Special Status Species
Great blue heron Photo: NOAA / SIMoN Common name: Great blue heron
Scientific name: Ardea herodias
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Range Habitat Movements
Abundance Natural History Threats
Conservation Research Gaps Recommended Actions
References Resources

Listing Status
Endangered Species Act (?)


Not listed

Migratory Bird Treaty Act (?)



California Endangered Species Act (?)


Not listed

California Department of Fish and Game (?)


Special Animal

California Department of Forestry (?)



California Natural Diversity Database (?)



IUCN (?)


Not listed



Not listed

The Audubon Society Watchlist (?)


Not listed

Geographic Range

The Great Blue Heron ranges along the Pacific coast from western Alaska to northern South America and the Galapagos Islands, along the Atlantic coast from Nova Scotia to the West Indies and northern South America, and along the entire coast of the Gulf of Mexico (Figure 1). This species is also found inland in large portions of the continental U.S., southern Canada, southern Mexico and Central America (Figure 1). In California, the Great Blue Heron is fairly common all year throughout most of the state (Figure 2).1 This species may appear on some coastal islands (e.g., the Channel Islands).


The location of nesting colonies can change in response to local conditions. Currently, Great Blue Herons are known to nest at various locations along the Salinas River, at the Elkhorn Slough National Estuarine Research Reserve (ESNERR), and Blue Fish Cove in Pt. Lobos State Reserve. Individuals are observed foraging and roosting in wetland area to the north and south of Elkhorn Slough including: Watsonville Slough, Struve Slough, Bennett Slough, Moro Cojo Slough, Alisal Slough, and Trembladero Slough.2 This species forages in coastal habitats throughout the longitudinal extent of the Sanctuary. Birds from nesting colonies to the north, (e.g., Bolinas Lagoon3), to the east (e.g., San Francisco Bay4), and to the south (e.g., Morro Bay State Park5) of the MBNMS may use Sanctuary resources in the non-breeding season.

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The Great Blue Heron feeds mostly in slow-moving or calm salt, fresh, or brackish water habitats. It forages in a variety of habitats including rocky shores, coastal lagoons, saltwater and freshwater marshes, mudflats, bays, estuaries, along the margins of rivers, lakes, and irrigation canals, and in flooded fields. It also perches on and forages from driftwood and kelp canopies in coastal waters. Foraging occurs in waters less than 30 cm deep.2 This species often roosts on the ground during the day and above ground in secluded tall trees at night.6 Nesting colonies are typically found in groves of large trees - often in the highest branches.4 Preferred nesting habitat is free of human disturbance and mammalian predators and near good foraging areas. This species often nests in mixed colonies with other herons, egrets, and cormorants.


This species forages in many coastal habitats in and adjacent to the MBNMS. Great Blue Herons are frequently observed foraging in tidepools along rocky shores and from floating objects (e.g., kelp beds and driftwood). They are seen foraging in coastal wetlands and even in the agricultural fields in the Salinas Valley. The rookery in the ESNERR is located in a mixed-grove of Monterey pine and blue gum eucalyptus that is next to a freshwater pond. Birds nesting in the ESNERR rookery forage in Elkhorn Slough, in fresh and brackish wetlands to the north and south of the Slough, along the Salinas River, and along the coast of central Monterey Bay.2,7 In Elkhorn Slough, Great Blue Herons are seen foraging in channels, mudflats and eelgrass beds and roosting on elevated structure and in pickleweed flats.2

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Migration and Movements

In parts of their range where food is not available in the winter due to ice cover, Great Blue Herons are migratory. They head south in the fall (mid-September to late-October) and then migrate northward again in the spring (April).6 Populations on the California coast appear to be non-migratory, though some post-breeding dispersal does occur.8,6 In coastal habitats, especially at saltwater marshes, there is an influx of birds beginning in mid-July (presumably post-breeding individuals and fledglings).5 Inland colonies in California, particularly the northeastern parts of the state, may be the source of some of these migrants.5 Numbers decline again prior to the start of the next breeding season as migratory birds leave the area.

From February to July, populations in California are concentrated near nesting colonies. Colonies tend to be situated near shallow-water feeding areas so that long distances do not have to be traversed many times each day. However, Great Blue Herons may travel up to 20 km to reach good foraging habitat.9 After fledging, individuals probably do not return to nesting colonies until the onset of reproduction.6 The number of birds in one nesting colony may vary dramatically from year to year.4 The likelihood that individuals return to the same nest or even the same rookery in subsequent years is not known.


Most adult birds observed in the Sanctuary during the breeding season are year-round residents in central California. In the non-breeding season, the population in the Sanctuary is composed of a mix of resident breeders, immature birds, and post-breeding dispersers. The rookeries to the north and south of the Sanctuary (e.g., San Francisco Bay, Bolinas Lagoon, Morro Bay) are a likely source of individuals into the MBNMS and surrounding habitats.10 Non-resident adults arrive in the Sanctuary in the fall and winter and then leave again for their breeding grounds in mid-march.11 In the ESNERR, Great Blue Herons arrive at the rookery from late Jan-early March. 

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The plume trade, which supplied feathers for ladies hats, drove many species of wading birds to the brink of extinction in the late 1800s and early 1900s. Plume-hunting drastically reduced the number of Great Blue Herons in North America, but populations have since recovered. In California, a statewide survey in 1978 found a steady increase in the number of nests, probably due in part to more thorough survey efforts and to an increase in the population size.7 Great Blue Herons have been surveyed each winter in the Audubon Society’s Christmas Bird Count. Based on these surveys, the number of Great Blue Herons in the state appears to have been stable since the early 1980s (Figure 3). The number of Great Blue Herons nesting in South San Francisco Bay appears to have been fairly stable over the last two decades at approximately 100 nests per season.12


Consistent with trends in the rest of the country, numbers in central California declined in the late 1800s and early 1900s and then rebounded in the mid and late 1990s. Nesting near Elkhorn Slough (at the current ESNERR rookery) ceased during the 1930s. One breeding pair re-colonized the ESNERR rookery in 1985. By 1992 and 1993 approximately 30 breeding pairs were present (Figure 4). Since 1996, numbers at the rookery have fluctuated between 14 and 25 nesting pairs (unpublished ESNERR monitoring data). The small rookery at Blue Fish Cove, established in the mid-1990s, produced 17 fledglings from 9 nests in 2001.11

The average number of Great Blue Herons that forage and roost in the coastal habitats of the Sanctuary is not known. Abundance in coastal habitats of the Sanctuary and the adjacent wetlands is strongly seasonal. Abundance increases beginning in June and July due to both the fledging of locally produced offspring and the arrival of post-breeding migrants.5,2 Christmas Bird Count data collected between winter 1987/88 – winter 2003/04 in the Elkhorn Slough/Moss Landing area recorded an average of 55 Great Blue Herons per year (Figure 5; average calculated from CBC data).

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Natural History
Click here to view the natural history information of this species.


Human disturbance: This species is very sensitive to disturbance from humans. Intrusions of humans (including researchers) into nesting colonies may lead to nest failure or abandonment of the colony site.6 Approach of outboard powered boats within 133 m of Great Blue Herons can cause feeding and roosting birds to flush.14

Habitat degradation: Drainage of wetlands and marshes has markedly reduced available habitat. Human alteration of waterways (e.g., channelization of rivers, dredging of estuaries) often leads to changes in water levels and erosion rates, which may lead to a reduction in suitable foraging habitat. For example the creation of Moss Landing Harbor at the mouth of Elkhorn Slough has substantially increased erosion rates and reduced the amount of shallow marsh habitat.15

Predation: Predators of eggs and nestlings include raccoons, corvids (crows and jays), gulls, vultures, falcons, hawks and owls. Human activity in and near wildlife areas often attracts corvids and raccoons and can lead to increased predation rates at nesting colonies.

Pollution: Ingestion of prey contaminated with pesticide residues can cause reduced breeding success, due to eggshell thinning, or even death of adults.6 This threat was reduced substantially in 1972 when many pesticides were banned in the U.S.


No threats are unique to the MBNMS, but all the “general” threats listed above have the potential to impact breeding or wintering birds in the Sanctuary.


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Conservation and Research

The Great Blue Heron is not listed under the Federal Endangered Species Act of 1973. This species is protected in the U.S., Canada, and Mexico by the Federal Migratory Bird Treaty Act (MBTA) of 1918, which prohibits pursuing, hunting, shooting, wounding, killing, trapping, capturing, or collecting any migratory bird, nest, or eggs without a permit from the U.S. Fish and Wildlife Service. The MTBA protects nesting habitat only when occupied by herons.


ESNERR Rookery Monitoring Program (Research Coordinator: Kerstin Wasson, ESNERR). The rookery in the ESNERR has been monitored sporadically since 1985 and consistently since 1996. The goal of monitoring is to estimate abundance, timing of reproduction, and reproductive success. In 2000, a camera was installed in the treetops of the rookery to videotape the nests and the monitoring program began collecting data on tree colonization and tree death to assess the impact of the birds on the nesting habitat.


The Great Blue Heron is not listed under the California Endangered Species Act. Resident birds, such as the Great Blue Heron, are protected from take and harassment under the California Code of Regulations (CCR Title 14, Division 1, Subdivision 2, Chapter 1 §250 and §251.1). Harassment is defined as an intentional act that disrupts an animal’s normal behavior patterns. The Great Blue Heron is designated a "Special Animal" by the California Department of Fish and Game.16 The California Department of Forestry classifies the Great Blue Heron as a “sensitive species”. The Board of Forestry assigns this classification to species that warrant special protection during timber operations.


San Francisco Bay Heron and Egret Monitoring – North Bay (Lead investigator: John Kelly, Audubon Canyon Ranch). In 1967, Audubon Canyon Ranch began a study of nesting herons and egrets at the Bolinas Lagoon heronry. In 1990, this effort was expanded to all the nesting colonies of herons and egrets in the northern San Francisco Bay area. Data collected includes colony size, nest survivorship, productivity, seasonal timing, and individual behaviors. Colony sites are recorded in a GIS (geographic information system) database for analysis of spatial relationships and proximity to feeding areas and human activity. This on-going project is a component of the Integrated Regional Wetland Monitoring (IRWM) Pilot Project, an interdisciplinary research effort examining wetland restoration in the North Bay and Delta regions of the San Francisco Estuary.

San Francisco Bay Heron and Egret Monitoring – South Bay (Lead investigator: Anne Hanson, San Francisco Bay Bird Observatory). Since 1981, SFBBO has been monitoring population dynamics of heron and egrets breeding in South San Francisco Bay. Each year between March and August colonies are monitored to determine the number of active nests and the survivorship of the young in each colony. This monitoring program is part of a larger project called the Colonial Waterbird Monitoring Project. Collaborators: Audubon Canyon Ranch.

Annotated Atlas of Heron and Egret Nesting Colonies in the San Francisco Bay Area (Project Leader: John Kelly, Audubon Canyon Ranch). The atlas evaluates current and historical distributions, reproductive performance, habitat values, human disturbance, and management concerns at all known heron rookeries (over 100 sites in nine counties) in the San Francisco Bay region. Atlas colony site accounts include information on location, ownership, nesting habitat, proximity to wetlands, human land use, historical trends, reproductive performance, conservation concerns, and management recommendations. 

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Research Gaps

On-going research programs monitoring nesting populations at the ESNERR and in the greater San Francisco Bay area. Additional research is needed in the following areas:

  • Tagging/banding studies are needed to determine movement patterns of birds that nest and forage in the Sanctuary. Movement studies would help determine the percentage of the birds in the Sanctuary that are local residents vs. migrants, the wintering locations of local breeders and the sources of winter migrants. Rates of movement between colony sites could also be assessed.
  • Genetic studies are needed to determine if there is population structure among the U.S. west coast nesting colonies.
  • Determine the location of critical habitat and develop conservation plans to protect those areas. Monitor changes in habitat characteristics and determine impacts on heron populations.
  • Monitor rates of predation and human disturbance in nesting colonies. Determine the cause of substantial changes in disturbance rates. 

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Recommended Actions
  • Promote preservation and restoration of nesting and foraging habitat to the north and south of the Sanctuary. These areas may be sources of birds that winter in the Sanctuary.
  • Enforce Sanctuary regulations that help prevent disturbance to Great Blue Herons and degradation of nesting and foraging habitat:
    • Existing “Restricted Overflight” zones prohibit low flying aircraft (<1,000 ft) over some Great Blue Heron nesting and foraging habitat in the Sanctuary. Use education outreach efforts to decrease low flying aircraft over important Great Blue Heron habitat that is not located in a restricted overflight zone.17
    • Prohibitions on discharging or depositing from beyond the boundary of the Sanctuary, any material that injures a Sanctuary resource or quality (e.g., oil, chemicals). Enforcement of this regulation will help maintain or improve water quality in the rivers and tidal wetlands that surround the Sanctuary that act as important foraging habitat for this species.
    • Prohibitions on take or injury to birds protected under the MBTA.
    • Promote preservation and restoration of critical habitat, particularly groves of large trees and shallow wetlands. Consider protection of suitable alternative colony sites near existing sites because herons may abandon colony sites when disturbed and establish new sites nearby if suitable habitat is available
    • Create education outreach programs and materials to help reduce human disturbance at nesting, foraging and roosting sites. Education outreach should focus on reducing or eliminating human attraction of predators near heron rookeries and reducing sources of shore-based and water-based disturbance. Many studies recommend that there should be no development or logging within 300 m of the edge of a heron rookery and no human disturbance in or near colonies from March to August.6,18
    • Support predator relocation and other control programs if needed to protect local heron rookeries from high levels of predation.

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Cited References

1. California Department of Fish and Game, California Interagency Wildlife Task Group (CDFG IWTG) (2005) California Wildlife Habitat Relationships version 8.1 personal computer program. Sacramento, California.

2. Byrnes PE (1997) Habitat use, behavior, and morphology of herons and egrets in Elkhorn Slough, California. M.S. Thesis, San Francisco State University.

3. Pratt HM, Winkler DW (1985) Clutch size, timing of laying, and reproductive success in a colony of Great Blue Herons and Great Egrets. Auk 102:49-63.

4. Kelly JP (2003) A Tale of Two Islands: 25 years of heron and egret monitoring at the Marin Islands. The Ardeid 2003:1-3.

5. Small A (1994) California Birds: Their Status and Distribution. Ibis Publishing Company, Vista, CA.

6. Butler RW (1992) Great Blue Heron. In: The Birds of North America, No 25 (Poole A, Stettenheim P, and Gill F, eds.). Philadelphia: The Academy of Natural Sciences; Washington, DC: The American Ornithologists’ Union.

7. Roberson D, Tenney C (eds) (1993) Atlas of the Breeding Birds of Monterey County, California. Monterey Peninsula Audubon Society, Carmel, CA.

8. Pratt HM (1970) Breeding biology of Great Blue Herons and common egrets in central California. Condor 72:407-416.

9. Kelly J, personal communication

10. Gill RJ, Mewaldt LR (1979) Dispersal and migratory patterns of San Francisco Bay produced herons, egrets, and terns. North American Bird Bander 4:4-13.

11. Roberson D (2002) Monterey Birds, 2nd ed. Monterey Peninsula Audubon Society, Carmel, CA.

12. San Francisco Bay Bird Observatory, Colonial Waterbird Monitoring

13. Butler RW (1997) The Great Blue Heron: A natural history and ecology of a seashore sentinel. UCB Press, Vancouver, B.C.

14. Rodgers JA, Jr., Schwikert ST (2002) Buffer-zone distances to protect foraging and loafing waterbirds from disturbance by personal watercraft and outboard-powered boats. Conservation Biology 16:216-224.

15. Wasson K, Van Dyke E, Kvitek R, Brantner J, Bane S (2001) Tidal erosion at Elkhorn Slough. Ecosystem Observations for the Monterey Bay National Marine Sanctuary 2001:13-14.

16. California Department of Fish and Game, California Natural Diversity Database, Special Animals List

17. Addressed in part by JMPR Wildlife Disturbance Issues - Marine Mammal, Seabird and Turtle Disturbance Action Plan: Low Flying Aircraft Disturbance Strategy. Joint Management Plan Review (JMPR). Proposed Action Plans. Draft report. Monterey Bay National Marine Sanctuary.

18. Addressed in part by JMPR Wildlife Disturbance Issues - Marine Mammal, Seabird and Turtle Disturbance Action Plan: Vessel Disturbance and Shore Based Disturbance. Joint Management Plan Review (JMPR). Proposed Action Plans. Draft report. Monterey Bay National Marine Sanctuary.

References and Resources
Click here for images, reports, and links to other websites for this species.

Acknowledgement of Reviewers


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