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BC Conservation Data Centre: Conservation Status Report

Ardea herodias fannini
Great Blue Heron, fannini subspecies

Scientific Name: Ardea herodias fannini
English Name: Great Blue Heron, fannini subspecies
Provincial Status Summary
Status: S3B,S4N
Date Status Assigned: March 25, 2022
Date Last Reviewed: March 25, 2022
Reasons: This subspecies is distributed along the coast of British Columbia with a relatively small population that is concentrated at a few breeding colonies in southern British Columbia. Population fluctuates but is thought to be more or less stable, however, threats from habitat loss, particularly breeding and foraging habitat, due to development, human disturbance, and eagle predation are ongoing, particularly in the southern part of the range where concentrations of birds are highest; recent threats from intensification of agriculture practices in the Lower Fraser Valley that are expected to continue and potentially expand onto Vancouver Island may also impact winter foraging habitat. Expansion of dykes, and rising sealevels are also considered to be threats in the near future.
Range Extent: G = 200,000-2,500,000 square km
Range Extent Estimate (km2): 274,216 square km
Range Extent Comments: 274,216 square km (COSEWIC in prep) Breeding on the coast is primarily in the Georgia Depression (south-eastern Vancouver Island, southern Gulf Islands, and the lower Fraser Valley, east to Hope) and isolated pairs and small colonies along the central and northern mainland coasts and Haida Gwaii (Campbell et a. 1990; COSEWIC 2008). Non-breeding birds are more widespread: mainly found along the coast, wandering occasionally to the interior. Major winter concentrations for this subspecies occur on the Fraser River delta (Campbell et al. 1990). Extent of occurrence for the entire Canadian range from GIS analysis is 244,000 km2 (COSEWIC 2008)
Area of Occupancy (km2): G = 501-2,500
Area of Occupancy Estimate (km2): >2000 km2
Area of Occupancy Comments: Initial minimum estimate of 1320 km2, likely exceeds 2000 km2 (COSEWIC in prep).
Occurrences & Population
Number of Occurrences: C = 21 - 80
Comments: Surveys completed along coastal British Columbia in 2005 found 1943 nesting pairs in 49 colonies (COSEWIC 2008). Most (46 of 49) of the colonies occurred in the Georgia Basin and represented 1833 nesting pairs; 68% of which were concentrated in 6 colonies of > 100 nesting pairs each. Four of these 6 colonies were located in the lower Fraser Valley, while the other 2 were located on southern Vancouver Island and the southern Gulf Islands. However, even large (> 100 nesting pairs), long-established heron colonies regularly relocate or become abandoned; the colony at Pacific Spirit Park was abandoned in 2004, while the Point Roberts colony was abandoned in 2003 and re-colonized at a new site in 2004 (COSEWIC 2008). Although regular surveys have only occurred in the core of the range in the Strait of Georgia, nesting sites have been documented across the range of the subspecies (Butler 1997). Colony size tends to be greater in the southern part of its range; in the northern part of its range, birds tend to nest in smaller colonies (< 10 nests) or individually. On Haida Gwaii, as of 2008, there were 24 known nests, including historic records (Dyment 2006; Chytyk et al. 2008). Most occurrences on Haida Gwaii were solitary nests; maximum colony size was 4 nests (Dyment 2006). Results from 2006 surveys on Haida Gwaii found only 2 active nests (Dyment 2006).
Number of Occurrences with Good Viability / Ecological Integrity: C = 4 - 12
Comments: Unspecified (COSEWIC 2008). However, colonies with > 100 nesting pairs probably have good viability, due to the significant positive correlation between colony size and higher productivity found at monitored colonies in the Georgia Basin (COSEWIC 2008). In 2005, there were 6 colonies with > 100 nesting pairs along coastal British Columbia (COSEWIC 2008). However, due to the temporal nature of colonies and their frequent habit of moving or being abandoned, current viability is difficult to determine and project into the future.
Number of Occurrences Appropriately Protected & Managed: C = 4 - 12
Comments: Seven colonies were located in protected lands and accounted for 14% of the 49 known active colonies in 2005 (COSEWIC 2008). Protected colony size ranged from 2 to 222 nesting pairs and represented 37% of the documented nesting pairs (721 of 1943 nesting pairs). Protected colonies include 4 in municipal parks, 1 in a provincial wildlife management area, 1 on a local conservation land, and 1 in a municipal conservation area. However, due to the dynamic nature of colonies frequently re-locating, currently protected colonies, in many cases, should be considered temporal. Under the Forest and Range Practices Act (FRPA), nesting colonies may be afforded protection through the establishment of Wildlife Habitat Areas (WHAs). No WHAs have been established for the coastal subspecies up to and including 2009. FRPA Section 7 Orders also afford some protection for colonies by setting objectives specifying amounts and distribution of areas for survival of the species. Licensees are required to prepare results and strategies in their Forest Stewardship Plans to address these objectives. Section 7 Notices for the Campbell River and North Island - Central Coast Forests Districts have set objectives for amounts of 160 ha and 240 ha, respectively, for protection of the subspecies. Nests, eggs, young and adults are protected from hunting by Article II:3 of the Migratory Birds Convention Act (1994) and Section 5(4) of the Migratory Birds Regulations. As well, the nest and nest trees are protected year-round by Section 34 of the Wildlife Act.
Population Size: E = 2,500 - 10,000 individuals
Comments: COSEWIC (in prep) estimates the current population between 5200 and 5940 mature individuals. In 2018, some known long-term colonies are now known to have been abandoned. There haven?t been updated population numbers. T. Note: Dyment (2006) estimated the Haida Gwaii population to be 100-175 adults. Population size is difficult to estimate since colony location and size fluctuate regularly (Butler 1997). . Extrapolating these densities across available coastal shorelines provide liberal population estimates of 200 nesting adults for Haida Gwaii (P. Dyment, pers. comm. cited in COSEWIC 2008) and 900 nesting adults on the mainland coast outside the Strait of Georgia (COSEWIC 2008). Survey data for the Strait of Georgia are more robust, with colony counts being conducted intermittently as early as 1920, and more regularly from 1970 to the present (Butler 1997; COSEWIC 2008, COSEWIC in prep).
Threats (to population, occurrences, or area affected)
Degree of Threat: BC = High - medium
Comments: NOTE 2021 Threats Assessment is available and should replace this one. From 2018-01-09 TC: Generation time 5.6 yr (COSEWIC 2008) therefore 3 gens = ~17 years used to score severity and timing. Estimated ~4766 birds in 2005: 1833 active nests x 2 = 3666 birds (77%) Strait of Georgia; 200 (4%) Haida Gwaii; 900 (19%) outside Strait of Georgia; 68% of the 1833 active nests (=46 colonies) concentrated in 6 colonies of > 100 pairs each, 4/these 6 in lower Fraser Valley [largest at Tsawwassen], 1 on southern Van. Island 1 on southern Gulf Islands; ECCC (2016): ~60% of pop occurs in Lower Fraser Valley, nesting habitat is limited but not limited elsewhere; population trend is uncertain; decline in nesting productivity in COSEWIC (2008) has intensified (ECCC 2016), especially in Lower Fraser Valley; winter counts suggest 39% decline over last 3 generations although overall population apparently stable. ECCC (2016): overall medium )1 medium (8.2); 5 low (1,2,4,5,6) [must have been adjusted down]. Distributional abundance used to score scope during teleconference: 50% Lower Fraser Valley, 40% Vancouver Island, 10% elsewhere including Haida Gwaii; further 12-15% of Canadian population (= ~ 400 nests) at Tsawwassen.

The major threats to Great Blue Heron, fannini ssp include nest failure and reduced productivity caused by Bald Eagle predation, human disturbance and habitat declines from development (COSEWIC 2008). With the detailed threats assessment that was completed in 2018, there have not been substantial changes to the major threats.

A threat calculator was completed in 2018 using the following parameters: Generation time 5.6 yr (COSEWIC 2008) therefore 3 generations = ~17 years used to score severity and timing with the following distributional abundance used to score scope during teleconference to complete the calculator: 50% Lower Fraser Valley, 40% Vancouver Island, 10% elsewhere including Haida Gwaii; further 12-15% of Canadian population (= ~ 400 nests) at Tsawwassen.

Bald 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 this period (Vennesland and Butler 2004). There is interesting current research that herons may be adapting by nesting within 200m of eagle nests to ensure protection from 'outside' raiding eagles (Jones et al. 2013). For both subspecies of herons across British Columbia, 17 of 27 heron colony abandonments were a result of human activity near the colony site, including: tree cutting, flooding, vehicle use and researcher activity (Forbes et al. 1985). However, some coastal heron colonies have become accustomed to routine human activities. Herons nesting in Stanley Park in Vancouver and Beacon Hill Park in Victoria appear habituated to frequent pedestrians and vehicles near their nests (Butler 1997; Vennesland 2000). . A human population around the Strait of Georgia increases, it is likely that the amount and the quality of available heron habitat in the Georgia Basin will decrease (Gebauer and Moul 2001). Additionally, herons are sensitive to exposure to environmental contaminants and pollutants that can lead to breeding failure (Elliot and Noble 1993).
Trend (in population, range, area occupied, and/or condition of occurrences)
Short-Term Trend: FG = Decline of <30% to relatively stable
Comments: Christmas Bird Count data suggests a decline of 25% over the last 3 generations, but Coastal Bird Survey and Breeding Bird surveys suggest that the population is more or less stable and the CBC trends can be explained by increased observer effort that doesn't result in increased birds counted (literature reviewed in COSEWIC in prep). Reasons for the differences in trends seen in Christmas Bird Count trends and Coastal Bird Survey trends are discussed in COSEWIC (in prep). Nesting productivity has increased from low levels (Environment Canada 2016) seen in the early 2000's (COSEWIC in prep).

From a 2010 review: Population trend data are variable (COSEWIC 2008). Some datasets show declines (some nesting productivity from colony surveys, Christmas Bird Counts (CBC)), others show stability (some nest counts from colony surveys, Breeding Bird Surveys (BBS), and in one case an increase (Coastal Waterbird Surveys). Although colony survey data were collected intermittently and, at times, inconsistently among years, nesting productivity at monitored colonies in south-coastal British Columbia showed a significant decline from 1971-2006, for both productivity per active nesting attempt and productivity per successful nesting attempt (COSEWIC 2008). From the 1970s to 2005, colony occupancy appears to be generally stable for coastal British Columbia (COSEWIC 2008), although some population retraction has occurred along the margins of the Strait of Georgia, particularly the Sunshine Coast (Butler 1997; Vennesland 2000) and possibly north-eastern Vancouver Island due to low productivity (Chatwin et al. 2006). CBC data show a significant 19% decline in herons observed per party hour from 1991-2005 (over 3 generations), along coastal British Columbia (COSEWIC 2008). Additional CBC analysis for the same period found a small significant increase of herons observed per party hour for the lower Fraser Valley, and significant declines for both Vancouver Island and the Sunshine Coast (COSEWIC 2008). BBS data indicated a significant decline of 5.7% in herons along coastal British Columbia from 1966-1994 (Downes and Collins 1996). However, results from one BBS census route were disproportionately contributing to this decline, and if removed from the dataset, the downward population trend was no longer significant (B. Smith, unpubl. data; cited in COSEWIC 2008). CWS data show a significant increase in herons along coastal British Columbia during the winters from 1999/2000 to 2003/04 (6.9% increase per winter) (Badzinski et al. 2005). Both CBC and CWS surveys are conducted during the winter and their data seem to contradict one another; however, the CWS data are from a shorter 5-year period that corresponds with a period of increasing heron numbers in the CBC data (COSEWIC 2008). More recent analysis of the Coastal Waterbird Survey suggest little net change, with relative stability during the past three generations overall. Ethier et al.
(2020) found a significant decline in numbers of Pacific Great Blue Heron recorded on the outer coast of British Columbia (from Victoria north to the Alaska border) from 1999 to 2019 (-6.76%/year; CI: -11.75 to -1.78), but given the relatively sparse numbers on the outer coast, this result would have only a very small impact on the overall numbers in BC. Conversely, there was a nonsignificant positive trend for the inner waters of the Salish Sea, where a most of the BC population occurs (1.41%/year; CI: -2.37 to 6.40) (Ethier et al 2020).
Long-Term Trend: G = Relatively Stable (<=10% change)
Comments: Christmas Bird Count data (1970-2019) suggest the population is relatively stable (Meehan et al. 2020).
Other Factors
Intrinsic Vulnerability: Rank Factor not assessed
Environmental Specificity: Rank Factor not assessed
Other Rank Considerations: This subspecies is non-migratory and confined to the northeast Pacific coast. Human population in this area is expected to continue to increase, potentially at an even faster rate, as migration from elsewhere continues westward into southern and coastal BC. Concomitant development and disturbance will continue. Most recent threats assessment (2021) indicates increasing threats to the species.
Information Gaps
Research Needs: Vennesland and Butler (2020) list important research topics for the species as a whole. Research on coastal Bald Eagle populations should continue from Jones et al. (2013) to continue investigation of impacts on heron colonies and heron adaptation, including: effects on nest productivity, colony viability, and mortality rates. Impacts from intensification of agriculture practices in the Lower Fraser Valley (e.g, increase greenhouse construction, laying grow clothes across fields for crops such as corn, strawberries and blueberries) should be investigated;.Research into the effects can lead to management recommendations that can form guidelines in permitting, where possible.
Inventory Needs: Larger colonies should be monitored annually to detect trends in population and reproductive success. Other colonies and winter concentrations should be censused on a regular basis using RISC standardized methods to monitor population trends and persistence of breeding colonies. The BC Breeding Bird Atlas (Butler and Vennesland 2015) helped inventory the breeding occurrences in BC (COSEWIC in prep). Encouraging subsequent atlas projects to proceed on time would be helpful in monitoring trends in occurrences.
Protection: Due to the temporal nature of some colonies, efforts to establish permanent protected areas around larger colonies, in some cases, may not be effective. Implementation of Best Management Practices around known colonies may be more effective or practical. For large, persistent colonies on Crown land, the establishment of Wildlife Habitat Areas may be effective at protecting nesting and/or foraging habitats. Predator management, specifically for Bald Eagle deterrence around vulnerable colonies, should be investigated.
Management: Managment Plan was published in 2016 (Environment Canada 2016). Human disturbance should be minimized at breeding colonies. Activities that negatively impact nesting herons should be avoided near breeding colonies during the six month period of March to August. Guidelines to reduce human disturbance near heron colonies (Summer 1996) should be implemented (Butler 1997). Significant tidal flat and old-field foraging sites should be protected from development by acquisition or long-term stewardship agreements with nearby landowners. Eggshells should be collected regularly from selected colonies and assessed for toxic levels of environmental contaminants. Levels of pollutants should be monitored periodically in conjunction with data on reproductive success (Butler 1992). Coastal eagle populations should be monitored to help assess predation impacts (COSEWIC 2008, in prep).
Author: Chytyk, P and A. VanWoudenburg, updated by D.F.Fraser
Date: January 17, 2022
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Butler, R.W. 1992. Great Blue Heron (Ardea herodias). in A. Poole, P. Stettenheim, and F. Gill, eds. The Birds of North America, No. 25. Acad. Nat. Sci., Philadelphia, PA. and Am. Ornithol. Union, Washinton, DC. 20pp.
Butler, R.W. 1997. COSEWIC status report on Pacific Great Blue Heron, Ardea herodias fannini. Comm. on the Status of Endangered Wildl. in Can. 18pp.
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Butler, R.W. 1997. The Great Blue Heron: a natural history and ecology of a seashore sentinel. UBC Press, Vancouver, BC. 167pp.
Butler, R.W. 1999. Status of the subspecies of Great Blue Heron, Ardea herodias fannini, in Canada. DRAFT. Comm. on the Status of Endangered Wildl. in Can. Available from Can. Nat. Fed. Ottawa, ON.
Campbell, R.W., N.K. Dawe, I.McT. Cowan, J.M. Cooper, G. Kaiser, and M.C.E. McNall. 1990. The Birds of British Columbia, Vol. 1. Nonpasserines: Introduction, Loons through Waterfowl. Royal B.C. Mus. in association with Environ. Can., Can. Wildl. Serv. 514pp.
Cannings, R.J. 1998. The Birds of British Columbia - a taxonomic catalogue. B.C. Minist. Environ., Lands and Parks, Wildl. Branch, Victoria, Wildl. Bull. B-86. 266pp.
Downes, C.M., and B.T. Collins. 2007. Canadian Bird Trends Web site Version 2.2. Canadian Wildlife Service, Environment Canada, Gatineau, Quebec, K1A 0H3.
Elliot, J.E., and D.G. Noble. 1993. Chlorinated hydrocarbon contaminants in marine birds of the temperate North Pacific. Pages 241-253 in K. Vermeer, K. Briggs, K. Morgan, and D. Seigel-Causey, eds. The status, ecology and conservation of marine birds in the North Pacific. Can. Wildl. Serv. Spec. Publ., Ottawa, ON.
Forbes, L.S., K. Simpson, J.P. Kelsall, and D.R. Flook. 1985. Great Blue Heron colonies in British Columbia. Unpubl. rep., Can. Wildl. Serv. Delta, BC. 78pp.
Moul, I.E. 1998. The location and status of heron colonies around the Strait of Georgia: Region 2, Lower Fraser Valley, Sunshine Coast and the Powell River Area. Unpubl. contract rep. B.C. Minist. Environ., Lands and Parks, Nanaimo, BC. 146pp.
Resource Inventory Committee. 1997d. Standardized Inventory Methodologies for Components of British Columbia's Biodiversity: Colonial Freshwater Nesters, version 1.1. Prepared for the Resour. Inventory Comm., B.C. Minist. Environ., Lands and Parks, Victoria, BC.
Simpson, K., J.N.M. Smith, and J.P Kelsall. 1987. Correlates and consequences of coloniality in Great Blue Herons. Can. J. Zool. 65:572-577.
Summers, K. 1996. Management Guidelines for Great Blue Heron Colonies in British Columbia. English 301 (Technical Report Writing) assignment. Univ. B.C., Vancouver, BC.

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Suggested Citation:

B.C. Conservation Data Centre. 2022. Conservation Status Report: Ardea herodias fannini. B.C. Minist. of Environment. Available: (accessed Apr 22, 2024).