Deadly Darwinian drama playing out in Stanley Park heronry

By Sandra Thomas-Staff writer

Call it survival of the fittest.

Many heron chicks in Stanley Park are dying as the result of siblicide, says Robyn Worcester, urban wildlife coordinator with the Stanley Park Ecological Society. Siblicide is the term used when animals and birds kill their siblings and is common among wading birds and raptors.

So far this year, 23 dead Great Blue Heron chicks have been collected from under the heronry by park rangers and parks board wildlife staff. As well, eight chicks were injured after being pushed out of the nest and were taken to wildlife hospitals. The Great Blue Heron is listed as at risk in B.C.

"Herons lay one egg and then another at intervals," said Worcester. "The heron will lay one egg while sitting on another. Because they hatch at different times they're different sizes."

Worcester notes siblicide is common in herons when the chicks are of varying ages and sizes. If a lack of food becomes an issue, the larger chick will turn on its sibling in a battle for survival. She adds it's thought herons lay their eggs at different times because the parents don't know if they will be able to supply enough food for three to five eggs. By laying eggs asynchronously, the parents let the yearly conditions and older siblings decide if all the chicks or only a few will survive.

"If there's lots and lots of food, all the chicks will live, but if there's not very much food, you can go from four to one chick quite quickly," said Worcester. "And that's pretty normal."

Worcester explains both parents spend all day and night finding food for their chicks. The adult herons in Stanley Park fish on the beaches and in the estuaries around Burrard Inlet and English Bay, but will travel as far as Richmond to find food. Heron chicks are raised on a diet of regurgitated fish. Once the food is dropped off at the nest, the parents take off again and are rarely around to watch the squabbling.

"Their investment is to lay the eggs," said Worcester. "They don't break up fights and they're not concerned about who gets what food. They just drop it off."

Worcester notes this year heron chicks appeared in at least 116 of the 182 nests in the heronry. Though leaf growth prohibits exact counts, the volunteer monitors who keep track estimate 300 chicks hatched this year. These chicks now range in age from two to 10 weeks. At its peak, the heronry will hold about 800 herons this year.

Worcester says some of the older chicks are attempting to fly and are pacing the higher walls of their nests, while some of the braver juveniles are carefully balancing along a branch.

"It's quite something to see three or four of these big clumsy chicks hanging onto a branch," said Worcester.

At eight weeks the young fledglings prepare to leave the colony by testing their wings so they can take flight from the treetops. The ones that don't make it into the air end up on the ground under the heronry where they'll learn to fly from the ground as long as they stay clear of dangers such as cars or predators. The day the Courier photographer visited the heronry, a dead chick lay on the road after being hit by a car.

Worcester adds herons aren't the only birds nesting in the city. In a report she wrote for the ecological society, she notes there are eaglets in at least six of the city's 14 active bald eagle nests. In the spring, there were adult eagles in 18 nests across the city, but four of the nests have since become inactive.

Worcester explains some of the inactive nests could be a secondary nest built in an eagle's territory if suitable trees are available. These nests are usually close to the primary nest and can be used for roosting and feeding.

While some eagle pairs breed each year, others take the occasional year off. While the reason for this is unknown, it's thought it may have to do with the number of occupied territories in an area, food supply or a disturbance at the nest site.

A nest in Stanley Park on Pipeline Road has at least one live eaglet, while another off Cathedral Trail is showing adult activity, but with no young visible. Eaglets born this spring at Vanier and Jericho parks did not survive.

For more information on Stanley Park's herons and the city's eagles, go to www.stanleyparkecology.ca.

published on 07/06/2007