Great blue heron (fannini subspecies) COSEWIC assessement and update status report: chapter 8
Biology
Life cycle and reproduction
In springtime, most Pacific Great Blue Herons gather in colonies where they court, nest, and raise young. During the nesting season the principal diet is small fish, while during the winter this primarily piscivorous diet is augmented with small mammals.
In south-coastal British Columbia, Pacific Great Blue Heron nesting is initiated between February and April (Butler 1992; Butler 1997; Vennesland 2000; Vennesland and Butler 2004). The initiation period is defined as the period of courtship before eggs are laid, and can last for over two months at some colonies (Butler 1997; Vennesland 2000). At one closely watched colony, males arrived at the colony site and established nest sites followed about a week later by the arrival of females (Butler 1991). Nest repair and/or building can take from as little as several days to about two months (Butler 1991, Vennesland 2000). Monogamous pairs are established for the season (Simpson 1984), and eggs are laid at about two-day intervals (Vermeer 1969; Pratt 1970; R. Butler, pers. obs.). Clutch size reported from Great Blue Heron colonies across North America ranges from one to eight eggs, with three to five being typical (Ehrlich et al. 1988; Campbell et al. 1990).Clutch size increases with latitude and the Pacific Great Blue Heron lays smaller clutches than expected for this latitude (mean clutch size is about 4 eggs compared to about 4.7 at other sites at this latitude; Butler 1997).Incubation begins soon after the first egg is laid and results in asynchronous hatching (Butler 1992). Hatching occurs after about 27 days of incubation (Butler 1992), though the incubation period in a colony can last for much more than a month as pairs often re-nest after predation or other disturbance events (Vennesland 2000). The chick rearing period lasts about 60 days (Krebs 1974; Simpson 1984).
Herons require about 95 days to complete a nesting cycle, but regularly take much longer than this if re-nesting or other delays occur. Herons can potentially breed more than once if their first attempt fails early.
In south-coastal British Columbia, the number of fledglings raised in a nest varies from 0 to 4 (Butler 1992; 1997; Vennesland 2000). Historically, the nesting productivity of herons in studies across North America has ranged from 1.3 to 2.7 fledglings per active nesting attempt, and from 2.0 to 3.0 fledglings per successful nesting attempt (reviewed by Butler 1997; see also Pratt 1970 and Vos et al. 1985). Fewer than 25% of juveniles survive to their second year after which survival increases to about 75% per year for adults (Butler 1997).
Nesting colony characteristics
Pacific Great Blue Herons in British Columbia are normally arboreal nesters and nest solitarily and in colonies (Butler 1992; 1997; Vennesland 2000). Nests are large stick platforms, usually 20 m to 30 m above ground (Butler 1997), but some have nested as low as 2 m in shrubs (Vennesland 2000). For south-coastal British Columbia in 1999, Vennesland and Butler (2004) reported a ‘colony’ size range of 1 to 400 active nests, with a mean of 62 active nests (SD = 94, n = 31) and a median of 26 nests. The most common tree species used for nesting are Red Alder (Alnus rubra), Black Cottonwood (Populus balsamifera), Bigleaf Maple (Acer macrophyllum), Sitka Spruce (Picea sitchensis) and Douglas Fir (Pseudotsuga menziesii) (Gebauer and Moul 2001). See Gebauer and Moul (2001) for a full list of tree species used.
Predation
In British Columbia, the Bald Eagle (Haliaeetus leucocephalus) is the primary predator of Pacific Great Blue Herons (Butler 1997; Gebauer and Moul 2001; Vennesland and Butler 2004). Bald Eagles prey on heron eggs, nestlings, juveniles and adults (Simpson and Kelsall 1978; Forbes et al. 1985b; Forbes 1987; Forbes 1989; Simpson et al. 1987; Norman et al. 1989; Butler et al. 1995; Butler 1997; Gebauer and Moul 2001; Vennesland and Butler 2004), and have been responsible for reduced nesting productivity at many colonies (Norman et al. 1989; Gebauer and Moul 2001; Vennesland and Butler 2004). Repeated eagle predation is the suspected cause of many colony abandonments (Forbes et al. 1985b; Simpson et al. 1987; Butler 1991; Butler 1997; Gebauer and Moul 2001; Vennesland and Butler 2004). The effects of Bald Eagles are covered in more detail in the Limiting Factors and Threats section.
Other birds of prey also have been observed preying on Pacific Great Blue Heron nest contents, including Red-tailed Hawks (Buteo jamaicensis) on both eggs and nestlings (Simpson 1984, Simpson and Kelsall 1978, Forbes et al. 1985b, Norman et al. 1989, Butler 1997, Vennesland and Butler 2004) and Northwestern Crows (Corvus caurinus) and Common Ravens (Corvus corax) on eggs (Butler 1989; Moul 1992). Raccoons prey on nesting herons, but in British Columbia disturbance from non-human mammals is rare (Butler 1997).
Physiology
Specific research on physiological requirements or tolerances of Great Blue Herons from a conservation perspective has been rare. Monitoring of contaminants in eggshells and fetal tissues of Pacific Great Blue Herons has been ongoing since about 1977 (Elliott et al. 1989; Elliott et al. 1996; Elliott et al. 2001; Harris et al. 2003). Currently, contaminants (e.g., organochlorine pesticides, polychlorinated biphenyls, dioxins, furans) are not seen as a significant conservation issue for this subspecies as concentrations have generally been in decline over recent years (Elliott et al. 1989; Elliott et al. 2001; Harris et al. 2003). Recently, however, attention has been paid to chemicals that have not previously been tracked. Concentrations of one class of chemicals in particular (polybrominated diphenyl ethers; PBDEs) has been found to be increasing exponentially in heron tissues 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 urban areas (Elliott et al. 2005). There are similar concerns over another emerging class of industrial pollutants, the perfluoro chemicals (PFCs). Environment Canada is currently in the process of investigating spatial and temporal trends of those chemicals, including in herons from the Georgia Basin (J. Elliott, pers. comm.).
Dispersal/migration/movements
The Pacific Great Blue Heron is non-migratory. Banded individuals are known to disperse between habitats in the Strait of Georgia and a few individuals have been found dead in the interior of British Columbia and in coastal Washington and northern Oregon (Butler 1997). Most individuals on the Fraser River delta and other south-coastal areas forage along beaches from March to October and along beaches and grasslands in winter (Butler 1995; 1997). Juvenile herons forage along beaches until about October and reside largely in grasslands in winter (Butler 1995; 1997). When not nesting, herons on the coast of British Columbia roost alone or in loose flocks of over 100 individuals on the ground, in trees, and on man-made objects near feeding grounds during the day (Butler 1992). Some roosts are used repeatedly (Butler 1992). At night, herons sleep in trees with dense foliage during high tide and forage on beaches at low tide (Butler 1992). Limited research has been conducted on annual movements between colonies. Simpson et al. (1987) concluded that considerable movement might occur (40% of nesting herons did not return to breed in the second year of the study). Movements between regions are largely unknown but assumed to occur, as birds have been seen by both authors flying across the Strait of Georgia. Colonies will suddenly grow when new arrivals settle, presumably from an abandoned nearby colony (R. Vennesland, unpubl. data).
Interspecific interactions
Interactions of Pacific Great Blue Herons with their predators are described previously in the Predation section. Prey includes a wide array of animals including fish, insects, mammals, amphibians, and crustaceans (Butler 1992; 1995; 1997). Fish are a mainstay food item during the nesting season, demonstrated by summer congregations of more than 600 herons feeding together in eelgrass meadows near Tsawwassen on the Fraser River delta (R. Vennesland, unpubl. data). In winter, small mammals in agricultural areas are also important, especially for juvenile survival (Butler 1991; 1995; 1997; Gutsell 1995). Little information is available on diseases, although some have been documented in Great Blue Herons (but not Pacific Great Blue Herons), including Giardia and Eustrongylides nematodes (Butler 1992).
Adaptability
Some Pacific Great Blue Herons can tolerate human activities near their nests, but many are sensitive to the presence of humans (reviewed by Vennesland 2000; Gebauer and Moul 2001). Human activity near colonies of herons compounds the threat posed by eagle predation to this subspecies (Vennesland 2000, Vennesland and Butler 2004). Butler et al. (1995), Carlson and McLean (1996) and Vennesland and Butler (2004) showed that the number of fledglings raised in Great Blue Heron colonies with frequent disturbances was significantly lower than at colonies with no disturbance. When disturbed, herons leave nests unguarded, especially early in the nesting season when humans enter colonies on foot or when loud noises occur nearby (Vennesland 2000). Corvids take eggs when the opportunity arises (Butler 1989; Moul 1992).
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