March is a marvellous month to enjoy the birds at Coquitlam's Colony Farm Regional Park, especially the great blue herons. These large and elegant birds can be reliably seen there throughout the winter months. During that time, the herons hunt for food in their typical solitary fashion. They can be seen in the fields searching for voles and mice or, alternatively, in the ditches where they prey upon salamanders, frogs and small fish.
But as spring approaches, the herons begin to prepare for the nesting season and undergo a dramatic change in their behaviour.
The great blue herons in this part of B.C. are a unique subspecies of herons that are found across Canada. Because of relatively mild coastal winters, local herons are not forced to undertake fall migrations south in search of an adequate food supply.
In the spring and summer, these herons are often observed locally fishing in shallow water. With their long necks recoiled like a snake, their hunting strategy is to patiently stand still, wait for fish to swim within reach and then strike. During the winter months, shallow water can freeze and fish are, in general, less abundant. As a consequence, the herons must switch their hunting activities to grassy meadows, where they find voles and mice to satisfy their appetite.
Of course, meadow habitat is becoming a rare feature in the lower mainland so places such as Colony Farm Regional Park, with its abundant fields, are especially attractive hunting areas for great blue herons.
In B.C., the non-migrating population of great blue herons, listed as a subspecies at risk, is thought to number less than 10,000. These herons, especially those that reside in the Lower Mainland, nest in large colonies - called a heronry - in a grove of tall trees. The species of trees seems not to matter as much as their height; either cottonwoods or coniferous trees are suitable. In contrast, within forested areas of southwestern B.C., great blue herons nest in smaller groups where their nests are more easily hidden in the canopy of mature forests.
The preference of herons in the Lower Mainland for a large heronry is not well understood. It could be related to the behaviour of predators or a general lack of suitable stands of trees in the rapidly developing urban areas of Metro Vancouver and agricultural areas of the lower Fraser Valley.
One thing is clear, though: The large heron colonies of the Lower Mainland, which number under a dozen in total, are notoriously unstable. It is not unheard of for a nesting colony of great blue herons to shift the location of their nesting colony after only a decade of use or less.
The reason for this dramatic abandonment of heronries is, again, not clear but appears to be related to the appearance of predators such as bald eagles or raccoons, which prey upon young herons in the nest. Herons are also sensitive to people moving under their nests, with the exception of the heronry established in Stanley Park, where these herons have become accustomed to human activity.
The habitat value of Colony Farm Park, which offers excellent winter foraging opportunities for herons, is augmented by a large stand of cottonwood trees at the mouth of the Coquitlam River that supports a heronry. This area, just south of the Mary Hill Bypass and adjacent to Colony Farm, was protected as a provincial Wildlife Management Area in 1995 specifically because of its heronry. This colony consisted of only 11 nests in 1992 but it rapidly increased in size when a heronry was abandoned at DeBoville Slough in 1994 and these herons moved to the mouth of the Coquitlam River. Since the Coquitlam site was protected, this heronry has continued to grow in size and is now thought to be one of the largest in the Lower Mainland.
TAKE A HERON WALK IN MARCH
You have two special upcoming opportunities to enjoy the great blue herons of Colony Farm Park when volunteers from the Burke Mountain Naturalists and Colony Farm Park Association co-host nature walks on Saturday, March 17 and Sunday, March 25 starting at 9:30 a.m. These walks will start from the parking lot in the park at the end of Colony Farm Road in Coquitlam. The walks, on level ground, will last for approximately two hours and are suitable for all ages. No registration is needed and the walks are offered free of charge. While the focus will be on the herons and their prenuptial behaviour, walk participants will be able to enjoy all the birds of Colony Farm Park including, possibly, some early spring migrants.
Elaine Golds is a Port Moody environmentalist who is vice-president of Burke Mountain Naturalists, chair of the Colony Farm Park Association and past president of the PoMo Ecological Society.
