Nesting and Courtship

Between February and April, most Pacific Great Blue Herons gather in colonies where they court, nest, and raise young. The male chooses a nesting site which may involve building a new nest or improving an already existing one. He looks for a nest site close to water and foraging grounds and away from humans and predators. Preferred nest sites are often in tall trees 20 to 30 m off the ground, and frequently where there are nests from former years. Once the location is set, the male puts on  energetic displays and shrieks loudly to attract a female. 

The courtship display pictured above is impressive, but the male is “not just a pretty face”. After he has chosen the ideal nesting location and attracted a female, he brings twigs to give to the female for building the nest. Usually, nests are about 1 m in diameter and have a central cavity 10 cm deep with a diameter of 30 cm. The herons sometimes line this internal cavity with twigs, moss, lichens, or conifer needles. Once the 3 to 5 eggs are laid, the male generally takes the day shift for 28 days incubating the eggs. Once the eggs hatch, he and the female spend the next three weeks warming (brooding) the young, feeding them with regurgitated food and guarding them so that the nest is never left without one adult. PGBH take, on average, 95 days to complete a nesting cycle.

Life Cycle

Once hatched, young PGBHs develop rapidly. Competition for food makes feeding time a raucous affair. Often only the strongest survive. Falling or being pushed out of the nest is a fatal happening as the parents will not feed young outside the nest. At 8 weeks the young birds attempt to fly clumsily from one tree to another but return to the nest to be fed. At around 10 weeks the young become independent.  Fewer than 25% of juveniles survive to their second year. PGBH mate at 2 years old and choose a new mate every breeding season. Great Blue Herons live long lives, some as long as 17 years. 

Threats and Predators 

Great Blue Herons are particularly sensitive to human disturbance. The noise and activity of humans as far away as 200 metres from a colony can cause birds to abandon nests. This leaves eggs and nestlings unguarded and vulnerable to predators. 

In addition, most of B.C.’s human population live in the heron’s favourite areas – the coast and the southern interior. Land clearing, marsh draining and urban development in these regions is limiting the number of available nesting sites and foraging areas. The human threat is twofold: disturbance that interrupts the reproduction cycle and habitat loss.

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The predator most likely to attack a heron nest or to take advantage of an unguarded nest is the bald eagle. Predation and the associated disturbance eagles cause result in significantly higher nest and colony abandonment. Although bald eagle numbers were in serious decline, there has been a robust resurgence in their numbers mainly attributed to a 1972 ban on the pesticide DDT. Eagle populations on the south coast have increased since the mid-1980s and the rate of attacks on nesting herons has more than doubled over the same time period (Vennesland and Butler 2004).

Population Numbers….how are Pacific Great Blue Herons doing?

Ross Vennesland, a Senior Species at Risk Biologist with Environment and Climate Change Canada, Pacific Region, very kindly provided several information sources for this article. Having studied herons since the 1990s, he is quoted in 2010 as saying, “bird counts indicate the population of Pacific Great Blue Herons is in a slow decline, and fewer than 10,000 remain around the “Salish Sea,” a name applied to the inland waterways of Washington and B.C. For reasons that aren’t clear  the average number of surviving young herons produced per nest each year has dropped from about two in the 1980s to about one now – a level that might be near the minimum needed to sustain the species.”

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Because of the Great Blue Heron’s sensitivity to human activity and its declining population, it has been placed on B.C.’s Blue List of vulnerable species.

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The Pacific Great Blue Heron was assessed as Special Concern in 1997 and again in 2008 by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) due to a small population size, declining productivity, threats related to Bald Eagle predation, habitat loss, and human disturbance. The species was listed on Schedule 1 of the Species at Risk Act (SARA) in 2010.