Conservation Crossroad: Herons and scientists adjust to a changing landscape

Adult great blue herons are a common spring sight along the suburban coast of the Strait of Georgia, near Vancouver, B.C., foraging in bays and estuaries for small fish to nourish their young in nearby breeding colonies. Some of the largest colonies of more than 400 nests have become well established and well known to local residents and naturalists.

The Strait of Georgia is a particularly important breeding region for the non-migratory faninni subspecies of the great blue heron, which is federally listed as a species of Special Concern by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC).

Forward-thinking conservationists recognized the importance of protecting the heron colonies on the threatened suburban landscape and, in collaboration with the Canadian Wildlife Service, the B.C. Ministry of the Environment and local municipalities, purchased the small patches of treed land that supported these colonies.

Nature Throws a Curve

In recent years, bald eagles have become numerous in and around Vancouver and B.C.'s south coast – the National Audubon Society's Christmas Bird Counts have shown an annual increase of about 5 per cent in bald eagle numbers over about the past two decades.

The recovery of the bald eagle along the Pacific coast is considered a North American conservation success story. Over the last few decades, the bald eagle has emerged from an era of reproductive failure – attributed to exposure to DDT – to a stage of abundance following reduced use of pesticides and a ban of the use of DDT in North America.

Catching biologists off guard, the rise in bald eagle numbers has also resulted in increased disturbance and abandonment of heron colonies, including those on purchased conservation lands. Conservationists are trying to determine exactly how the relationship between the eagles and herons is affecting the colonies.

According to one theory, harassment of heron nestlings by increasing numbers of adult eagles has forced herons to make an important breeding decision: Should they raise their young in a large colony where they are obvious to eagles, but somewhat protected since that nest is only one of several hundred an eagle can choose from? Or should they nest elsewhere, perhaps alone and hidden among the trees, where an eagle is unlikely to find the nest, but if it does, will likely kill off the heron's entire young?

It seems that in the area studied, herons have made both choices. Some protected colonies, such as the 125-nest McFadden Creek colony on Salt Spring Island, have been completely abandoned and researchers don't know where the adult herons have subsequently chosen to nest. No new large colonies in the vicinity of the abandoned colony have been discovered.

In other cases, the bald eagle population is affecting heron colonies in another way. An abandoned 400-nest colony on Point Roberts in Washington State, just south of Tsawwassen, B.C., has re-established nearby in a small patch of trees near the B.C. Ferry Terminal. In this case, the herons have established a colony as close as possible to a rich marine coastal foraging area on the edge of an expanding suburban landscape – not ideal heron habitat. The colony now shares a remnant patch of forest with a nesting pair of bald eagles. One theory explaining this unlikely nesting association is that the territorial pair of adult eagles in the middle of the heron colony provides protection by preventing a large number of juvenile eagles from attacking heron nestlings.