Great blue heron (fannini subspecies) COSEWIC assessement and update status report: chapter 10

Limiting factors and threats

The major factors currently limiting the persistence of heron populations are nesting failure and reduced nesting productivity arising from eagle predation, human disturbance and habitat declines from development (Norman et al. 1989; Butler et al. 1995; Gebauer and Moul 2001; Vennesland and Butler 2004).

Human disturbance

Moore (1990) showed that for every increase of 1000 people in the population of the Lower Fraser Valley, 89 hectares of rural land was converted to urban uses. On Vancouver Island, human population growth has been only slightly lower than near Vancouver. The human population around the Strait of Georgia is projected to increase by more than 50% from 1990 to 2020 (from about 2.5 million to about 4 million; Georgia Basin Ecosystem Initiative 2002). Under present conditions, we can expect increased human disturbance at heron colonies and reduced habitat availability/quality (Gebauer and Moul 2001).

Human activity disturbs nesting Great Blue Herons (Werschkul et al. 1976; Simpson and Kelsall 1978; Vos et al. 1985), and has been linked to reduced nesting productivity (Forbes et al. 1985b; Gebauer and Moul 2001; Vennesland and Butler 2004; Vennesland 2000; reviewed by Parnell et al. 1988). Carlson and McLean (1996) found that the distance of heron colonies from human activity and the width or efficacy of the buffer zone around colonies were positively related to nesting productivity (buffer zones included vegetation, water and fencing). Watts and Bradshaw (1994) reported herons nesting further from human development than would be expected by chance, and Parker (1980) observed that colony size increased with distance from roads.

Several studies have linked the abandonments of colonies to human activity, including housing and industrial development, highway construction, logging, vehicle traffic, and repeated human intrusions (Bjorklund 1975; Mark 1976; Werschkul et al. 1976; Simpson and Kelsall 1978; Kelsall and Simpson 1979; Forbes et al. 1985b; Leonard 1985; Vennesland and Butler 2004; Eissinger 2007; see also reviews by Parnell et al. 1988; Hockin et al. 1992; Rodgers and Smith 1995; Nisbet 2000; Vennesland 2000).

Vennesland (2000) found that humans were likely involved in 4 of 14 abandonments from 1998 to 1999, but the effect of humans also could not be separated from the effect of eagles. A possible explanation for the interaction between these factors is that forest fragmentation caused by humans is allowing easier access to sites by predators such as eagles (Vennesland and Butler 2004). Forbes et al. (1985b) concluded that 17 of 27 colony abandonments occurred due to human activity near the colony site, including tree cutting, flooding, vehicle use and researcher activity. Simpson (1984) documented construction work that resulted in adult herons leaving nests and ended with a large loss of nestlings to eagles. Simpson and Kelsall (1978) found that housing construction near to a colony in Sechelt in 1978 resulted in the abandonment of about 73% of nests.

Some colonies of Great Blue Herons in British Columbia have become acclimatized to routine human activities. Herons nesting in Stanley Park in Vancouver and Beacon Hill Park in Victoria seem habituated to the frequent human pedestrians and vehicles directly below their nests (Butler 1997; Vennesland 2000). However, colonies in more rural settings may respond to disturbances at a great distance. At a sensitive colony on Vancouver Island (Quamichan Lake, Duncan), adult herons flushed from their nests when a researcher approached within 200 m before eggs had been laid, 100 m after eggs had been laid, and 10 m after chicks were present (Butler 1991). Although no noticeable response is observed by herons at some urban sites, productivity at these locations is negatively correlated with the local level of human activity (Vennesland 2000; Vennesland and Butler 2004).

Bald Eagle predation

Bald Eagles are the primary predator of Pacific Great Blue Herons (Butler 1997; Gebauer and Moul 2001; Vennesland and Butler 2004) and represent a significant limiting factor for heron populations. Predation and associated disturbance results in significantly higher nest and colony abandonment (Butler et al. 1995, Vennesland and Butler 2004).

The influence of eagle attacks generally has been described in the context of recovering raptor populations after many populations were decimated by human pollution such as dichlorodiphenyltrichloroethane (DDT) (Bednarz et al. 1990; Kjellen and Roos 2000; Butler and Vennesland 2000; Elliott and Harris 2001). This recovery has been ongoing for several decades. The number of nesting eagles increased 30% in the Gulf Islands (Vermeer et al. 1989), and 34% in the Puget Sound (McAllister et al. 1986) from the mid-1970s to the mid-1980s. Eagle populations on the south coast have increased since the mid-1980s (Elliott and Harris 2001) and the rate of attacks on nesting Pacific Great Blue Herons has more than doubled over this time period (Vennesland and Butler 2004). Eagle nesting productivity from 1992 to 1995 was higher in the Strait of Georgia than on the west coast of Vancouver Island or in Johnstone Strait, and was producing a ‘considerable’ surplus of juveniles (Elliott et al. 1998). The reasons for this increase are unclear, but were probably due to increasing prey populations (e.g., gull populations enhanced by human refuse), declining contaminant levels in prey (Vermeer et al. 1989; Elliott et al. 1998), and possibly reduced persecution (Vennesland 2000). It is also possible that reduced fish populations have caused eagles to search out alternative sources of food (Vennesland 2000). It is unclear how current eagle populations compare with historical numbers, or how eagle populations compared to heron populations prior to the influence of Europeans. Given the magnitude of increase over the past few decades, it is unlikely that they are significantly below historical levels.

In addition to predation, Bald Eagles might be negatively affecting habitat use by Pacific Great Blue Herons (Butler and Vennesland 2000; Vennesland and Butler 2004). For example, soon after a landowner felled trees that exposed a colony on Vancouver Island in 2005, eagles began to enter the colony at which time the herons abandoned the site (Vennesland 2006). Eagle attacks on nesting herons have escalated in recent years (Vennesland and Butler 2004) and this increased level of predation stimulated many herons to search for new nesting habitat. Interestingly, some herons also are nesting near eagle nests where they might be afforded a reduced level of disturbance from other predators (Koonz 1980; Butler 1995; Vennesland 2000).

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