bodies from the cold. But if winters continue trending to be warmer overall with less snow cover (which is likely given climate change), that would make snow roosting more difficult for grouse and could increase stress on the population (Shipley et al. 2019).
West Nile Virus
Another potential factor affecting survival could be disease-related. West Nile Virus, in particular, is spread by mosquitoes during the summer and is deadly to many bird species. With additional stressors, such as insufficient/poor quality habitat and changing weather conditions, the disease could be more devastating, as noted in Pennsylvania. Many states are now asking hunters to submit blood, heart tissue, and feather samples from grouse they shoot so the presence of West Nile Virus can be detected and tracked more closely.
To summarize, grouse drumming surveys are still very useful for resource managers as they help track general population trends over time. However, they have their purpose and limits. It seems that in today’s woodlands, other factors are affecting grouse survival that are more difficult to identify. Ted mentioned that even though Minnesota has some of the best public land hunting opportunities and ruffed grouse habitat, “it appears that nationwide we’ve been in some long-term harvest and hunter satisfaction declines even though the long-term drumming indices don’t show a significant negative trend statewide.” More research is needed to identify those issues.
Dougherty, Eric M. 2008. Ruffed Grouse Drumming Counts: An Examination of Observer and Roadside Effects. School of Environment and Natural Resources, Ohio State University.
Jones, Benjamin C., Craig A. Harper, David A. Buehler, and Gordon S. Warburton. 2005. Use of Spring Drumming Counts to Index Ruffed Grouse Populations in the Southern Appalachians. Proc. Annu. Conf. Southeast. Assoc. Fish and Wildl. Agencies 59:135–143.
Minnesota Department of Natural Resources (MDNR). 2017. 2017 Grouse Survey Report. Available online at: https://www.dnr.state.mn.us/hunting/grouse/reports.htmlShipley, A.A., M.J. Sheriff, J.N. Pauli, and B. Zuckerberg. 2019. Snow roosting reduces temperature-associated stress in a wintering bird. Oecologia. 2019 Jun; 190(2):309-321.