A Pacific Great Blue Heron rests in a nest in Vancouver's Stanley Park. (CBC)

One of the West Coast's most iconic birds may be in trouble, according to a Parks Canada biologist.

This has been one of the worst breeding years on record for Pacific Great Blue Herons in the Vancouver area and experts don't know why.

In the last few years, each nest on average produced four chicks, and one of them survived to fledge, said Ross Vennesland, a species at risk recovery specialist at Parks Canada.

But this year, the survival rate was half of that, he said, meaning the almost 2,000 nests around the Georgia Basin will produce fewer than 1,000 chicks.

"The Lower Mainland this year was the worst year we have on record so far. So that's quite worrying. If that continues we're in really big trouble," said Vennesland.

The large colonies of the Lower Mainland and Fraser Valley are normally the healthiest in the province, he said.

"Chilliwack, Maple Ridge, and Coquitlam, they've always done really well, and this year those ones all either failed or did very poorly .… I haven't seen that before," he told CBC News.

Cause of low survival unknown

It's not clear why this year has been so hard on the heron, but increased predation by eagles or disturbance by humans are both possibilities.

The low survival rates on the mainland could also point to potential troubles for struggling populations on Vancouver Island, where the colonies have been doing poorly for a while.

"Normally I wouldn't be so concerned about one bad year, but the fact that the Lower Mainland did so poorly this year is something new, so we do have a fairly significant fear that that's a new trend," said Vennesland.

The Pacific Great Blue Heron is a subspecies of the Great Blue Heron, and found only on the coast.

It's considered vulnerable by the B.C. government, and a species of special concern by the Committee on the Status of Endangered Wildlife in Canada.

But Vennesland said if the decline seen this year does become a trend, that designation will have to be revisited.

With files from Lisa Johnson