The mystery surrounding dozens of paralyzed birds that were discovered in B.C.'s northeast has deepened after veterinarians ruled out West Nile virus but found wing and leg fractures.

Last month, dozens of paralyzed ravens and crows were dropped off at a Dawson Creek rehabilitation clinic, sparking concerns about West Nile, which can also affect humans.

Despite efforts to save them, all 30 birds eventually died.

Veterinarians have now ruled out West Nile, saying no viruses of any kind were found in the eight or so birds sent for a necropsy.

But the young birds showed wing and leg fractures, they said.

"Bones can be broken when they fall out of nests … or if they are coming out of the nest earlier than they should before the wings can really support them. So, maybe that's not that unusual," said wildlife veterinarian Helen Schwantje. 

"We just don't see large numbers of them very often," she added.

A spokesperson for the province's Fish and Wildlife Branch said officials don't believe there is a risk to human health.

Last week, dozens of black birds started falling  from the sky in Winnipeg. 

Residents described a "blanket of black" that looked like something out a Hitchcock movie.

Animal experts are also trying to figure out the cause of those deaths.