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Snowy Owls thrive in winter, confounding conventional wisdom

One of the wintering Snowy Owls takes flight.
Snowy Owl, male, by mayhaga.

A study just published in the journal The Auk: Ornithological Advances is challenging a widely held assumption about Snowy Owls that spend the winter on the cold Canadian prairies.

Conventional wisdom holds that the owls are forced south by a lack of prey in the Arctic, and therefore are starving.

Researcher Karen Wiebe, of the University of Saskatchewan, with a Snowy Owl.
Researcher Karen Wiebe, of the University of Saskatchewan, with a Snowy Owl. Photo by K. Jacobsen.

Eager to test this, researchers Alexander Chang and Karen Wiebe, of the University of Saskatchewan, studied the body condition and relative mortality of 537 Snowy Owls over 18 frigid field seasons in south-central Saskatchewan.

Because female Snowies are typically larger than males, the researchers predicted that wintering females would be in better condition than males and that adults would be in better condition than juveniles, and the results bore this out: Females tended to be in better condition than males, and adults, presumably reaping the benefits of greater hunting experience, tended to be in better condition than juveniles.

Even more interesting, few of the adults captured in the wild showed signs of starvation. Indeed, counter to expectations, subcutaneous fat seemed to increase in adults during winter.

Snowy Owls aren’t starving

“Clearly, in our study, some individuals of both sexes became emaciated while wintering on the Canadian prairies,” Chang and Wiebe write, “but the average body mass of adult wild-trapped individuals was 73 percent above the emaciation threshold, so most overwintering individuals on the prairies were not starving.”

The researchers speculate that adults are able to build up or at least maintain their energy reserves because they are experienced hunters, and well insulated against the cold.

“Furthermore, winter is a time when they are not engaged in energetically costly activities such as reproduction,” write Chang and Wiebe. “Thus, given sufficient food sources, winter may actually be a time for Snowy Owls on the prairies to increase energy stores prior to migration, especially in adult females preparing for reproduction.”

According to the researchers, much of the data for the study was collected by a pair of retired farmers who discovered a love of owls and pursued raptor banding as a hobby.

“They had no intention of analyzing data but spent numerous hours in the field in grueling sub-zero weather honing their trapping techniques and drinking liters of coffee while waiting — sometimes hours — for an owl to come to a trap,” says Wiebe.

The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union.

Read the paper

Alexander M. Chang and Karen L. Wiebe (2016). Body Condition in Snowy Owls Wintering on the Prairies Is Greater in Females and Older Individuals and May Contribute to Sex-based Mortality. The Auk: Ornithological Advances, Volume 133, pp. 738–746.

eBird maps show Snowy Owl’s summer and winter ranges.

How at least one Snowy Owl benefited from 2013 Razorbill irruption.

See reader photos of Snowy Owl.

 

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