Great Blue Heron Ardea herodias
Version: 1.0 — Published March 4, 2020
Text last updated April 28, 2011
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Conservation and Management
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Effects of Human Activity
Historically, several industrial contaminants (organochlorine pesticides, polychlorinated biphenyls, dioxins, furans) had negative effects on the species – lowered reproduction and apparent survival (Vermeer and Risebrough 1972, Ohlendorf et al. 1979b, Blus et al. 1980, Laporte 1982, Fleming et al. 1984a, Elliott et al. Elliott et al. 1988, Elliott et al. 1989a), Bellward et al. 1990, Moul 1990, Hart et al. 1991).
In the past two decades, however, these contaminants seem to pose fewer problems for most populations (Mora 1996, Rodgers 1997, Elliott et al. 2001, Harris et al. 2003, Champoux et al. 2006, Straub et al. 2007, Baker and Sepulveda 2009), presumably because environmental levels are lower owing to better regulation of industrial wastes. Nevertheless, Parsons and McColpin (1995) and Thomas and Anthony (1999) found some evidence of continuing contamination from some of these chemicals in Great Blue Herons in Delaware Bay and coastal Oregon/Washington, respectively. In addition, Thomas and Anthony (2003) found that nest attendance and visitation rates were lower for colonies with elevated PCB and DDT concentrations, though no impact on reproductive success was observed.
15-20 years ago, mercury levels in Florida Great Blue Herons appeared below toxic levels (Beyer et al. 1997), but see Rodgers (1997). Spalding et al. (1994) found that Occidentalis group herons that died from multiple chronic diseases had higher mercury levels than birds that died from more acute causes and speculated that mercury may be a contributing factor to population declines in southern Florida.
Overall, no clear evidence has been found of adverse effects of contaminants on the reproductive success of this species (Harris et al. 2003), but measures of reproductive success might commonly have been too crude to detect subtle differences (see Demography and Populations: measures of breeding activity).
Although society has achieved success in reducing many contaminants, new chemicals are emerging that could pose a threat to the species in the future. For example, concentrations of polybrominated diphenyl ethers (PBDEs) have been found to be increasing exponentially in heron tissues in British Columbia and may be close to toxicologically significant levels (Elliott et al. 2005). The implications of this finding currently are not fully understood, but the situation is seen as a potential emerging threat in areas impacted by humans (Elliott et al. 2005).
Disturbance By Humans
Humans impact Great Blue Herons by destruction and degradation of habitat, by disturbance of nesting colonies and key feeding areas, and by causing direct mortality (reviewed by Parnell et al. 1988, Butler 1997, Vennesland 2000).
Great Blue Herons likely suffered from hunting in the past, for both the plume trade and food (e.g., egg harvesting), although data are sparse. For example, Frohring et al. (1988) provided a thorough review of the history of wading bird population trends in southern Florida back to the 1800s, but Great Blue Herons are not mentioned – perhaps owing to their small populations in this region (Crozier and Gawlik 2003). Whatever the losses, the species has now returned to much of its former range, albeit likely at smaller population sizes than in historical times (Powell et al. 1989, Fleury and Sherry 1995, Crozier and Gawlik 2003). But all in all, attitudes have now changed substantially, and the conspicuousness of this bird that led to its hunting in the late 1800s and early 1900s may now act in its favor through conservation programs.
Overall, loss of habitat, particularly wetland nesting and feeding areas, may have had the strongest negative impact on the species through time (English 1978, Parnell et al. 1988, Rosenberg et al. 1991). For example, half of the Florida Everglades have been lost to development, with historical loss of about 70% of the wading bird numbers there (Crozier and Gawlik 2003); furthermore, by the 1970s more than 50% of the total wetlands in the US had been drained (Bancroft 1989). Nevertheless, Great Blue Herons appear to have rebounded from earlier habitat and mortality impacts better than many other ardeids (David 1994, Crozier and Gawlik 2003), demonstrating their adaptability.
Human activity can disturb nesting Great Blue Herons (Werschkul et al. 1976; Simpson and Kelsall 1978; Vos et al. 1985, Parnell et al. 1988, Skagen et al. 2001), though Nisbet (2000) cautioned that disturbance does not always lead to adverse impacts at the population level. Nevertheless, human disturbance in general has been linked to reduced nesting productivity (Carlson and McLean 1996, Vennesland 2000, Gebauer and Moul 2001, Vennesland and Butler 2004). And several studies have linked abandonment of Great Blue Heron colonies to human activity, including housing and industrial development, highway construction, logging, vehicle traffic, and repeated human intrusions (Kelsall and Simpson 1979, Drapeau et al. 1984, Forbes et al. 1985b, Leonard 1985, Vennesland and Butler 2004; see also reviews by Parnell et al. 1988, Rodgers and Smith 1995, Carney and Sydeman 1999, Vennesland 2000). Bjorkland and Holm (1997) reported declines in breeding populations in Illinois due to human induced flooding in a river system, and Simpson (1984) documented construction work that resulted in adult herons leaving nests and ended with a large loss of nestlings to eagles. Some colonies splinter and attempt to settle nearby following abandonment (Parker 1980a). Watts and Bradshaw (1994) reported herons nesting further from human development than would be expected by chance, and Parker (1980) and Gibbs and Kinkel (1997) observed that colony size increased with distance from roads.
Great Blue Herons tolerate some human activity near nesting areas, and show more tolerance for repeated non-threatening mechanical disturbances than for pedestrian traffic (Vos et al. 1985; Carlson and McLean 1996; Rodgers and Smith 1995; Vennesland 2000, 2010). Demauro (1993) found that an observation tower established 229 m from a rookery in Illinois did not result in any obvious disturbance to nesting birds. Heron response to disturbance stimuli depends on the timing, frequency and magnitude of the stimulus and the sensitivity of the birds, and can vary between sites and the stage of the breeding season (Vos et al. 1985, Vennesland and Butler 2004, Vennesland 2000, 2010); e.g., early in the nesting season, herons flush easily from nests; after eggs have been laid, they fly more reluctantly and tend to return quickly to nests; once chicks have hatched, few adults will flush unless they are heavily disturbed by loud and/or novel activities (Vennesland 2000). Vennesland (2010) used a behavioral experiment in the field to show that although herons can habituate to a low-level human disturbance stimulus (in this case a single pedestrian), factors based on the stage of the nesting cycle also affect response so caution needs to be exercised when assessing the species' potential for habituation to particular disturbance stimuli.
Buffers are commonly recommended to limit human disturbance near colonies, both on land (e.g., Vennesland 2004) and in the air (e.g., Markham and Brechtel 1978). Carlsen and McLean (1996) showed that breeding productivity was higher at sites with stronger barriers (e.g. ditches and fences), suggesting that isolating colonies can be effective. Many studies have suggested a set-back distance of 250 - 300 m from the edge of the colony (Parker 1980, Vos et al. 1985, Murphy 1988, Quinn and Milner 1999, Vennesland 2004), although there is wide variation in suggested set-backs. Some authors (e.g., Bowman and Siderius 1984, Vennesland 2004) have recommended a tiered buffer approach, where some activities (e.g., tree cutting) are curtailed year round within an inner tier, but are allowed during the non-breeding season in an outer tier. The inner tier therefore acts to maintain habitat and reduce disturbance, while the outer tier acts solely to prevent disturbance during the breeding season.
Rodgers and Smith (1995) developed a formula to calculate setback distances that would prevent flushing of birds caused by specific human activities near colonies. Calculated setbacks for nesting Great Blue Herons in Florida were 100 m for pedestrian activity and 82 m for motorboats. Vennesland (2000) used the formula of Rodgers and Smith (1995) to recommend a nesting colony setback for pedestrian activity of 165 m in the heavily developed southwest of British Columbia. Using a similar methodology, Rodgers and Schwikert (2002) determined that foraging and loafing birds in Florida require a 180 m buffer to mitigate potential disturbances from personal watercraft and motorboats. It should be noted that these calculations are for particular stimuli in particular locations, and should not be used to predict buffers needed for other stimuli or in different locations (i.e., the response of herons to specific stimuli in a specific area should be used to calculate setback distances).
Disturbance By Bald Eagles
Bald Eagles are a primary predator of Great Blue Herons (Butler 1997, Gebauer and Moul 2001, Vennesland and Butler 2004) and in some geographic areas predation and associated disturbance results in significantly higher nest and colony abandonment (Butler et al. 1995, Vennesland and Butler 2004). Eagle populations on the south coast of British Columbia have more than doubled since the mid 1980s (Elliott and Harris 2001, Jones 2010) and the frequency of incursions into Great Blue Heron colonies also has more than doubled over this time period (Norman et al. 1989, Vennesland and Butler 2004). The interaction between herons and eagles is dynamic and nuanced. Some herons nest near eagle nests where they might be afforded a reduced level of disturbance from other predators, including other eagles (Butler 1995; Vennesland 2000, Jones 2010). Butler (1995) and Vennesland (2000) could find no correlation between colony reproductive success and proximity to nesting eagles in earlier studies, but Jones (2010) recently reported a significant negative correlation. Vennesland (2000) reported the apparent habituation of herons to eagle activity at one colony site (in 1998 eagle attacks on the heron colony were witnessed and herons responded to eagle flights in and near the colony, while in 1999 no eagle attacks were witnessed and the herons ignored the activity of the eagles). Butler and Vennesland (2000) and Kenyon et al. (2007) have suggested that colony configuration on the landscape should change with predation danger.
Habitat protection for the species is not well covered in the literature, although many jurisdictions have programs in place for this heron (e.g., Bowman and Siderius 1984, Quinn and Milner 1999, Vennesland 2004), a bird commonly used as a flagship species for habitat protection programs. Protecting colony-sites and foraging areas are seen to be key priorities (Butler 1997). Kenyon et al. (2007), Jones (2010) and Knight (2010) pointed out that because this species is fairly mobile and changes colony locations frequently, habitat protection measures need to be adaptive to ensure that current locations are protected but also that alternative locations are maintained for when the birds choose to move. Kelly et al. (2007) suggested habitat protection measures should be concentrated at large colonies because they tend to persist for longer than small colonies.
Although water levels are manipulated in many habitats used by Great Blue Herons, the utility of human-altered habitats to the species has not been studied in detail (Butler and Vennesland 2000). Fleury and Sherry (1995) concluded that the presence of crayfish ponds in Louisiana has likely contributed to the increase in Great Blue Herons there. Restoration of natural hydrologic cycles and vegetation communities in the Everglades may benefit Great Blues (Bancroft et al. 2002). Nutrient enrichment in the Everglades might have some benefit to Great Blue Heron foraging in the short term, but over the long term may be detrimental to populations by degrading foraging habitats (Crozier and Gawlik 2002). Based on this, Crozier and Gawlik (2002) concluded that enrichment ultimately is incompatible with ecosystem restoration in the Everglades.
Kelly et al. (2007) reported local population increases that coincided with the restoration of tidal marshes in the San Francisco Bay area. Supplemental feeding by humans of Occidentalis group herons in Florida may have increased the heron's reproductive success (Powell and Powell 1986).
Popotnik and Giuliano (2000) reported that Great Blue Herons did not use habitat actively grazed by livestock, though no effect on reproductive success was observed. In Mississippi, flooding delayed nest initiation and reduced Great Blue Heron clutch size (Custer et al. 1996).