Great Blue Heron - Ardea herodias
State Rank Reason (see State Rank above)
Small breeding population size, evidence of recent declines, and declining regeneration of riparian cottonwood forests due to altered hydrology and grazing.
- Details on Status Ranking and Review
ScoreE - 2,500-10,000 individuals
CommentBased on recent waterbird surveys of standing water bodies and surveys for Great Blue Herons on the Lower Yellowstone River, which documented approximately 1,000 pairs, we estimate that there are between 2,500 and 10,000 individuals of reproductive age.
ScoreG - 200,000-2,500,000 km squared (about 80,000-1,000,000 square miles)
Comment380,531 square kilometers based on Natural Heritage Program range maps that appear on the Montana Field Guide
Area of Occupancy
ScoreU - Unknown
ScoreE - Relatively Stable (±25% change)
CommentDeciduous and mixed deciduous riparian forests have been relatively stable since European arrival.
ScoreD - Declining. Decline of 10-30% in population, range, area occupied, and/or number or condition of occurrences
CommentBreeding Bird Survey data is of moderate credibility in Montana and shows a significant decline of -1.5% per year or 14% decrease per decade. Surrounding states and provinces show both positive and negative trends. A decline of around 50% has been observed in the lower Yellowstone River Basin between 1988 and 2009.
ScoreD - Moderate, non-imminent threat. Threat is moderate to severe but not imminent for a significant portion of the population or area.
CommentLoss of cottonwood forests due to altered hydrology and grazing.
SeverityModerate - Major reduction of species population or long-term degradation or reduction of habitat in Montana, requiring 50-100 years for recovery.
CommentDeclines in riparian forest roosts will take a long time to recover.
ScopeModerate - 20-60% of total population or area affected
CommentMore than 20% of riparian deciduous forests are decadent and/or have altered hydrological regimes that are impacting cottonwood recruitment.
ImmediacyLow - Threat is likely to be operational within 5-20 years.
CommentThreat is not fully operational now, but is likely accelerating.
ScoreB - Moderately Vulnerable. Species exhibits moderate age of maturity, frequency of reproduction, and/or fecundity such that populations generally tend to recover from decreases in abundance over a period of several years (on the order of 5-20 years or 2-5 generations); or species has moderate dispersal capability such that extirpated populations generally become reestablished through natural recolonization (unaided by humans).
CommentModerately Vulnerable. Species exhibits moderate age of maturity, frequency of reproduction, and/or fecundity such that populations generally tend to recover from decreases in abundance within 5-20 years or 2-5 generations. Species has good dispersal capabilities such that extirpated populations generally become reestablished through natural recolonization.
ScoreB - Narrow. Specialist. Specific habitat(s) or other abiotic and/or biotic factors (see above) are used or required by the Element, but these key requirements are common and within the generalized range of the species within the area of interest.
CommentNarrow Specialist. Dependent on large mature stands of riparian cottonwoods for nesting colonies.
Raw Conservation Status Score
3.5 - 0.25 (population size) + 0.0 (geographic distribution) - 0.25 (short-term trend) - 0.25 (threats) = 2.75
Largest heron in North America, 60 cm tall, 97 to 135 cm long, 2.1 to 2.5 kg mass. Wings long and rounded, bill long and tapered, tail short. Upper parts are gray, fore-neck is streaked with white, black, and rust-brown. Bill yellowish. Legs brownish or greenish. In flight, folds neck in an "S" shape and extends legs along the body axis; wing beats are deep slow wing. Adults have long occipital plumes (Butler 1992).
For a comprehensive review of the conservation status, habitat use, and ecology of this and other Montana bird species, please see Marks et al. 2016, Birds of Montana.
No other heron in Montana is the size or color of the Great Blue Heron, nor are other herons likely to be encountered in Montana during winter.
Western Hemisphere Range
Great Blue Herons breed from southern Alaska southeast across central Canada to Nova Scotia and south to Guatemala, Belize, and the Galapagos Islands. They winter in most of the breeding range (being absent in the interior of Canada and in the northern Great Plains) and throughout Central America to Venezuela and Colombia. The Great White Heron, once considered a full species but now treated as a subspecies of the Great Blue Heron, is a permanent resident in southern Florida and in parts of the West Indies.
Observations in Montana Natural Heritage Program Database
Number of Observations:
(Click on the following maps and charts to see full sized version)
Map Help and Descriptions
(direct evidence "B")
(indirect evidence "b")
No evidence of Breeding
(regular observations "W")
(at least one obs. "w")
(Observations spanning multiple months or years are excluded from time charts)
Fairly common to common permanent resident, with more than 100 nesting colonies scattered across the state. Numbers are greatly reduced in winter, especially on the plains north of Lewistown and east of Billings. Bozeman area movement periods are 25 March to 15 April and July to 1 October, with a fall peak of 15 August (Skaar 1969). Mean arival in the Fortine area 4 May, ranging from 27 March to 27 May during nine years (Weydemeyer 1973).
Great Blue Herons are equally at home in urban wetlands and wilderness settings. Most Montana nesting colonies are in cottonwoods along major rivers and lakes; a smaller number occur in riparian ponderosa pines and on islands in prairie wetlands. Nesting trees are the largest available. Active colonies are farther from rivers than inactive colonies. The number of nests in the colony corresponds to the distance from roads (Parker 1980). Great Blue Herons build bulky stick nests high in the trees when nesting near the shores of rivers and lakes and on the ground or in low shrubs when nesting on treeless islands.
Ecological Systems Associated with this Species
- Details on Creation and Suggested Uses and Limitations
How Associations Were Made
We associated the use and habitat quality (common or occasional) of each of the 82 ecological systems mapped in Montana for
vertebrate animal species that regularly breed, overwinter, or migrate through the state by:
- Using personal observations and reviewing literature that summarize the breeding, overwintering, or migratory habitat requirements of each species (Dobkin 1992, Hart et al. 1998, Hutto and Young 1999, Maxell 2000, Foresman 2012, Adams 2003, and Werner et al. 2004);
- Evaluating structural characteristics and distribution of each ecological system relative to the species' range and habitat requirements;
- Examining the observation records for each species in the state-wide point observation database associated with each ecological system;
- Calculating the percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system to get a measure of "observations versus availability of habitat".
Species that breed in Montana were only evaluated for breeding habitat use, species that only overwinter in Montana were only evaluated for overwintering habitat use, and species that only migrate through Montana were only evaluated for migratory habitat use.
In general, species were listed as associated with an ecological system if structural characteristics of used habitat documented in the literature were present in the ecological system or large numbers of point observations were associated with the ecological system.
However, species were not listed as associated with an ecological system if there was no support in the literature for use of structural characteristics in an ecological system, even if
point observations were associated with that system.
Common versus occasional association with an ecological system was assigned based on the degree to which the structural characteristics of an ecological system matched the preferred structural habitat characteristics for each species as represented in scientific literature.
The percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system was also used to guide assignment of common versus occasional association.
If you have any questions or comments on species associations with ecological systems, please contact the Montana Natural Heritage Program's Senior Zoologist.
Suggested Uses and Limitations
Species associations with ecological systems should be used to generate potential lists of species that may occupy broader landscapes for the purposes of landscape-level planning.
These potential lists of species should not be used in place of documented occurrences of species (this information can be requested at: mtnhp.org/requests
) or systematic surveys for species and evaluations of habitat at a local site level by trained biologists.
Users of this information should be aware that the land cover data used to generate species associations is based on imagery from the late 1990s and early 2000s and was only intended to be used at broader landscape scales.
Land cover mapping accuracy is particularly problematic when the systems occur as small patches or where the land cover types have been altered over the past decade.
Thus, particular caution should be used when using the associations in assessments of smaller areas (e.g., evaluations of public land survey sections).
Finally, although a species may be associated with a particular ecological system within its known geographic range, portions of that ecological system may occur outside of the species' known geographic range.
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Great Blue Herons eat mostly fish but also amphibians, invertebrates, reptiles, mammals, and birds (Palmer 1962, Kushlan 1978, Verbeek and Butler 1989, Butler 1992). Voles comprised 24-40% of the diet of nestlings in one Idaho study, small mammals were also important for juvenile survival in British Columbia (Butler 1992).
The first statewide survey of nesting colonies occurred in 1979 (Thompson 1982) and determined that at least 1,915 active nests were present in 84 colonies (2-156 nests per colony). Most colonies were located in riparian cottonwoods, several were in conifers and willows, and two (Lake Bowdoin and Medicine Lake NWRs) were on the ground on islands. The highest densities were in cottonwood floodplain forests in the Flathead, Bitterroot, Beaverhead, upper Missouri, middle Yellowstone, Tongue, and Bighorn valleys, and the largest colony was on Tongue River Reservoir. Some heron colonies contained nesting Double-crested Cormorants, which arrived later than herons and often took over unoccupied heron nests with no apparent detrimental effects on the herons (Thompson 1981). In 2008 and 2009, MDFWP biologists surveyed suitable nesting habitat along the lower Yellowstone, Tongue, and Powder rivers (Waltee and Rauscher 2010). In total, 563 nests were located at 49 colonies: 302 nests in 29 colonies along the Yellowstone River, 211 nests in nine colonies along the Tongue River, and 50 nests in 11 colonies along the Powder River. All colonies were in stands of mature cottonwoods, 41 on the mainland and eight on islands. Prior surveys along the lower Yellowstone River documented 11 and 24 heron colonies in 1976 and 1988, respectively (Waltee and Rauscher 2010). Counts at nine of the colonies in 1976 and at 16 colonies in 1988 yielded 126 and 268 nests, respectively. The data suggest that since 1976, colony abundance in the lower Yellowstone Valley has increased continually, whereas abundance of adult herons fluctuated up and down (Waltee and Rauscher 2010). Colonial Waterbird surveys of 133 total sites in 2009 and 2010 (Wightman and Tilly 2010) reported herons at 80 (60%), including 53 sites in 2009 (719 breeding pairs, with 4-50 pairs per colony) and 27 sites occupied in 2010 (333 breeding pairs, with 1-62 per colony). BBS data indicate a significant decrease in numbers of 6.3% per year in Montana from 1980 to 2007, although the data suffer from deficiencies (Sauer et al. 2012).
Mostly monogamous. Birds choose new mates each year (Simpson 1984). Clutch size is 2 to 6 eggs (Butler 1992). Reproductive success was not affected by the number of breeding birds in the colony (Parker 1980). Courtship in Montana occurs in March, egg laying from early April to early May, hatching from early May to early June, and fledging from early July to early August. Most colonies are devoid of herons from September to February.
No management activities specific to Great Blue Heron are currently occuring in Montana, although annual colony counts have been conducted for the past several years as a follow-up assessment to Thompson (1981). Effects of human disturbance at 22 colonies in northwestern Montana was examined in 1978 and 1979 (Parker 1980). Larger colonies tended to be farther from roads, and some colonies close to rivers were abandoned when disturbed by recreational activity early in the nesting season. Larger colonies from the late 1960s had splintered into smaller colonies that were occupied for only 5-10 years; colony relocations may have resulted from deterioration of habitat quality and increased disturbance from humans. Most studies recommend a minimum 300 m buffer zone from the periphery of colonies in which no human activity should take place during courtship and the nesting seasons, with the exception of scientific studies (Butler 1992).
Threats or Limiting Factors
Disturbance by humans and loss of protected colony sites (trees or islands) are major threats. Chemical contaminants continue to be a problem related to egg-shell thinning and direct mortality of young and adults.
- Literature Cited AboveLegend: View Online Publication
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- Additional ReferencesLegend: View Online Publication
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- Additional Sources of Information Related to "Birds"