Montana blue heron survey depends on citizen science, Google Earth

Those who frequent the Kim Williams trail are probably familiar with the sight of great blue herons standing in the Clark Fork River, especially in winter. After the ice has scabbed over the river, you’ll occasionally see one sentineling a patch of shallow, open water – head cocked and still, eyes scanning for lunch, on stilt legs impervious to cold.

Woe to any fish that swims beneath a great blue heron.

Eagles may be regal, but great blue herons are elegant with slate gray plumage and a long slender black feather coif on the back of their heads to balance their long spiked beak. However, unlike eagles, whose populations are on the comeback, great blue heron populations may be on the decline. That’s what Montana Audubon and the Montana Natural Heritage Program are trying to confirm with a three-year citizen science survey.

This was the first year of the survey, which ran in May and June in spite of COVID-19, said Carmen Borchelt, Big Sky Watershed Corps worker with Montana Audubon.

“This was the pilot year and a successful one at that,” Borchelt told the Bitterroot Audubon chapter last week. “We had over 50 volunteers surveying over 30 locations across 12 watersheds throughout Montana.”

Montana Audubon decided to conduct the study because the great blue heron is a species of concern in the state, meaning it’s in a bit of trouble.

Great blue herons are considered colonial water birds because they nest in colonies or rookeries of five to 20 nests in the tops of trees, often cottonwoods or ponderosa pines, along the waterways where they fish.

The first record of great blue herons nesting west of the Continental Divide was recorded in 1931 along the Clark Fork River near Drummond and the first winter nesting was recorded in 1948 along the Bitterroot River near Stevensville.

So the birds have been around for a while, but some worry they may not stay around. Biologists and birders noticed that some rookeries were being abandoned.

Breeding Bird Surveys conducted between 1966 and 2010 show heron population declines of more than 2% a year.

Biologists aren’t sure what is causing the decline but suspect a number of factors, including the decline of riparian cottonwood stands, increased urban sprawl, human disturbance around rookeries, especially during the nesting season, changing conditions of waterways, logging and road construction, Borchelt said.

With the help of eBird, a University of Cornell database of bird sightings, Montana Audubon found 80 to 100 locations along Montana’s rivers that herons seem to favor, including hotspots on the Yellowstone and Bitterroot rivers.

But, of six rookeries recorded in the Bitterroot the past five or six decades, none were found to be active during this year’s pilot survey.

“Although that is disheartening to hear - that those rookeries were abandoned - this is actually the up-to-date information that we are looking for when doing this survey,” Borchelt said.

The good news is the 2020 pilot survey did find 161 herons and active 278 nests in 23 of the 50 locations they visited. The other 27 rookeries were found to be abandoned, Borchelt said.

When disturbed, herons will abandon rookeries. Sometimes, they move to other more remote locations away from humans, but that makes it harder for biologists to survey them. So are they really declining? Or have they just moved and is there a way to find them?

That’s where wildlife technician Bo Crees of the Montana Natural Heritage Program comes in.

“For most of the year, blue herons are solitary,” Crees said. “Because herons are so big and charismatic, they’re very visible around cities and towns. But in rural parts of Montana, they forage in habitats that are not well represented in structured surveys. That’s why it can be helpful to know where rookeries are in the state.”

Crees and Scott Bloom, another Heritage Program employee, found they could see large blue heron nests in photos from aerial surveys. Then they took it one step further and found they could use the photos from GoogleEarth.

Crees figured out he could see the white circles of nests by choosing an elevation of about 1,000 feet above the ground and then he’d run through the photo grid using a serpentine pattern so he wouldn’t miss anything.

The only drawback is some of the grid images are more blurry than the surrounding ones, but he can chose photos from different years to see if the nests are there. He can also see if an area has been disturbed over time.

“By switching back and forth between years, you can see if the colony is growing, is shrinking or just disappears,” Crees said. “That makes it more effective than a one-time survey.”

This was beneficial discovery because surveys conducted on the ground are time-consuming, expensive and can be limited when rookeries are on private property. On the other hand, aerial surveys are even more expensive and difficult to set up.

But now, Crees can sit at his desk and fly over streams in Google Earth images to find rookeries for citizen scientists to verify. It took him just three days to look for rookeries along five rivers, and he not only found 94% of the rookeries that are documented, but he also found 8 new ones. He estimates he could survey the entire state in less than a month.

“One of the reasons we want to send people out there is to ground-truth the surveys and make sure rookeries we couldn’t find aren’t being overlooked,” Brees said. “Depending on our findings during the upcoming volunteer surveys, it may be worthwhile to continue mapping rookeries on all major waterways in Montana to establish a rookery location baseline to help us determine population trends.”

Borchelt said Montana Audubon would be looking for more volunteers and will offer training next spring to do more surveys across the state.

“Coming this year, we are definitely looking at new sites and updating the old ones,” Borchelt said.

Contact reporter Laura Lundquist at lundquist@missoulacurrent.com.