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August 18-31
VOL.14 ISSUE. 26
HOME / STORY

Secrets Of Owl Barf

Gregory Beatty
Published Thursday January 21, 04:46 pm
Biologists study prairie life one regurgitated lump at a time

You might want to read this one after you’ve eaten.

So: owl barf.

Like other carnivores, raptors such as owls and hawks poop as part of their digestive process. But that’s not the only way they get rid of indigestible bits of their latest meal (typically a small rodent). They also regurgitate “pellets”.

You could compare it to a cat coughing up a hair ball — except instead hair, the birds gag up their prey’s skull, teeth and fur.

The birds are forced to do that because, while they have powerful beaks, they don’t have teeth to chew their prey, so they must swallow it whole. The stuff their digestive system can’t handle gets separated out and compacted into a pellet.

Freshly excreted, the pellets, as you can probably imagine, are gunky. When they’re dry they look like greyish-white turds. They’re gross — but wait! They hold scientific value, as gross things often do.

In fact, the Royal Saskatchewan Museum has a collection of owl pellets dating back 20 years from across the Prairies.

Quest For Pellets

Leanne Heisler is a PhD science student at the University of Regina. This summer, along with assistant Amber Burnett, she spent several months in the field gathering owl pellets to add to the RSM’s  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,” she says. “With the conversion of native grassland [to farming and ranching] you see loss of habitat that way.”

Scientists sometimes use traps to measure animal populations. But rodents such as mice and voles have a limited range, says Heisler. “I think the average is 200 to 500 m. So when you use traps, you can get very different results depending on where you set them.

“Great horned owls forage up to five km 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.”

Saskatchewan is home to around 10 owl species. In addition to great horned owls, which are widespread in the province, Heisler is collecting burrowing owl pellets. They’re a much smaller species that inhabits the southern grasslands. Burrowing owls are endangered, and are being monitored by researchers at a Moose Jaw interpretive centre, and they pass their pellets on to the RSM.

“Rodents are a pretty good indicator of how the prairie eco-system 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 hanta virus that can be transferred from animals to humans.”

Oil and gas activity has impacted significantly on the prairie landscape too — especially in the past two decades. Then there’s the longer terms effects of climate change to consider.

Many more years of pellet-collecting will be needed to get enough data to detect firm trends in rodent populations. But by examining the remaining patches of native grassland in Saskatchewan, says Heisler, scientists hope to get a baseline picture of rodent numbers to compare with their current data.

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

Abandoned barns and grain bins are popular roost sites for owls, and if they find a spot they like they settle in and make it their home, says Heisler. “They’re mostly nocturnal, and hunt between dusk and dawn. 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.”

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 Draino) to dissolve the fur. Then the skull, teeth and mandibles are extracted.

DNA analysis to identify species is expensive. Fortunately, many rodents can be identified through visual cues such as the shape of their teeth and lower jaw. Prey 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 shrew — and even 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.”

Some are highly adaptable, she says, 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, it’s pretty varied.

“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. 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.

Despite their name, for instance, burrowing owls don’t actually burrow. Instead, they roost in abandoned burrows dug by ground squirrels.

Science Is Cool

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

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. Just make sure you wear gloves, and if necessary, a mask, because of the dust associated with the pellets. If you send them in, we can process them and send back the results.”

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