Great Blue Heron Ardea herodias
Version: 1.0 — Published March 4, 2020
Text last updated April 28, 2011
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Demography and Populations
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Measures of Breeding Activity
Age At First Breeding; Intervals Between Breeding
Most Great Blue Herons apparently begin breeding in their second spring (i.e., >22 mo), but this conclusion depends on the ability of observers to age herons accurately by plumage, which can be difficult (see Appearance; RWB). Juvenile-plumaged birds have been observed attempting to breed (Pratt 1973a, RWB).
2 to 6 eggs. Mean clutch size from individual studies include: California, mean = 3.16, SE = 0.04, n = 297 (Pratt and Winkler 1985); Oregon, mean = 4.19, SD not given, n = 32 (Henny and Bethers 1971); Nova Scotia, mean = 4.2, SD = 0.8, n = 36 (McAloney 1973); Quebec, weighted mean = 4.1, n = 77 (DesGranges et al. 1979); British Columbia, mean = 4.2, SD = 0.6, n = 23 (Butler 1989); British Columbia, mean = 3.9, SD not given, n = 47 (Butler 1997); Alberta, mean = 5.0, SD not given, n = 11 (Vermeer 1969d).
Clutch size increases with latitude (Rubolini and Fasola 2008), is higher for freshwater habitats than for salt or brackish-water habitats (Rubolini and Fasola 2008), and decreases through the season (Pratt and Winkler 1985).
For the Occidentalis group: Florida, mean = 2.9, SD = 0.6, n = 64 (Powell 1983). Clutch size in Florida was higher for nests with herons that received supplemental feeding than for nests with un-supplemented herons (Powell 1983, Powell and Powell 1986).
Annual And Lifetime Reproductive Success
Annual reproductive success varies widely. Historically, most authors have reported only the mean number of fledged young per successful nest (i.e., only for nests that fledged young), which shows less annual variation and results in the loss of data on nest failures, a potentially important variable (Vennesland and Butler 2004). More recent studies are now providing measures of reproductive success separately for both all initiated and for all successful nesting attempts, which is positive.
North American reports of annual mean reproductive success for the Herodias group have ranged from 0.5 to 2.7 fledglings per initiated nesting attempt and from 1.5 to 3.3 fledglings per successful (fledging ≥ 1 young) nesting attempt (Pratt 1970, Quinney 1983, Vos et al. 1985, Burkholder and Smith 1991, Butler et al. 1995, Butler 1997, Vennesland and Butler 2004, Witt 2006, Kelly et al. 2007, Baker and Sepulveda 2009, RGV). Early nesting birds had higher reproductive success than later nesting birds in British Columbia and Washington (Butler 1995, Vennesland 2000, Stabbins et al. 2006).
In Canada, herons in large colonies had higher reproductive success and lower variability than those in smaller colonies (Forbes et al. 1985a). Likewise in British Columbia, reproductive success for all initiated nesting attempts was higher in large colonies than in small colonies (Vennesland and Butler 2004, COSEWIC 2008), but one study reported that reproductive success was higher in small colonies than in large colonies only in successful nests (COSEWIC 2008). Conversely, Forbes et al. (1985a) and Butler et al. (1995) in British Columbia and Kelly et al. (1993, 2007) in California found no significant correlation between colony size and reproductive success. These differences might reflect different nesting responses of herons to changes in predation risk. Reproductive success also has been positively correlated with the duration of colony occupancy (Butler et al. 1995).
Henny and Bethers (1971) estimated for Oregon that 1.91 fledglings per initiated nesting attempt were required to maintain a stable population, but this estimate needs to be restudied. And the figure is likely to vary among populations – e.g., migratory vs. non–migratory.
Few data are available for the Occidentalis group. In Florida Bay, 1981-1983 (Powell and Powell 1986), mean reproductive success equaled 2.5 fledglings per successful nesting attempt; excluding nests that were given supplementary feedings by humans, only 1.46 fledglings were produced per successful nesting attempt. Productivity including failed nests was lower, at 0.35 fledglings per initiated nesting attempt (the lowest reported figure for this measure) due to the failure of 76% of all nesting attempts. Powell and Powell (1986) also found that nest failure was more common for un-supplemented nesting herons (66% failure) than for those getting supplemental food (17% failure); supplemented herons also had larger clutch sizes.
Productivity increases with latitude (Rubolini and Fasola 2008). Lifetime reproductive success is unknown.
Life Span and Survivorship
Mortality estimates from band recovery data include: first year 69.0%, second year 36.3%, and subsequent years 21.9% (Henny 1972). Mortality has been reported to vary regionally (Bayer 1981b). The oldest banded Great Blue Heron recovered was 23 yr old (RWB). COSEWIC (2008) calculated the average age of a breeding adult heron in British Columbia to be 5.6 years. Assumptions in their model included: a first winter mortality rate of 0.727, a second year mortality rate of 0.5, and an annual adult mortality rate of 0.273 (Butler 1995); an annual reproductive success of 1.12 fledglings per initiated nesting attempt (RGV unpubl. data); and a maximum life span in the wild of 23 years (RWB). Herons banded on refuges in the U.S. appeared to live longer than those banded off refuges (Bayer 1981b).
Disease and Body Parasites
Giardia has been found in an adult male Great Blue Heron in New York (Georgi et al. 1986). The species also has been reported to carry the nematode Eustrongylides ignotus, which can kill herons (Locke 1961, Ziegler et al. 2000). Infection by Avian Poxvirus (Adams et al. 2005) and various hemoparasites (Telford et al. 1992) also have been documented. Transmission of fish pathogenic viruses may occur via the Grey Heron (A. cinerea, Peters and Neukirch 1986).
Population Spatial Metrics
Initial Dispersal From Natal Site
Few data. Band recoveries suggest dispersal in all directions 2 to 3 months post-breeding, then southward movement in autumn when the species migrates. One banded juvenile from the US was found in Belize (Henny 1972). A study of band recoveries from across North America (n = 3297 records; 1914-1994) concluded that juveniles disperse an average of 758 km in their first winter (Melvin et al. 1999).
Simpson (1984) found that 40% of herons banded at a British Columbia colony in 1978 returned to breed at the same site.
Fidelity To Breeding And Winter Home Range
Few data, needs study -- especially with marked birds. However, similar numbers of herons from 6 abandoned colony sites in BC nested 3.1-7.6 km away one year later (RWB).
The mean distance Great Blue Herons fly from their nesting colony to their principal feeding sites has been reported in the 2.3 - 6.5 km range (Dowd and Flake 1985b, Thompson 1978c, Parris 1979, Butler 1991, Knight 2010). One study concluded that breeding adults ranged within about 30 km of their colony, though most stayed within 3 km (Parris 1979, Butler 1991, Butler 1997). Little is known about home ranges outside of the breeding season.
Estimates Or Counts Of Density
Figure 5 shows Great Blue Heron breeding and wintering densities. During the breeding season, the highest densities occur in Eastern North America from south of the Great Lakes to the Gulf of Mexico. Spendelow and Patton 1988 reported that the highest density of coastal breeding herons occurred in Texas, the Mississippi delta, w. Florida, and Chesapeake Bay. The highest wintering population densities occur in se. and sw. North America.
For the Herodias group, it has been estimated that 83,000 breeding adults (124,500 individuals) occur across North America (Kushlan et al. 2002). Earlier studies showed that for the south and east coasts of the U.S., about 32% of all breeders (36,248 birds) occurred on the Atlantic Coast, < 10% in Florida, 40% in Louisiana and Texas, and 18% along the Great Lakes (Spendelow and Patton 1988).
In the Pacific Northwest, A. h. fannini has been estimated at 10,000 individuals (Butler 1997), with about 4,000 in sw. British Columbia and 4,000 in nw. Washington (COSEWIC 2008). Counts for the Herodias group in individual regions have included the following: about 12,211 nests in 248 colonies and 34 isolates in Ontario in the early 1980s (Peck and James 1983); a minimum of 13,022 pairs in Ontario in 1980 to 1981 (Dunn et al. 1985); 744 colonies in Ontario in 1990-1991 (Graham et al. 1996); 1,064 nests tallied in 32 colonies in 1987 on the Great Lakes in Michigan (Scharf, unpubl. data); about 5,340 nesting pairs in 35 colonies in Illinois (Kleen 1987); and 4,000 nests counted in 56 colonies on the Canadian Prairies between 1967 to 1972 (Vermeer 1973c). Little information is available elsewhere in the range of the Herodias group. New, more recent surveys are clearly needed for many regions.
For the Occidentalis group, it was estimated (1970s and 80s) that 450-600 breeding adults and 1,200-1,400 individuals winter in Florida Bay, with another 1,500 elsewhere in Florida (Powell et al. 1989, Powell and Bjork 1989). About 1,500 individual Occidentalis herons were estimated for Florida Bay in 1985 (Powell et al. 1989).
Great Blue Heron population trends are equivocal across North America (Butler et al. 2000, Delaney and Scott 2002). Figure 7 shows trends in populations of Great Blue Herons as reported on Breeding Bird Surveys. BBS data (1980-2007) show a stable trend for North America (0.0, P = 0.92, n = 2,475), a decline in Canada (-2.3, P < 0.001, n = 357), and an increase in the US (0.9, P = 0.01, n = 2,118). There are localized declines and increases in breeding numbers across North America, but Sauer et al. (2003) caution putting faith in trends from these volunteer-driven data sets.
Christmas Bird Count data (1980-2007) – early winter surveys -- show an increase in Great Blue Heron numbers for North America (P<0.01, n = 28 yr), a decline for Canada (P = 0.05, n = 28 yr), and an increase in the US (P<0.0001), but these surveys are volunteer counts made once a year with uneven effort. One notable decline during this period was in the Pacific Northwest coast (for A.h. fannini), where declines were also reported using BBS data (-4.2, p<0.01, n = 17), CBC data (19% decline from 1991/92 to 2006/07; COSEWIC 2008), and Bird Studies Canada's Coastal Waterbird Survey (P. Davidson pers. comm.). As with the BBS data, researchers have recommended caution when interpreting CBC data because the conspicuousness of the bird may mean that recent increases in effort through time may not lead to increased observations, which might lead to the reporting of false negative trends (COSEWIC 2008).
Detailed reports on breeding numbers in Texas (http://txtbba.tamu.edu/accounts/gtbh/gtbhacc.html; Telfair 2007) suggest declining trends (-3.7%/yr; 1973-2000) in costal areas, but mean annual increases of 1.7%, 1980-2005, statewide.
The Occidentalis group shows a steep decline in winter numbers -- CBC data (1980 to 2007; P<0.0001, n = 28 yr). If real, this trend is worrying as there has not been a marked increase in survey effort to bias the analysis. More study is needed.
Numerous trend figures for localized areas are available from the literature from various time periods back to the middle of the 20th century. Populations were stable or showed increases in Tennessee (Fleming et al. 1984a), Illinois (Kleen 1987, though see contradictory report below), and central California (Kelly et al. 2007). Populations have increased in NY State Andrle and Carroll 1998; (New York BBA), Louisiana (5 censuses over 37 yr show probable increase; Ogden 1978a), Fleury and Sherry 1995), Michigan (1980s to 1990s; W. C. Scharf, unpubl. data), Ontario (Graham et al. 1996), and the Chesapeake Bay area of Virginia and Maryland (a 6 fold increase in nesting pairs over 25 years; Williams et al. 2007). Populations in one study decreased after dams were built on the Colorado River (Rosenberg et al. 1991). Probable declines have been documented in Iowa (Dinsmore et al. 1984), Illinois (Robbins et al. 1986a), and Saskatchewan (Hansen 2000).
Widespread nest and colony abandonment was reported on the south coast of British Columbia in the late 1990s and early 2000s, but potential population declines associated with this loss in reproductive success are equivocal (Vennesland and Butler 2004, COSEWIC 2008). Reproductive success in this region has declined by about half since the 1980s (COSEWIC 2008).
The Occidentalis group was decimated by hunting for food and plumes in the late 1800s, but had rebounded by the 1930s and continued to increase (apart from rare but large mortality events associated with hurricanes) into the 1970's (Powell et al. 1989), when it was thought to be stable or increasing (Ogden 1978a). Also see above for more recent trend information from Christmas Bird Counts.
Powell and Powell (1986) compared reproductive success of Occidentalis herons between 1923 (thought to be prior to human impacts in Florida Bay) to data from the 1980's and concluded that reproductive success for unsupplemented individuals was lower than in 1923, but for supplemented individuals it was comparable to 1923 (in this area some herons receive supplemental feedings from humans). Powell et al. (1989) reported that current populations likely are not as large as they were before the impacts of the plume trade.
Severe winters may reduce northern populations of Great Blue Herons when foraging sites freeze (Blus and Henny 1981). Occidentalis heron populations in Florida were decimated by hurricanes, but recovered to former levels (Powell et al. 1989). Evidence for population regulation during the breeding season is weak or non-existent (Butler 1996). Apart from some localized areas (e.g., see COSEWIC 2008), nest sites are not considered limiting for this species in most regions, though Williams et al. (2007) concluded that increases in forest cover have been partly responsible for a 6-fold increase in Great Blue Heron nests in the Virginia/Maryland area from 1977 to 2003. The number of breeding pairs in colonies has been positively related to the area of nearby wetlands (Bayer and McMahon 1981, Gibbs et al. 1987b, Butler 1995), suggesting food supply is a critical limitation, perhaps not surprising in a species so strongly territorial.