Biologists study prairie life one regurgitated lump at a time

SCIENCE by Gregory Beatty

Leanne Heisler, photo by Darrol Hofmeister


Owl pellets. Perhaps not the most repulsive thing to come out of an animal but, well, they still come out of animals and that’s a big strike against them.

Nevertheless, owl pellets are helping one Saskatchewan researcher put together a picture of some of the prairies’ most hard-to-study inhabitants.

Wait, let’s back up. What the hell are owl pellets?

Owls have powerful beaks but no teeth. When they catch prey (typically small rodents) they swallow it whole. But an owl’s digestive system isn’t as efficient as that of other birds of prey. Before the prey enters the stomach, indigestible material such as fur, bones and teeth gets separated out and compacted into a “pellet” that the owl regurgitates.

The academy will probably frown at our characterization as inaccurate and childish, but since that’s how we roll at Prairie Dog, too damn bad. Owl pellets are basically bird barf.

And if you’re a scientist, you can learn a lot from bird barf. Just ask Leanne Heisler.

Quest For Pellets

For 20 years, the Royal Saskatchewan Museum has collected owl pellets from across the Canadian prairies. This summer, as part of her PhD research, University of Regina science student Leanne Heisler spent several months in the field where she and assistant Amber Burnett gathered scores of new pellets to add to the collection.

Heisler is plenty knowledgeable about owls. But the focus of her research is the unfortunate critters whose remains are in the pellets.

“My PhD looks at the impacts of agriculture, and loss and fragmentation of habitat,” says Heisler. “With the conversion of native grassland [to farming and ranching] you see loss of habitat.”

Scientists sometimes use traps to measure animal populations, but tiny mammals such as mice and voles have such small ranges traps aren’t a viable study tool. “I think the average range is 200 to 500 metres,” says Heisler. “So when you use traps, you can get very different results depending on where you set them.”

But the predators that eat mice and voles? Much easier to study. And owls tell us about habitats every time they cough up a corpse.

“Great horned owls forage up to five kilometres from their roost, and the pellets get compiled at the roost site, so all the rodent populations in that range will be represented. That gives me a broader scale of what rodent species are in the area,” says Heisler.

In fact, Saskatchewan is home to around 10 owl species [see sidebar]. Heisler studies widespread species, like the aforementioned great horned owls. She also looks at owls inhabiting more limited ranges, such as burrowing owls. Their pellets can tell us a lot, too.

Burrowing owls are a much smaller species that inhabits grassland areas in the south. As an endangered species, they’re the focus of other researchers, such as at the Burrowing Owl Interpretive Centre in Moose Jaw.

But their pellets? Those are passed on to the RSM.

“Rodents are a pretty good indicator of how the prairie ecosystem has been affected by our activities — especially agriculture,” says Heisler. “It’s changed the vegetation that rodents rely on for food and shelter. When that happens, we suspect that leads to changes in the species. That could change the distribution of zoonotic diseases, such as hantavirus, that can be transferred from animals to humans.”

Oil and gas is another activity that’s impacted significantly on the prairie landscape — especially in the past two decades. Then there’s the longer-term effects of extreme weather and climate change to consider.

More years of pellet collecting will be needed to get enough data to discern firm trends in rodent populations. But by examining the remaining patches of native grassland in Saskatchewan, says Heisler, scientists hope to get a picture of rodents’ current numbers, ranges and habitats.

“We’re assuming those communities would be similar to what existed in the province before agriculture,” she says. “So once we make those comparisons we’ll be able to see how rodent distributions have changed.”

Roosts And Regurgitation

Abandoned barns and grain bins are popular roost sites for owls, and if they find a spot they like they’ll use it for multiple years, says Heisler.

“They’re mostly nocturnal and hunt between dusk and dawn,” she says. “Then they sit in a roost during the day. It usually takes 12 hours to cough up a pellet, and they’re ready to go out the next evening.”

And researchers who know where owls roost will be ready to collect these pellets.

Fresh owl pellets are shaped like grayish-white turds. Whenever Heisler and Burnett found a roosting owl, they’d scoop up the intact and degraded pellets. Back at the lab, they’re mixed with sodium hydroxide (similar to Drano) to dissolve the fur. Then Heisler extracts the skull, teeth and mandibles.

DNA analysis can be used to identify species, but it’s expensive. Fortunately, many rodents can be identified through visual cues — such as the shape of their teeth and lower jaw. A partial list of species Heisler’s found in owl pellets include deer mice, northern grasshopper mice, meadow and sage-brush voles, Richardson’s and Franklin’s ground squirrels, muskrats, northern pocket gophers and northern short-tailed shrews — not to mention the Ord’s kangaroo rat, which is an endangered species.

When grasslands habitat is fragmented by agriculture and other human activities, says Heisler, edge habitat increases.

“You might see an increase in predators moving along the habitat edges, or the growth of weeds, and that could affect rodent species in different ways,” Heisler says.

Some species, she says, are highly adaptable and might regard cultivated farmland and pastures as new habitat. If they’re “grassland specialists”, though, they might struggle to adapt and suffer a population decline.

Food, water, ground cover and nest/hibernation sites are some of the habitat requirements for rodents. As far as diet goes, Heisler says there’s a lot of variety.

“The northern grasshopper mouse is carnivorous — it will eat insects and other mice. Other rodent species are omnivorous, so they’ll eat insects, grains, vegetation, pretty much anything they can find.

“Some species are granivorous and focus on grains,” she says. “Then there are vole species that mostly eat vegetation.”

Farmers and ranchers might regard rodents as pests. But if rodent populations, as a whole, are in decline that would be bad news for a lot of other species (such as owls) that rely on them for food — and other ecological services.

Burrowing owls, for instance. They don’t actually burrow. Instead, they roost in abandoned burrows dug by ground squirrels.

Science Is Cool

The research Heisler and other scientists are doing will determine the health of different rodent populations, and if any changes are needed to our current farming and ranching practices to give rodents the habitat they need to survive and thrive as part of healthy ecosystems.

Heisler invites anyone who knows the location of great horned owls in Saskatchewan to help out with her research. “If you’d like to send the pellets to us, you could totally do that,” she says. “Just make sure you wear gloves, and if necessary, a mask, because of the dust associated with the pellets.”

So basically, if you’re hell-bent on collecting owl barf for science, take precautions. Still, it can be an interesting pastime.

“If you send them in, we can process them and send back the results,” says Heisler.

All About Owls

Owls are found on all continents outside of Antarctica and a few remote islands. Biologically, they belong to the order Strigiformes, which consists of about 200 species of nocturnal and generally solitary birds of prey.

Paleontologists have traced owls back to the Paleocene epoch around 60 million years ago. Like other birds, they have ancestral ties to dinosaurs. Aside from great horned owls and burrowing owls, other species found in Saskatchewan include snowy owls, short and long-eared owls, northern, eastern and western screech owls, great grey owls and saw-whet owls.

As nocturnal predators, owls have special adaptations to help them hunt. One example is their necks, which have 14 vertebrae (compared to seven in humans), which permits them to rotate their heads 270 degrees, giving them an extra wide range of vision. That’s important because unlike many animal species owls can’t shift their eyes to look in different directions. Instead, they swivel their heads.

Sizewise, owl eyes are disproportionately large in comparison to their skulls, and that translates into superior night vision. When hunting, great horned owls perch and scan the surrounding area for prey. Other species hunt from the air.

Sight isn’t a hungry owl’s only advantage. “There’s a group of owls that have very good hearing,” says biologist Leanne Heisler. “Their ears are off-centred, and they have acute hearing in certain frequencies that allows them to distinguish their prey as it moves in the grass without any light.”

As if that wasn’t enough, owls also have specially adapted feathers with serrated edges that give them stealth capability — letting them swoop down silently and snatch their prey with powerful talons.

And what is that prey? Start with around 20 species of small mammals, says Heisler. What else?

“We’ve found ducks, grebes and other birds in the pellets too, along with insects, snakes, bats, skunks and weasels. We’ve even found some cats, but no dogs, which is good. But great horned owls can take things that are pretty big.”

Due to their small size, burrowing owls often digest their prey in stages, coughing up two pellets with each successful hunt. Great horned owl pellets, conversely, sometimes contain more than one kill. Regardless, all owls have to regurgitate pellets from their prey before they can feed again the next night.

“I have found pellets with undigested mice in them,” says Heisler. “They’re not great to see, but it’s a good indicator of how much food is available. If they’re not digesting everything it means they’ve either just eaten something, and they’ve had to cough up a pellet, or they’ve eaten too much and didn’t have time to digest the mouse.” /Gregory Beatty