Authorities believe they're one step closer to identifying a mystery illness causing unusual behaviour and ultimately death in some juvenile birds on Toronto's waterfront.

A swab sample taken from a visibly sick double-crested cormorant this week tested positive for a highly contagious avian viral condition called Newcastle disease, according to Brian Stevens, a pathologist at the Canadian Wildlife Health Cooperative.

Brain cells taken from the same animal also show signs of Newcastle disease, he added. Since the affliction can be devastating for both wild birds and poultry, a brain sample will now be sent to the Canadian Food Inspection Agency (CFIA) for further testing.

"It's all preliminary at this point, but there are signs suggesting that it is indeed Newcastle disease," said Stevens, adding that the process could take a number of weeks to complete.

The initial result provides some insight into what might be causing dozens of Toronto's most controversial birds to act so strangely.

About three weeks ago, people using the area around Tommy Thompson Park started reporting finding double-crested cormorants behaving oddly. Individual birds were seen walking down trails nearby people — unusual for the species. Others were found acting as though their wings or necks were broken.

One video posted to Facebook shows a cormorant with its head tilted sharply to the side, standing in a shallow puddle, seemingly unaware of human activity around it. The user who posted the video said in the comments that the bird stayed that way for hours.

"The early symptoms are sort of that abnormal behaviour, where they are coming close to humans. Then the muscles start to deteriorate," said Andrea Chreston, project manager for the Toronto and Region Conservation Authority's restoration of Tommy Thompson Park.

That deterioration causes limbs to hang limply, creating the appearance of broken wings or other joints, and it appears as though juveniles are especially susceptible, she adds.

"It is very difficult to watch," Chreston said.

About 17 sick cormorants have been taken to the Toronto Wildlife Centre (TWC) for treatment, however all of them eventually died of the illness.

"After none of the first ones we admitted survived ... we started referring subsequent calls about sick birds to local animal controls for humane euthanasia as there seemed to be no way to treat them and have them survive," said Nathalie Karvonen, executive director of the TWC, in an email.

Tommy Thompson is home to North America's largest double-crested cormorant breeding colony, with some 14,500 breeding pairs and between 60,000 and 70,000 individuals.

But it was not always that way. The introduction of toxic insecticides during the early decades of the last century devastated cormorants' ability to reproduce, and by the 1970s, they were nearly extirpated in southern Ontario.

Their return in huge numbers to the Great Lakes region has been deeply controversial. Many commercial and sport anglers, as well as aquaculturists, maintain that cormorants' gluttonous appetites negatively impact fish populations.

As many as 70,000 double-crest cormorants currently live in Tommy Thompson Park on Toronto's waterfront. It is the largest breeding colony anywhere in North America. (Mark Blinch/Reuters)

They can also significantly change ecosystems. Cormorant excrement, or guano, is extremely acidic. Where they nest in large numbers, their guano can change the soil chemistry, ultimately killing the trees they call home.

"There's that aesthetic component that people don't appreciate when it comes to cormorants," Chreston said.

Some jurisdictions in Ontario and the U.S. use culls to keep cormorant populations in check. The illness currently spreading among juveniles in the Toronto area could be the ecosystem's way of doing just that.

The guano that double-crested cormorants excrete is very acidic, and changes local soil chemistry and coats trees. Eventually, trees where cormorants nest will die. (Wilson Ring/Associated Press)

"In a colony of this size, it is normal and natural for there to be illness and disease among some of the individuals in that population. It's nature's way of balancing a natural population," said Chreston.

In total, about 100 cases have been reported to the TWC and city, though it's likely there are more infected cormorants that have gone unreported by humans.

"That sounds like a lot, but when you're talking about 70,000 individuals that's actually a small proportion," she explained.

Newcastle disease presents a "minimal" risk to the public, according to the CFIA. Humans who handled infected birds can contract conjunctivitis, commonly called pink eye. It is usually found in some cormorants in Canada each year in late summer and fall, the CFIA says.

Stevenson, the wildlife pathologist, said it's a good thing — from a wildlife management perspective — that the culprit seems to be Newcastle disease.

"At least it would mean we're not dealing with something unknown that could affect cormorants and other birds or humans," he said.

If you're near the waterfront and happen to come across a cormorant displaying symptoms of illness, "leave it alone," said Chreston.

"I know it can be incredibly difficult to leave an animal that appears to be suffering. But this is nature, it's not human induced, as far we know. So this is a natural process."