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Feature Story: Snowy Owl Genetics

snowy owl eye

Snowy Owl Genetics and Gender

DNA Gives a Hoot

by Jacquelyn K. Beals

“Everybody was thrown a curve ball by the irruption of snowy owls this winter. Wildlife biologists and centers had a lot of work to do very quickly – including genotyping ... along the East coast and in the Northern prairie states.”

                                                                                                                       – Paula Goldberg, City Wildlife

Among the heartwarming critter-stories attracting public attention during the winter of 2014 was the snowy owl that perched at the corner of 15th and K Streets in NW Washington, DC.  Usually Arctic residents, snowy owls were sighted this winter in Arkansas, the Great Lakes, the Dakotas, Massachusetts, and even Florida. Was this exotic visitor a male or a female?  Who could tell?

Like many more-common birds, a snowy owl’s gender often isn’t obvious (except, perhaps, to other snowy owls).  Young males’ feathers show dark-brown bars but these lighten with age. The “snowiest” owls are adult males – for example the owls playing Hedwig in Harry Potter films.  By contrast, adult females tend to be larger and may retain dark markings all their lives, although the darkest males and lightest females look much alike.

Unfortunately, this particular owl garnered more headlines January 30, when it was hit by a DC Metro bus – or possibly by a bus and then an SUV.  After initial treatment at the National Zoo, the DC owl was taken to City Wildlife, which specializes in emergency and critical care.  This facility, opened in mid-2013, offers care and rehabilitation for wild animals, which are actually involved in a surprising 25%–50% of the District’s animal calls. 

“Actually, the majority of DNA testing in wildlife  rehabilitation is done to determine gender.” 

“This was either a very lucky bird or a really unfortunate bird,” observed Paula Goldberg, executive director of City Wildlife.  “If only this owl could talk! We never found out exactly what happened to it … We tested its DNA for gender and ran routine blood tests that included rodenticide screening.”

The owl’s blood tests revealed a low red blood cell count, possibly due to bleeding from its injuries or from ingesting rat poison. Although the results were negative for rodenticide, it may have been metabolized  before the sample was collected or counteracted with Vitamin K.  But when the owl was examined at the care facility, wouldn’t its gender be obvious?  Actually, the majority of DNA testing in wildlife rehabilitation is done to determine gender. “We test for gender in bird species when there is no apparent way to discern the gender of the animal,” explained Goldberg. “You usually see this in some bird species when males and females are very similar in size and have no distinguishing markings.”

The DC snowy had only moderate barring and was small, making it a hard call – even more difficult because of its general feather damage and the significant singeing of its primary flight feathers. “I think everybody, including one of the biologists for the DC Government, thought that this bird was an immature male that had migrated South,” Goldberg said.

“Research organizations use genetic analyses to understand the broader framework of owl taxonomy.” 

In humans, the cells of females contain two similar sex chromosomes (XX), while males’ cells contain two different sex chromosomes (X and Y).  But birds have another system for determining sex:  Cells of females contain two different sex chromosomes (W and Z) while the cells of males contain two similar sex chromosomes (ZZ); the Z chromosome is large, but the W chromosome is very small.  DC’s snowy owl proved to be a female.

Gender determination is also useful for scientists observing a certain bird’s behavior, as well as for returning birds to the wild.  For example, if a bird is female, should she be released with a group of males?  Research organizations such as Project SNOWstorm use genetic analyses to understand the genetic make-up of snowy owl populations and the broader framework of owl taxonomy. And analysis of mitochondrial DNA (mtDNA) aids Vassar College researchers’ population studies of several owl species.

After spending several days at City Wildlife for critical care and stabilization, the DC snowy owl was transferred to a facility where her care could continue in a larger outdoor enclosure. The new facility has a 15-foot ceiling and no heat but a brooding lamp where the snowy can warm herself.  A rope-wrapped pole angles down to the ground from an upper shelf, so she can walk down to reach her food and water.

“One of the reasons that she hasn’t been released yet is because of concerns about damage to the feathering – but she also has a broken toe, making hunting difficult,” said Goldberg.  At latest report in mid-February, the snowy is doing well – eating four small rats at a sitting, each injected with medication as well as extra fluids for hydration.

“The most heartening story we were told by one of the rehabilitators,” said Goldberg, “is that the morning after the East coast was hit with the big snowstorm, someone saw small tracks, small footprints at the bottom of the cage, along with a body print. Owls make body prints in the snow when they play – so she was being playful!  It was really sweet.”


(1) Project SNOWstorm: Tracking Snowy Owls.

(2) A Webcast and Teaching Resources With Bird Detective Carla Love.

(3) The Secret of X and Z - How Sex Chromosomes From Humans and Chickens Found Common Ground. Discover.

(4) Tips for Helping Injured Wild Birds. Washington Crossing Audobon Society.