Great blue heron (fannini subspecies) COSEWIC assessement and update status report: chapter 2


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Great Blue Heron
Ardea herodias fannini

Fannini subspecies

Species information

The Great Blue Heron, Ardea herodias, is the largest wading bird in North America, standing over 1 m in height. On the coast of British Columbia the subspecies, Ardea herodias fannini, referred to as the Pacific Great Blue Heron in this report, resides year round. This subspecies is non-migratory and isolated in part by high mountain ranges to the east and a slightly earlier breeding season, compared to more continental herons. The Pacific Great Blue Heron is darker plumaged, smaller in size and has a smaller clutch size than continental herons.


Distribution

The Great Blue Heron breeds across most of North America south of Alaska, and on the Galapagos Islands. The non-breeding distribution is south of freezing areas in the north, to as far south as Panama. The distribution of the Pacific Great Blue Heron is confined to the Pacific Coast from Prince William Sound, Alaska south to Puget Sound, Washington, where it resides year-round.


Habitat

The Pacific Great Blue Heron forages along the seacoast, in fresh and saltwater marshes, along rivers and in grasslands. Smaller numbers of herons forage in kelp forests, from wharves and at anthropogenic waterbodies (e.g., ornamental ponds and fish farms). Most herons nest in woodlands near large eelgrass (Zostera marina) meadows, along rivers, and in estuarine and freshwater marshes. Nesting colony locations are dynamic, especially in areas of high disturbance. Some colonies are used for many years, but most colonies and especially those with fewer than 25 nests, are relocated every few years. In autumn, juvenile herons occupy grasslands on the Fraser River delta and southern Vancouver Island, and adults occupy estuarine marshes, riverine marshes and grasslands.

The size of Great Blue Heron populations is correlated with the area of foraging habitat available locally, and consequently the largest concentrations of Pacific Great Blue Herons occur around the Fraser River delta where extensive mudflats and eelgrass beds provide abundant foraging locations. Local declines in foraging habitat likely have been greatest in south-coastal British Columbia because most of the province’s human population is located in this area. Further, the magnitude of use of some foraging locations currently may be limited by the amount of suitable nesting habitat that remains undeveloped.

Suitable tall trees as nesting habitat near foraging areas have declined in some parts of British Columbia over the past century due to increases in the size of human populations and industry. Especially hard hit is south-coastal British Columbia and especially the lower Fraser Valley, where the human population is large and still growing. In this region, nesting habitat might be limiting the size of the heron population. Habitat destruction in south-coastal British Columbia has resulted in the abandonment of at least 21 colonies (from 1972 to 1985 and from 1998 to 1999).


Biology

In springtime, most herons gather in colonies where they court, nest, and raise young. The principal diet is small fish during the breeding season augmented with small mammals in winter. Typically four eggs are laid and less than two chicks on average reach the fledgling stage and leave the nest to become juveniles. Fewer than 25% of juveniles survive their first winter, after which survival increases to about 75% per year for adults. Nests are generally in trees and are made using large sticks.


Population sizes and trends

Population size has been difficult to estimate for the Pacific Great Blue Heron because colonies are not stable and are difficult to track in a standardized fashion. The best available estimates suggest that the Pacific Great Blue Heron population size in Canada is 4000-5000 nesting adults. The global population of the Pacific Great Blue Heron is likely between 9 500 and 11 000 nesting adults.

Christmas Bird Count data show population declines over the past three generations, while Coastal Waterbird Surveys show increases over a recent five-year period. Colony surveys suggest that productivity has declined significantly since the 1970s.


Limiting factors and threats

Declines and other issues with productivity and population size are thought to primarily be due to Bald Eagle predation, human disturbance and destruction of nesting and foraging habitat. The projected doubling in the human population in the next 30 years in the core of the range threatens to exacerbate the human disturbance problem and habitat loss. In addition, the influence of predators may be reducing habitat quality by causing herons to move to new, and ever more limited, sites.


Special significance of the species

The Pacific Great Blue Heron has high public appeal as a symbol of wetland conservation and environmental quality.


Existing protection or other status designations

All Great Blue Herons are protected from hunting and molestation by the Migratory Birds Convention Act, Migratory Bird Regulations and the British Columbia Wildlife Act. Both subspecies of Great Blue Heron inhabiting British Columbia are at present on the provincial ‘Blue List’ compiled by the British Columbia Ministry of Environment. The Pacific Great Blue Heron was listed in 1997 as Special Concern by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC).

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