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BC Conservation Data Centre: Conservation Status Report

Ardea herodias herodias
Great Blue Heron, herodias subspecies

 
Scientific Name: Ardea herodias herodias
English Name: Great Blue Heron, herodias subspecies
   
Provincial Status Summary
Status: S3?
Date Status Assigned: May 15, 2017
Date Last Reviewed: March 15, 2017
Reasons: This subspecies is distributed through the central and southern interior of British Columbia with a relatively small population that is concentrated at scattered colonies in the Central Interior, Southern Interior, and Southern Interior Mountains ecoprovinces. Current population trends are not known with certainty. Threats from eagle predation, habitat loss and human disturbance are ongoing.
 
Range
Range Extent: F = 20,000-200,000 square km
Range Extent Comments: Breeding in the interior is primarily along the Kootenay River, in the Okanagan Valley, and in the Princeton/Merrit/Kamloops areas, with additional records along the Columbia River, Shuswap Lake, the Thompson and North Thompson rivers, and the Cariboo-Chilcotin (Gebauer and Moul 2001; Butler and Vennesland 2015).  Potential breeding records exist up to Prince George and the Peace River area, though the latter likely represent birds originating from Alberta (Butler and Vennesland 2015).  The majority of interior herons occur in the Southern Interior and Southern Interior Mountains ecoprovinces, with fewer records through the Central Interor, Sub-boreal Interior, and Boreal Plains ecoprovinces (Butler and Vennesland 2015). Non-breeding birds occur in many of the same areas, though are restricted in winter to ice-free areas, and are thus more frequent in select areas in the southern portion of their overall range, including the Okanagan Valley, Columbia River, and Kootenay River. Winter distribution is localized, for example along entire Kootenay and Columbia River systems the majority of birds can be found in the vicinity of Creston and Castlegar (Hentze 2017). The extent of occurrence is estimated based on breeding locations, and will be greater if breeding is confirmed in northern areas. 
Area of Occupancy (km2): EF = 26-500
Area of Occupancy Comments: Area of occupancy is uncertain. Gebauer and Moul (2001) list 47 interior nesting locations since 1941. In 2008 a total of 20 active breeding sites were documented in the West and East Kootenays (Machmer 2009). The BC Breeding Bird Atlas documented 51 squares with confirmed breeding, 16 squares with probable breeding, and, 143 squares with possible breeding (Butler and Vennesland 2015). Area of occupancy is estimated using colony locations as a proxy for the number of 4-square km grid cells. Winter occupancy coincides with breeding range, but is contracted relative to summer distribution.
 
Occurrences & Population
Number of Occurrences: CD = 21 - 300
Comments: Dedicated colony surveys have not been completed across the entire BC interior range. The BC Breeding Bird Atlas found 51 squares with confirmed breeding, with a roughly equal split in confirmed nest squares between the Kootenay and Columbia River drainages together, and the southern interior west of the Columbia system (e.g., Okanagan Valley). Machmer (2009) lists 20 active colonies (as of that year) in the East and West Kootenays, among 43 total known colony locations (historic and active). Those 20 colonies represented 324 active nests (137 in the West Kootenays and 187 in the East Kootenays) with a mean colony size of 16.2 ± 4.8 (SE) nests (Machmer 2009). Colonies are dynamic, with frequent changes in locations and/or sizes.
Number of Occurrences with Good Viability / Ecological Integrity: U = Unknown
Comments: It is not known how many occurrences have good viability. For the coastal subspecies, 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), but few interior colonies reach this size. 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: CD = 4 - 40
Comments: In the Columbia and Kootenay watersheds, fourteen colonies (active or historic) (33%) were located in lands that had some level of protection: 10 in Wildlife Management Areas, 2 in lands managed by the Nature Trust of BC or the Nature Conservancy of Canada, and 1 in a provincial park (Machmer 2009). Machmer (2009) reports that for active nests only, three (17.6%) were located in Wildlife Management Areas, with most nest sites on private land, followed by crown land. An urban colony on private land in Vernon is protected by a conservation covenant. 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). 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: D = 1,000 - 2,500 individuals
Comments: Current population estimates for the entire interior range are not available. Gebauer and Moul (2001) suggest interior populations of 300-700 birds. Machmer (2009) found 324 active nests in 2008 in the Kootenays, suggesting populations of about 650 adult individuals for that region alone. Assuming that the Kootenays represent roughly half of known breeding sites, the interior population is most likely greater than 1000 individuals. There have been no range-wide, systematic surveys of this subspecies in the province, limiting the ability to accurately determine population size.
 
Threats (to population, occurrences, or area affected)
Degree of Threat: C = Medium
Comments: Potential threats in the province, identified following the IUCN categorization, include: 1.1 Housing and urban areas; 1.2 Commerical and industrial areas; 7.1 Fire and fire suppression; and 8.2 Problematic native species/diseases.
 
The major threats to interior heron populations in British Columbia are not known with certainty, but are expected to be related to nest failure and reduced productivity caused by Bald Eagle predation, human disturbance and habitat declines from development. These are the same general threats faced by the coastal subspecies (COSEWIC 2008). Bald Eagle attacks more than doubled in frequency in association with increased Bald Eagle populations on the south coast since the 1980s (Vennesland and Butler 2004). There is evidence that Bald Eagle poulations have also increased in the BC interior, and attacks on nesting birds have been noted (Van Damme and Colonel 2007; Machmer 2009). 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). Some colonies may habituate to human disturbance (Vennesland and Butler 2011), such as the urban colony in Vernon. Heron breeding habitat in the interior is typically along valley bottoms, and as such the quality and availability of habitat is expected to decrease as human populations increase in these areas (Gebauer and Moul 2001). Additionally, herons are exposed to environmental contaminants such as polybrominated diphenyl ethers (PBDEs) which are an emerging threat to heron populations (Elliott et al. 2005).
 
 
Trend (in population, range, area occupied, and/or condition of occurrences)
Short-Term Trend: G = Relatively Stable (<=10% change)
Comments: Population trends across North America are variable, with localized increasing or decreasing trends, and stable populations (Vennesland and Butler 2011). Overall numbers of breeding birds in BC interior appears stable, though declines have been noted over specific periods (Machmer 2009). Some areas in particular may be experiencing declines, such as in the West Kootenays where there was a reduction in active colonies from eight to three over a six-year period (Machmer 2009). Declines on wintering grounds have also been noted, such as around Castlegar, but the lack of continuous data prevent rigorous analysis (Machmer 2003; Hentze 2017). It is also not known if declines represent population reductions, or distribution shifts. Christmas Bird Count data from the region are likewise variable, and no clear trend is obvious. BBS data for the region have low reliability for this species, but for the two Bird Conservation Regions applicable (9 and 10) have annual trends of +2.23% (with confidence limits of -3.62 to +7.37%), and +2.41 (-3.05 to +9.55%) respectively, for the 2002-2012 period.  A region-wide nesting colony survey monitoring productivity is needed to determine whether apparent localized declines are indicative of a regional trend.
Long-Term Trend: I = Increase of >25%
Comments: The interior Great Blue Heron appears to have increased considerably relative to the early 1900s (Gebauer and Moul 2001). There were no known breeding records prior to 1925, and relatively few documented nesting sites until the mid-1980s (Brooks and Swarth 1925; Campbell et al. 1990). Gebauer and Moul (2001) surmise that the interior Great Blue Heron populations are ?much larger than those of the early 1900s?; however, Campbell et al. (1990) caution that increases through the 20th century may have been an artefact of increasing observer coverage and effort. Breeding Bird Survey data over the period from 1970-2012 indicate annual increases of 2.56% (lower and upper limits of -0.067% and +5.48%) for BCR 9 and 2.12% (-0.59% to +4.94%) for BCR 10. An analysis of Christmas Bird Count data during the 1990s for the interior indicated stable populations over that time period (Gebauer and Moul 2001).
 
Other Factors
Intrinsic Vulnerability: B=Moderately vulnerable
Comments: There is very little information available regarding annual adult survival rates or reproductive success and productivity related to interior BC populations. Heron colonies are prone to abandonment after disturbances, especially early in the nesting cycle (Gebauer and Moul 2001; Vennesland and Butler 2011). Even where colony abandonment does not occur, any departure from nests due to disturbance increases the risk of egg or chick predation (Gebauer and Moul 2001). However, Great Blue Heron nesting colonies are dynamic, and birds appear to easily shift colony sites over time, assuming that suitable habitat is retained close to foraging areas (Gebauer and Moul 2001; Machmer 2009; Vennesland and Butler 2011). The average age of a breeding adult in BC has been calculated to be 5.6 years (COSEWIC 2008). Over the period 2002-2008 average annual reproductive success in the Kootenays was calculated as between 1.25 ± 0.27 and 2.18 ± 0.43 chicks per active nest varying by year (Machmer 2009).
Environmental Specificity: C=Moderate. Generalist or community with some key requirements scarce.
Comments: Great Blue Heron distributions, both during breeding and wintering, are highly dependent on access to open water (Gebauer and Moul 2001). As such, heron occurrences are typically along or near lakes, rivers, streams, and other waterbodies. Nesting locations in the interior are typically along the edges of waterbodies, such as lakes, rivers, and other wetlands (Gebauer and Moul 2001). Interior heron colonies have been in both coniferous and deciduous trees. Species of coniferous trees hosting nests in the interior of BC have included lodgepole pine (Pinus contorta), ponderosa pine (Pinus ponderosa), Engelmann spruce (Picea engelmannii) and hybrid spruce (Picea glauca x engelmannii), Douglas Fir (Pseudotsuga menziesii), western larch (Larix occidentalis), western red cedar (Thuja plicata), and western hemlock (Tsuga heterophylla). Deciduous tree use, especially in the southern part of its BC interior range, is often exclusively in black cottonwood stands (Populus balsamifera). Distance to water was a significant predictor of colony productivity on the coast (Knight et al. 2016). Knight et al. (2016) report no association between colony location and human development, but studies elsewhere have documented colonies being further from development than expected by chance (Watts and Bradshaw 1994). During the winter herons occur where access to open water is available. This limits their range relative to their summer extent, and concentrations may occur in specific areas (e.g., Castlegar and Creston in the Kootenays) (Hentze 2017). While typically associated with water, herons will also forage in agricultural fields in the interior where available (Hentze 2017). Roosting locations are typically near foraging spots, and may be in trees (either coniferous or deciduous), along edges of waterbodies (including sewage treatment plants), or on gravel bars or other islets in rivers (Hentze 2017). 
Other Rank Considerations: This subspecies is migratory. Some individuals remain in the interior through the winter where suitable foraging habitat exists. It is not known what proportion of birds migrate or disperse out of the province, or how far they travel.
 
Information Gaps
Research Needs: Research on interior Bald Eagle populations should be conducted to investigate their impacts on heron colonies, including: effects on nest productivity, colony viability, and mortality rates. Human-caused disturbances also need to be investigated for impacts on nest productivity, nest locations, and colony viability. Potential nesting habitat should be mapped with regards to proximity to foraging areas, to identify what proportion of available habitat currently receives some level of protection, as colonies are dynamic. Migratory pathways and connectivity between nesting colonies and wintering areas are currently unknown. Research also needs to focus on the availability of fish prey relative to water management practices downstream of hydro dams. The impacts of contaminants and pollutants in the region should be monitored for Great Blue Herons in conjunction with nesting productivity data.
Inventory Needs: Region-wide monitoring programs should be implemented to monitor population trends and persistence of breeding colonies. Some restricted-area monitoring programs have been effectively carried out (e.g., Machmer 2009), and these should be continued and expanded to cover at least the Okanagan Valley, Thompson and North Thompson Rivers, Shuswap Lake, and Kamloops/Merritt areas. Larger colonies should be monitored annually to detect trends in population and reproductive success. Recent surveying of breeding and wintering areas has been focused on the West and East Kootenays. Complementary studies should be conducted elsewhere in the BC range. Locations outside of these areas where ?Probable? nesting has been identified based on the BC Breeding Bird Atlas or other sources (e.g., Peace River lowlands), should be formally surveyed to confirm if nesting is occurring.
 
Stewardship
Protection: Due to the temporal nature of some colonies, efforts to establish permanent protected areas around 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. On private land the establishment of conservation covenants can be considered around active colonies.
Management: Human disturbance should be minimized at breeding colonies. Activities that negatively impact nesting herons should be avoided near breeding colonies during from March to August (Gebauer and Moul 2001). Best Management Practices exist for Great Blue Heron and should be followed (MWLAP 2004).
 
Version
Author: N. Hentze and L. Ramsay
Date: March 17, 2017
 
References
Bevanger, K. 1998. Biological and conservation aspects of bird mortality caused by electric power lines: a review. Biological Conservation 86:67?76.
COSEWIC. 2008l. COSEWIC assessment and update status report on the Great Blue Heron fannini subspecies Ardea herodias fannini in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vii + 39pp.
Elliott, J.E., L.K. Wilson, and B. Wakeford. 2005. Polybrominated Diphenyl Ether Trends in Eggs of Marine and Freshwater Birds from British Columbia, Canada, 1979-2002. Environ. Sci. Technol. 39:5584-5591.
Gayton. D. 2008. Impacts of climate change on British Columbia?s Diversity: a literature review. FORREX Forest Research Extension Partnership, Kamloops, B.C. FORREX Series 23. http://www.forrex.org/sites/default/files/forrex_series/FS23.pdf
Gibbs, J.P. and L.K. Kinkel. 1997. Determinants of the size and location of Great Blue Heron colonies. Colonial Waterbirds 20:1-7.
Harris, M.L., J.E. Elliott, R.W. Butler and L.K. Wilson. 2003. Reproductive success and chlorinated hydrocarbon contamination of resident great blue herons (Ardea herodias) from coastal British Columbia, Canada, 1977 to 2000. Environmental Pollution 121: 207?227.
Hentze, N.T., V.C. Hawkes, D. Robichaud, J. Novoa, and D. Terbasket. 2017. CLBMON-49. Lower Columbia River Effects on Wintering Great Blue Herons. Draft Final Report ? 2016. LGL Report EA3590. Unpublished report by Okanagan Nation Alliance and LGL Limited environmental research associates, Sidney, B.C., for BC Hydro Generation, Water License Requirements, Burnaby, B.C. 74 pp + appendices.
Knight, E.C., R.G. Vennesland, and N.N. Winchester. 2016. Importance of proximity to foraging areas for the Pacific Great Blue Heron (Ardea herodias fannini) nesting in a developed landscape. Waterbirds 39:165-174.
Machmer, M.M. 2003. Evaluation of fall and winter use of Waldie Island by Great Blue Herons: Final Report. Unpublished report. BC Hydro and Power Authority, Burnaby, BC. 21 pp + appendices.
Machmer, M.M. 2009. Great Blue Heron and Bald Eagle Inventory and Stewardship in the Columbia Basin (2008-2009). 2009. Columbia Basin Fish & Wildlife Compensation Program, Nelson, BC. 23 pp + appendices.
Rioux, S., J.-P.L. Savard, and A.A. Gerick. 2013. Avian mortalities due to transmission line collisions: a review of current estimates and field methods with an emphasis on applications to the Canadian electricnetwork. Avian Conservation and Ecology 8: 7. http://dx.doi.org/10.5751/ACE-00614-080207
Vennesland, R.G. 2004. Accounts and Measures for Managing Identified Wildlife - Accounts V. 2004. Great Blue Heron (Ardea herodias). http://www.env.gov.bc.ca/wld/frpa/iwms/documents/Birds/b_greatblueheron.pdf
Vennesland, R.G., and R.W. Butler. 2011. Great Blue Heron (Ardea herodias), The Birds of North America (P.G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/grbher3
Watts, B.D. and D.S. Bradshaw. 1994. The Influence of Human Disturbance on the Location of Great Blue Heron Colonies in the Lower Chesapeake Bay. Colonial Waterbirds 17:184-186.
 

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Suggested Citation:

B.C. Conservation Data Centre. 2017. Conservation Status Report: Ardea herodias herodias. B.C. Minist. of Environment. Available: https://a100.gov.bc.ca/pub/eswp/ (accessed Jul 22, 2026).