Table of Contents
By Louisa Shepard

Secrets of the Snowy Owl

As the first Snowy Owl sightings in the Mid-Atlantic region were reported in recent weeks, veterinarians at Penn Vet’s New Bolton Center were examining the owls that didn’t survive last winter.

A year ago, the “irruption” started with a migration of Snowy Owls from their native Arctic to the unfamiliar Mid-Atlantic, Northeast, and Great Lakes regions. Hundreds of birds came down to the Lower 48, the most at one time in decades, and some did not survive.

Snowy OwlAbout 50 Snowy Owls that died have been examined, more than half of them at New Bolton Center, as part of “Project Snowstorm,” created last year in response to the rare influx of the species. The Project’s purpose is to systematically gather population and medical information on Snowy Owls that make it to this region – information previously nonexistent.

The necropsies are conducted at New Bolton Center under the supervision of Dr. Sherrill Davison, Director of the Laboratory of Avian Medicine and Pathology: the tissue samples go through toxicologic testing by Dr. Lisa Murphy of Pathobiology, histopathology testing by Dr. Perry Habecker of Pathology, and parasite identification by Dr. Tom Nolan at Penn Vet’s Ryan Hospital.

“We want to better understand the biology of these magnificent birds,” Davison said.

Dr. Erica Miller, Penn Vet Associate Adjunct Professor of Wildlife Medicine, is one of the veterinarians performing the necropsies at New Bolton Center.

“We want to find out what’s normal for the Snowy Owls and what problems they are having or encountering. We don’t usually see Snowy Owls because they are up in the Arctic, so this is a chance for us to investigate,” Miller said, as she carefully took samples from the body of a Snowy Owl found in Maine.

Dr. Erica Miller examines a wing during the necropsy.“What diseases do they have? What parasites do they have? What size is a normal spleen? We don’t know,” Miller continued. “This is an opportunity to collect as much data as possible.”

The deceased birds were shipped to New Bolton Center from around the country, some from other veterinary schools, including the Cummings School of Veterinary Medicine at Tufts University, as well as from state wildlife veterinarians, and the National Audubon Society. The birds were sent frozen, and have since been systematically necropsied, using protocols determined by Project Snowstorm organizers.

“They did a really complete and thorough job on the necropsies at New Bolton Center,” said Dr. Justin Brown, Wildlife Veterinarian for the Pennsylvania State Game Commission. “They used a standardized approach to look at birds in the same way. By using one standard protocol, we can pool information on a greater number of birds, and therefore understand it much better.”

Samples are taken for Pathology to study.Dr. Brown said the data is still being analyzed, although all necropsies and lab work are largely completed. In addition to better understanding what “normal” looks like in the species, Brown says the Project’s goal is to provide information on why these irruptions of Snowy Owls into southern areas might be occurring.

The necropsies have confirmed that most of the Snowy Owls died from trauma. The birds are attracted to open areas offered by airports, where they fly into things, are hit by cars or planes, or get electrocuted. Surprisingly, a few were shot. Others died from ingesting poison when they eat rodents. Contrary to myth, starvation is rare and is usually caused by underlying diseases.

Project Snowstorm is also doing what it can to track the live birds, raising money to buy transmitters. Last year, the telemetry devices were put on 22 live owls to track their movements.

Penn Vet Examines the Owls

The necropsy of a Snowy Owl takes about half an hour. On a day in late November, Davison worked with Miller; Dr. Cindy Driscoll, Wildlife Veterinarian at the Maryland Department of Natural Resources; and Dr. Nancy Ting, a first-year resident in avian medicine and pathology at Penn Vet.

“It is more consistent to have them all processed in one place. From a veterinary and pathology perspective, it is the smartest thing to do,” Driscoll said. “We are trying to establish baselines. If there are any disease patterns, we want to know that.”

Drs. Nancy Ting, Sherrill Davison, and Erica Miller examine a Snowy Owl.The veterinarians start the necropsy by weighing the bird, then taking measurements of the body: the wingspan, the bill, the tail, and the feet. Noted are feather markings, body condition, and muscling. Any trauma is observed. Extensive photographs are taken.

Because these birds are so valuable for scientific investigation, Miller said she does not “lay the whole bird open,” but instead carefully removes the organs one by one to be measured and analyzed. Samples are taken for testing: toxicology, histopathology, microbiology, virology, and even genetics. The contents of the stomach are examined to try to determine what they are eating. Any abnormal lesions are noted.

Early Results

Davison and her colleagues are identifying some consistent characteristics in the birds as they conduct the necropsies. For example, the birds that died in the very northern states, like Maine, are thin with poorer body condition than the ones that made it all the way south to Pennsylvania and beyond.

Dr. Nancy Ting and Dr. Erica Miller prepare more samples for Pathology to study.From an infectious disease perspective, Davison said that she found several types of internal parasites in the birds that died in New England, but not in the birds that made it farther south to the Mid-Atlantic region. One particularly interesting finding is that all the birds have lice. The team is working with parasitologists to try to determine the origin – whether the lice were from the Arctic, or if the birds picked them up here.

“We’ve found out a lot about these birds that we didn’t know,” Driscoll said.

Penn Vet parasitologist Nolan also worked to identify a parasite that was found in the birds’ tissues. The roundworm found in the stomach of the Snowy Owl was challenging to identify, Nolan said, noting that this was the only time he had been asked to look at a parasite from a Snowy Owl. But he was finally able to make a determination.

“The nematode from the Snowy Owl is a mature female of the Genus Capillaria. Capillaria longispucula has been reported from Snowy Owls in Canada,” Nolan said. “The presence of only one immature worm in the owl would suggest it is not a significant pathogen.”

Pathologist Habecker at New Bolton Center microscopically examined the major visceral organs and brain. “Most tissues exhibit no disease. We automatically test for West Nile Virus using immunohistochemistry since raptors are especially vulnerable to the virus,” he said, but added that, so far, the testing hasn’t found any positive cases.

Dr. Nancy Ting shows the wingspan of the Snowy Owl.Toxicologist Murphy said that out of 27 birds that were tested, 10 livers (36 percent) were positive for one or more anticoagulant rodenticides, and 13 livers (46 percent) contained detectable quantities of pesticides. Only eight birds (29 percent) were negative for both anticoagulant rodenticides and pesticide residues. Liver and kidney samples also were tested for 14 minerals and heavy metals, and data analysis is currently in progress.

Project Snowstorm is gathering all of the medical data provided by the necropsies and analyses. Davison said they plan to publish a paper in the next few months detailing the findings. 

“If Snowy Owls continue to migrate to the U.S., Penn Vet will continue to work with colleagues from Project Snowstorm to gather as much data as we can in order to protect this incredible species,” Davison said. “It has been an exciting opportunity to be a part of such a prestigious team of researchers, pathologists, wildlife veterinarians, and wildlife biologists.”

Table of Contents