Biologists searching for the cause of a sharp drop in the survival rate of heron chicks in Stanley Park say the city’s booming population of urban eagles may be to blame.

Over the past ten years, the number of chicks surviving to the fledgling stage in the colony on the edge of Vancouver’s West End has dropped from 258 in 2007 to just 61 this year.

That’s raising concerns for biologists, who have been watching the size of the noisy colony above the park’s tennis court decline in recent years.

“We really don’t know. They do fluctuate from year to year. There are good years and there are bad years in these colonies,” says Vancouver park board biologist Nick Page.

What is known is the city’s population of eagles has been increasing since the 1960’s when there were only one or two nests in the whole city.

That population is now booming with about 14 known nests in Vancouver, including three active nests in Stanley Park, says Page.

“It’s a fairly complicated thing to tease out one cause, but I think the main thing is we are seeing more eagles across the city in general, and this year we saw more predation from eagles.”

Greg Hart with the Stanley Park Ecology Society confirms biologists have observed the eagles are preying on both the eggs and the chicks in the colony more than ever this year.

15 mins ago: Herons’ nests and eggs got attacked by an eagle! Watch out as it can happen anytime. LIVE: https://t.co/SgX1BeQiyD #HeronTalk pic.twitter.com/O7EWQ2Xkvp

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@ParkBoard

Hart and others believe that’s cut the success rate for the 70 active heron nests down to 52 per cent, with about 61 chicks making it to the fledgling stage this year.

In comparison, in 2007 there were 139 active nests with 258 chicks surviving to the fledgling stage, he notes.

No eagle defence

What’s not yet clear is how many eagles are targeting the Stanley Park colony for food, Page says. It could be a select number or an adaptation by the wider population.

“Eagles are predators of lots of different birds and herons are obviously a good food source in terms of larger eggs and chicks as well,” says Page.

The closest eagle’s nest to the heron colony is near Lost Lagoon, but there is also another near the Canadian Coast Guard base across the water at Vanier Park.

“The herons can’t defend themselves against the eagles. They look quite large, but they are quite a light bird and definitely not robust enough to fight off an eagle.”

Eagles feed primarily by scavenging along the ocean shoreline and some won’t even prey on herons at all, he notes.

“The reason for the change — that’s the concern for us. Are we seeing a pair of eagles that have started to see herons as a food source? Are we seeing a change where we have new eagles moving in?”

A bald eagle sits on a great blue heron’s nest in the Stanley Park colony. (Vancouver Park Board Heron Cam)

Page notes if the herons lose an egg to a predator they will often lay another within a few days. But if the eagles take the larger chicks, the parents are unlikely to raise another to replace it.

People not the problem

Hart says the birds have been flocking to the site in Stanley Park since 2001 , and the nests make up one of North America’s largest urban Pacific great blue heron colonies. They typically stay until July, the end of breeding season.

While staff are concerned about the dramatic drop in the overall heron population at the rookery, Page says park staff have no plans to interfere.

Instead they plan to continue to let nature take its course and monitor the colony’s stability.

“I think our main concern is colony abandonment, where they all move out. This can be for a number of reasons. Sometimes the trees start to fall down or something like that.”

The Stanley Park herons are quite used to human activity, which is often a cause of colony abandonment in other locations, he notes.

“There are tennis players there and there are always people parking underneath them. That might actually provide some protection from eagles,” says Page.

He also notes the booming population of urban eagles is part of an overall trend that has seen other predator species return in greater numbers to the shores of Vancouver in recent years, including killer whales, river otters and seals.