gb_heron

English

Great Blue Heron
Ardea herodias
This information was scanned from The Birds of British Columbia (Campbell et al.), Volume I, pages 236-241. Volumes I, II and III of The Birds of British Columbia can be ordered electronically at: orders@ubcpress.ubc.ca from UBC Press in Vancouver, British Columbia.

Range
Great Blue Heron breeds from southeastern Alaska and coastal British Columbia east to the Gaspe Peninsula and Nova Scotia south to Mexico, the West Indies and Galapagos Islands. Post-breeding dispersal occurs north into nonbreeding areas. Winters from coastal British Columbia, central United States, and southern New England south to northern South America.

Status
Sedentary or migratory. Resident on the coast, where it is very common in the south, common in the north and fairly common on the Queen Charlotte Islands. Fairly common resident in the southern interior where waters do not freeze. Widespread breeder in southern British Columbia; local breeder on north coast.

Status Change
No change.

Nonbreeding
The Great Blue Heron is widely distributed throughout the year along the coast, including Vancouver Island and the Queen Charlotte Islands, and throughout the interior south of latitude 52°N. Occurrences elsewhere probably represent nonbreeding wanderers.

The Great Blue Heron is found in a variety of salt, brackish, and freshwater environments. On the coast, it frequents sheltered and shallow bays, lagoons, inlets, coves, tidal mud flats, sloughs, marshes, rivers, and irrigation ditches. Jetties and log booms are frequently used as communal roosting sites. Solitary birds and small groups often roost in coniferous trees. In the interior, herons may be seen around lakeshores, rivers, sloughs, marshes, and ponds. On the south mainland coast, southeastern Vancouver Island, and occasionally in the interior, wet and dry agricultural fields are used for foraging. Birds have been recorded from sea level to 2,100 m elevation.

There is little information available on seasonal movements or fluctuations in abundance. Data suggest spring migrants arrive at colonies in March and April. Nonbreeding birds probably disperse northward in May and June. In late July and August, there is a further dispersal of herons, probably comprised of young dispersing from breeding colonies in British Columbia and the northern United States. For example, 16 recoveries from young herons banded in Vancouver (University of British Columbia Endowment Lands colony) were remarkably scattered: 9 were found in the Fraser Lowlands, 3 were found in Washington, 3 were found in the interior of British Columbia as far north as Kamloops, and 1 was found at Astoria, Oregon (Campbell et al. 1972b). Migrants depart southward in September and October. The Fraser River delta is the major wintering area in the province. The largest wintering populations in the interior are found in the Okanagan valley.

Herons are most numerous on the coast during the summed months when foraging aggregations from nearby colonies maybe approach 300 individuals. In the interior, the largest numbers are found during spring migration, mostly in March.

Breeding

Nests

Eggs

Young

On the coast, the Great Blue Heron breeds primarily along south-eastern Vancouver Island, the southern Gulf Islands, and the Fraser Lowlands, east to Hope. Isolated pairs breed nearby Prince Rupert and on the southern Queen Charlotte Islands.

There are no other documented breeding records for the northern mainland coast, and none from northern or western Vancouver: Island. In the interior, the heron breeds from the Okanagan valley north to Clearwater, in the west Kootenay north to New Denver: and in the east Kootenay north to Golden. The centres of abundance are the Fraser Lowlands, southeast Vancouver Island, and the southern Gulf Islands. Breeding occurs at elevations between sea level and 1,100 m.

Colonies are usually located in mature forests (deciduous, coniferous, or mixed) that are relatively free from disturbance and near suitable foraging areas. Occasionally, colonies can be found in areas of high disturbance (e.g. Stanley Park, Vancouver). On the coast, colonies are usually situated on islands or mainland sites near tidal mud flats. Inland colonies are usually situated on the banks of slow-moving rivers, sloughs, or marshy lakes. Colonies can be ephemeral, abandoned one year only to be used the next. A colony at Brisco was abandoned in 1979 when an Osprey used one of the nests, and ably new colony was built about 500 m south. In 1980, the herons returned to the old site.

Forbes et al. (1985a) list 110 breeding sites in British Columbia: through 1984, of which 76% were still active when last visited. Eighty-four sites were along the coast; the remaining 26 were east of the Cascades. Sizes of 95 colonies ranged from 1 to 169 nests (1 to 10 nests-44, 11 to 20 nests-14, 21 to 30 nests-11, 31 to 50 nests-11, 51 to 100 nests-10, 101 to 169 nests-5).

Heronries may occupy up to 2 ha of forest (e.g. University of British Columbia Endowment Lands. Up to 39 nests may be situated in a single tree; 1 colony contained nests in 83 individual trees.

Nests:

Nest-building begins in March on the coast (27 Mar 1984, Derby Reach) and in April in the interior (21 Apr 1984, Nelson). Nests were large stick platforms (n=93) up to 1 m in diameter and 38 cm in depth. The nest cup was a shallow depression lined with small twigs, bark strips, fresh evergreen boughs, or rushes. Fifteen species of trees were reported as nest sites. On the coast, red alder (33%) was the most often used nest tree; black cottonwood (70%) was the main species used in the interior (Forbes et al. 1985a). The most frequently used conifer was Douglas-fir. Other tree species used on the coast included big-leaf maple, arbutus, western hemlock, western red-cedar and Sitka spruce. In the interior, poplar, western white pine, and ponderosa pine were also used as nest trees. Heights of 926 nests ranged from 7 to 70 m, with 67% recorded between 17 and 30 m.

Eggs:

Dates for 137 clutches ranged from 1 April to 2 July, with 66% recorded between 16 April and 10 June. Calculated dates indicate that eggs could be found as early as 23 March. Clutch size ranged from 1 to 8 eggs (1E-12, 2E-26, 3E-24, 4E-39, 5E-28, 6E-6, 7E-1, 8E-1), with 66% having 3 to 5 eggs. Incubation period is 25 to 29 days (Harrison, C. 1978).

Young:

Dates for 583 broods ranged from 20 April to 31 August, with 55% recorded between 2 and 20 June. Brood size ranged from 1 to 8 young (1Y-89, 2Y-271, 3Y-137, 4Y-76, 5Y-2, 6Y-5, 7Y-1, 8Y-2), with 70% having 2 to 3 young. In southwestern British Columbia, 15 colonies fledged 2.5 young per successful nest (1977 to 1981), and in the southeast, 6 colonies fledged 2.7 young per successful nest (1981 to 1983; Forbes et al. 1985b). Fledging period is about 60 days (Harrison, C. 1978).

Remarks
Prior to 1947, the Great Blue Heron was known to breed in coastal areas of the province, but only at 2 breeding sites in the interior (see Cannings, R.A. et al. 1987). By 1974, 6 interior sites were documented by Mark (1976), and a decade later 26 colony sites were known (Forbes et al. 1985a). Numbers appear to be increasing throughout southern areas of the interior but the magnitude of the increase is not known. Even in the early 1920s, Brooks (Bent 1926:114) reported an increase there. It could be that the recent increases are more apparent than real and are due mainly to better coverage by more observers.

Recently, high levels of organochlorine residues have been found in egg contents from heronries on the south coast and: Vancouver Island. Of particular concern were the elevated leveled of dioxins. In 1987, biologists found every egg destroyed beneath the nests of a colony adjacent to an industrial area near Crofton, which also held the highest dioxin levels of the 5 heronries sampled. While predation had not been ruled out, the elevated dioxin levels are a matter for concern. Studies are now being planned that will assess the embryotoxic effects of dioxins in heronries in south coastal British Columbia (see Elliott et al. 1983).

The Great Blue Heron appears on the "Blue List" for 1980 and 1981 (Fate 1981). In 1982, the species was delisted, but maintained as a species of "special concern" (Fate and Tate 1982).

Two subspecies are recognized in British Columbia (American Ornithologists' Union 1957). A. h. fannini breeds along the coast, while A. h. herodias breeds in the interior.

Forbes et al. (1985a) discuss a number of recommendations for management of the Great Blue Heron in British Columbia.

POSTSCRIPT: On 9 May 1989, a colony with 9 active nests was found at Tahsis - the first reported colony on the west coast o f Vancouver Island. It was situated in red alder trees within 0.5 km of the Tahsis shopping area (A.M. Breault pers. comm.).