Project SNOWstorm

Irruptions and Innovation

By Zoey Greenberg, Science Outreach Coordinator
Hawk Mountain Sanctuary

Stella, a snowy owl tagged by Project SNOWstorm earlier this year.

Stella, a snowy owl tagged by Project SNOWstorm earlier this year.

In 2007, Dr. JF Therrien deployed 12 transmitters on snowy owls on Bylot island in the Canadian Arctic. Snowy owls, like other raptors, can be difficult to monitor due to their territoriality and low densities. Arctic-breeding raptors typically establish nesting sites in remote locations, adding to the logistical dilemma of gaining insight into their life history traits, and the role they play within the Arctic ecosystem.

Satellite transmitters, citizen science, and long-term life history studies are extremely valuable tools that allow researchers to examine the movement ecology of birds, including snowy owls. Mixing and matching these tools can open doors to compelling scientific questions, and in the case of Therrien’s snowy owls, has resulted in papers covering a range of topics. New information on survival, reproduction rate, dispersal of adults, irruptions and winter movements are among recent Hawk Mountain publications that were made possible through this type of innovation. 

Two such papers were led by former conservation science trainees, in collaboration with others, including Dr. JF Therrien. These papers delved into the mystery of snowy owl irruptions, defined as the “massive movement of individuals over large distances, associated with large fluctuations in food supply.” Some of us have been lucky enough to see a snowy owl in Pennsylvania, on those rare winters when the birds venture further south than usual. I myself drove two hours to catch a glimpse, and I will never forget the sight of that owl hunkered down in the middle of a field, glittering in evening light. As I drove away, I couldn’t help but wonder; what brings them here?

Two primary hypotheses have been proposed to explain irruptions; the “lack-of-food” hypothesis suggests that snowy owls leave their normal wintering grounds because of food shortage in certain years. This implies that the snowy owls we see in Pennsylvania should be in poor shape, and likely close to starving. The alternative “breeding success” hypothesis instead links irruptive movements to a surplus of food on the breeding grounds.

A snowy owl that was spotted in a local PA field last year.

A snowy owl that was spotted in a local PA field last year.

Snowy owls specialize in lemmings, a prey resource that occurs in pulses, due to their population dynamics. One year there may be a plethora of lemmings scurrying around the tundra, the next year very few, due to high predation and other factors. Lots of lemmings means lots of chicks, which means lots more owls. The breeding success hypothesis implies that the reason we see snowies in Pennsylvania is because following a high lemming year, there are so many owls dispersing at the end of the summer that some young birds push south.

To add depth to our knowledge of irruptions, Teja Curk, a conservation trainee from 2016, assessed the body condition (mass relative to size) of snowy owls during both irruptive and “regular” years. Teja assessed body condition of snowies on both regular (Great Plains) and irregular (Northeaster North America) wintering grounds, to see whether the birds we see during irruptive years are, in fact, starving. She found that most owls (male, female, juvenile and adult) were in good shape during irruptive years and even discovered that body condition was better on the bird’s irregular wintering grounds. “Good shape” in this context varies depending on sex and age of the bird, however less than 2% of the owls approached the weight threshold that would deem them a starving bird. Her results provide support for the breeding success hypothesis.

Pablo Santonja and Irene Mestre, trainees from 2015, looked specifically at the age of the birds found south during irruptive years. Their results supported the same hypothesis, showing that the majority of owls (up to 90%) seen in eastern North America are juvenile birds, meaning they are less than 1 year of age. As with some other migratory species, the more dominant birds within a population don’t move as far. Adult snowy owls often out-compete young birds, so it makes sense that in a year where adults are laying up to 11 eggs, some of those young owls will need to leave to avoid competition from more successful, experienced adults.

Bylot Island, where the Hawk Mountain team tagged and tracked several snowy owls.

Bylot Island, where the Hawk Mountain team tagged and tracked several snowy owls.

This leads to an interesting point: perhaps these two competing hypotheses are not, as Teja notes in her paper, mutually exclusive. After a highly productive lemming year, many lemmings will be eaten due to a surge in predator numbers, and therefore the next year could produce low lemming numbers. If this happens, snowy owls could be forced to look elsewhere for food. So, it turns out, that both a surplus of food and a lack of food seem to play a role in the patterns of movement among snowy owl populations.

If you ask me, these papers are a beautiful example of the scientific method at work; formulating ideas, testing those ideas, and polishing the conclusions in pursuit of certainty. Hats off to our trainees and their collaborators for hard work and intriguing findings. Teja is currently at the Max Planck Institute for Ornithology in Germany pursuing a PhD. Pablo is living in Spain, and Irene is now returning from three years in Australia.

Both of these papers utilized a data set resulting from a 25 year-long study in which live snowy owls were weighed, sexed, and aged. Christmas Bird Count (CBC) was also a valuable asset in both studies. For those of you unfamiliar with CBC, every winter thousands of volunteers gather together on the same day, collecting bird records within a territory and submitting them to a database that provides an impressive summary of which birds were seen, where, and how many. These papers demonstrate the utility of both citizen science and long-term studies, and in a rapidly changing world, this cross-pollination of resources is critical. We are very proud of our trainees for their hard work, and grateful to citizen scientists around the globe who give their time to participate in the expansion of our raptor knowledge.

Gaining a holistic understanding of an ecosystem requires acknowledgment of moving parts within, not separate from, the whole. Hawk Mountain’s research is part of several collaborative efforts to better understand the role of snowy owls within the big picture, including Project SNOWstorm and the Bylot Island Ecological Studies and Environmental Monitoring.  

Future projects will include building a model that combines reproductive and mortality rates to assess population trends of snowy owls. This past August, three transmitters were deployed on chicks before they left their breeding grounds. Investigating the movement patterns of these birds will hopefully allow for a comparative look at owls throughout the Eastern tundra, Western tundra, and those in between.

Check out Project SNOWstorm’s website (including a blog written by J.F Therrien on the most recently tagged snowy owls in Barrow, Alaska, as well as interactive maps): projectsnowstorm.org/posts/tracking-young-snowies-in-the-arctic/

A Back-to-Back Snowy Owl Adventure

By Rebecca McCabe,  PhD Student, McGill University
Hawk Mountain Research Assistant

The view on Amherst Island

The view on Amherst Island

It was 6:15 a.m., and the sun had not yet come up as we stepped from our cars onto the ferry deck on the ice-covered waters of Lake Ontario. We were on our way to Amherst Island, a 70 square-kilometer island off the coast of Kingston, Ontario, which is a known hotspot for wintering snowy owls and the reason why we were there.

Myself, Jean-François Therrien (Hawk Mountain), Tom McDonald (Rochester, NY), and Dave Okines (Prince Edward Point Bird Observatory) arrived on the island with the hopes of deploying transmitters on snowy owls as part of the collaborative research of Project SNOWstorm (www.projectsnowstorm.org).

The sun rises over the frozen Lake Ontario.

The sun rises over the frozen Lake Ontario.

As the sun finally rose, we were already driving the main roads searching for white silhouettes in the distance. Luckily for us, Amherst had plenty of snowy owls! We estimated over 25 owls on the island, and now all we had to do was trap a few. We came up short in the morning and afternoon, but as soon as the sun started to set, our luck changed. Within 20 minutes both Dave and Tom had trapped an owl.

Snowy owl Stella, a female trapped and tagged by Project SNOWstorm in Amherst

Snowy owl Stella, a female trapped and tagged by Project SNOWstorm in Amherst

We were fortunate to have a wonderful host, Janet, who allowed us to bring the owls back to her place so we could process them out of the cold. The two immature females, later named Emerald and Stella, were in good condition and each weighed over 2 kg making them eligible to receive a transmitter before being released back into the night.

In just a short amount of time we have learned that the neighboring birds  established territories approximately 3 km from one another on the island, and Emerald has spent more time inland whereas Stella has been going offshore onto the ice during the day. I look forward to following these two individuals as they make their way north in the upcoming months.

One week and 2,370+ km later, I am scanning for snowy owls from the passenger seat again, but this time I am looking out on the prairies of Saskatoon, Saskatchewan. My co-supervisor Karen Wiebe (University of Saskatchewan) and I follow behind raptor bander Marten Stoffel in our pickup truck as we search the tops of telephone poles and fence posts. We spent six hours that day driving the survey route and had easily seen over 30 snowy owls. We were also very fortunate to trap and band six, a record day of trapping snowy owls on the prairies for Marten!

Male snowy owl trapped and tagged in Saskatchewan. 

Male snowy owl trapped and tagged in Saskatchewan. 

The next day we decided to go out again to see if luck was still on our side, this time driving about 150 km northeast of where we were the day before. I was shocked at the stark difference between locations and by the lack of owls we encountered on our second day. The habitat appeared similar but we only saw one adult male that afternoon. Marten decided to try trapping it, even though it was a more difficult catch with blustery winds and the male sitting high atop a transmission tower. Moments before calling it quits, I watched intently (and completely mesmerized) as the owl leapt from its perch and with a few strong flaps, came streaming in towards the bal-chatri before getting trapped.

Spending just a few days on Amherst Island and in Saskatoon and getting to see where these owls spend their winter was not only exciting but informative. I was able to observe them on their territories, take note of their favorite perches, and see first-hand the conditions they were in. As I move forward with my PhD at McGill University and analyze the movements of our GPS tagged snowy owls, my time spent in the field has allowed me to gain a better understanding of how these magnificent birds move throughout the landscape, the habitats they occupy, and the threats they face.

Becca with a recently trapped and tagged snowy owl right before release.

Becca with a recently trapped and tagged snowy owl right before release.

To learn more about these efforts or to support the project, visit hawkmountain.org/snowyowls.