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How West Nile Virus Impacts Grouse Populations

Updated: Jun 6, 2023

A strutting Ruffed Grouse

Note: Although I wrote this piece almost three years ago, the research data and information are still very relevant. As you know, hunter flush rates and brood observations in Pennsylvania have continued to plummet, and so the late season/post-Christmas grouse hunting season has been canceled the past couple of years. This webinar also points out that a key component of battling WNV is quality habitat management, which is something that I addressed in my last post, For Grouse, It Starts with Habitat. So in a way, this post is "Part 2" of what we need to do to ensure huntable grouse populations for future generations.

Here's the original version of the article based solely on information provided in the PGC webinar:

On February 15, 2018, the Pennsylvania Game Commission (PGC) hosted a free webinar that discussed the effects of West Nile Virus (WNV) on ruffed grouse populations. In short, WNV is acting as a suppressor of grouse populations, but research also suggests populations might be able to overcome WNV in the future.

Common to Africa, Europe, the Middle East, and West Asia, WNV first made its appearance in this country in New York in 1999 and has quickly spread throughout North America. Carried by mosquitoes, WNV effects over 250 species of wildlife, although it’s primarily known as a bird disease. There are at least 176 species of mosquitoes in the United States, but according to PGC Game Birds Biologist Lisa Williams, the species Culex restuans is most responsible for grouse declines.

When the PGC, in conjunction with the Department of Environmental Protection (DEP), began sampling mosquitoes commonly found in prime grouse brood habitat (low-lying, moist bottomlands), Culex restuans was discovered in every location and made up 25% of the mosquito population. “When these sites started going positive for West Nile in July and August, it was always this species that tested positive,” said Williams.

Looking at abundance data accumulated over the past 50 years by grouse hunters and cooperators, “you can see a sort of tipping point that occurred in the early 2000s where we had dramatic declines followed by a pretty weak recovery, and then continued dramatic declines,” said Williams in the webinar. “We have these record lows kind of accumulating at this point. The most worrisome thing about this is that we know this data is coming from good habitat. Populations aren’t recovering even when their circumstances are good.”

The PGC has also run a brood survey since 1981. Prior to the arrival of WNV, broods that tracked well in June typically tracked well in July, too. In the early 2000s, that began to shift. Although brood production has been relatively good in June since 2002, brood survival rates have plummeted, with 2016 producing a record low survival rate.

An immature Ruffed Grouse
Is West Nile Virus responsible for the decline of nesting success among Ruffed Grouse?

In 2015, the PGC initiated a WNV Challenge Study aimed at finding out to what extent WNV was responsible for these declines. They began collecting eggs from wild bird nests and transporting them to a propagator in Idaho who had a quarantine facility. When the chicks hatched and reached four to five weeks of age, they were then shipped to Colorado State University, home of one of the premiere WNV labs in the country. Once acclimated to their new surroundings, the chicks were then inoculated with WNV.

“Forty percent of the chicks died within the first week,” said Williams. “Another 40-50% of chicks lived through that second week, but when we opened those chicks up, they had organ damage to pretty much all of their major systems.”

Williams quoted a recent study by Nemeth et al 2017, published in the Journal of Veterinary Pathology, that found that, based on clinical and pathological responses, mortality of ruffed grouse may be as high as 90%. As Williams noted, though, these were statistics based on birds infected in a lab. The PGC’s goal was to find out how wild bird populations were faring. Once again, they turned to grouse hunters and cooperators for data collection. Hunters were asked to provide the PGC with a feather set and a filtered paper strip soaked in blood from harvested birds.

“We were looking, specifically, for antibodies to West Nile Virus,” said Williams. “We sampled over a two-year period. We got more than 400 blood samples, and the average over that two years was that 19% of the harvest had antibodies to West Nile Virus. This means that those birds were exposed to the disease but had survived. The lab folks tell me this also means that they’re likely immune, and likely immune for life.”

A closer look at the actual numbers, though, is even more disheartening. During 2015 and 2016, of approximately 51,000 harvested grouse, roughly 10,000 were WNV survivors. Taking in account a 30-50% survival rate (a very generous number compared to lab studies), that means that 10,000-30,000 (or more) grouse died prior to hunting season, according to Williams.

Unfortunately, the summer of 2017 was the worst on record for WNV, so the short-term outlook for grouse is pretty grim. However, the PGC study suggests that survival rates are directly related to good habitat.

“If we really want to maximize benefits to grouse, certainly abundance of habitat is key,” said Williams. “The National Grouse Plan and the Pennsylvania Grouse Plan call for about 15% of our landscape to be in young forest. Currently, we’re at 7%, and I think that’s about standard for Mid-Atlantic states. By my calculations, that means we’re about 800,000 acres short in Pennsylvania, not just on Game Lands, but across the state and across different types of ownerships.”

Creating good brood habitat presents a conundrum. Yes, grouse need those low-lying, moist bottomlands to successfully raise broods, but that same habitat is also a breeding grounds for WNV-carrying mosquitoes.

Ruffed Grouse tracks in the snow
Tracks of a Ruffed Grouse in the snow.

“Working with DEP, they gave us 800,000 data points for where Culex restuans occurred in Pennsylvania,” said Williams. Preliminary findings indicate there may thresholds and barriers that suppress this species of mosquito.

One of those thresholds occurs at the 1,200-foot level of elevation. This information will help guide the PGC in selecting habitat improvement sites that are less likely to be contaminated with the disease.

“We also know, unfortunately, that West Nile Virus mortality occurs before the hunting seasons,” said Williams. “So, it’s reducing that population surplus, that summer surplus, but we don’t know by how much.”

Tracking the effects of WNV during those summer months, though, will assist the PGC in determining the length of hunting season. “The grouse population is changing, and we’re going to change with it,” said Williams. “I’ve developed a Responsive Harvest Framework for setting future seasons.”

This framework takes into account decades of abundance data from hunter diaries as well as grouse production data from summer sightings, with almost 20 years of disease surveillance by the DEP. Generally speaking, high WNV activity and low grouse productivity will result in shorter grouse seasons. Low WNV activity and high grouse productivity will result in more liberal seasons. It’s a consistent and transparent system that informs hunters as to how and why decisions about hunting seasons are being made.

In truth, there is still much to be learned about WNV. But, as Williams noted, “These strong grouse data sets and this growing collaboration with DEP, the Human Health Surveillance Program, are assets that few other states have available. So, if any state is going to try to turn this around for ruffed grouse, it makes sense for Pennsylvania to try.”

Anyone interested in viewing the entire webinar can do so for free on the PGC’s YouTube Channel.

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