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GREAT BLUE HERON
<br />Predation. Northwestern Crows (Corvus
<br />caurinus) and Common Ravens (C. corax) eat
<br />unattended eggs (Butler 1989). Predators of
<br />nestlings include eagles (Kelsall and Simpson 1979,
<br />Koonz 1980, Forbes 1987, Norman et al. 1989),
<br />raccoons (Lopinot 1952, Hjertas 1982), bears (Foss
<br />1980, Parker 1980), Turkey Vultures (Meitner 1951)
<br />and Red - tailed Hawks (Buteo jamaicensis;Simpson
<br />1984). Colony -sites abandoned after predators kill
<br />adults (Butler 1991) and nestlings (Kelsall and
<br />Simpson 1979, Simpson et al. 1987).
<br />BREEDING
<br />MATE SELECTION
<br />Little information. Of 5 marked pairs in 1978,
<br />all had new mates the following year (Simpson
<br />1984). Herodias and occidentalis form pairs and
<br />produce viable offspring with white and blue
<br />plumages (Bent 1926, Powell et al. 1989).
<br />PHENOLOGY
<br />Pair formation. At some northern latitudes,
<br />adults gather in flocks on the ground for several
<br />days in spring before entering colony -sites
<br />( "gathering grounds" sensu Mock 1976). Birds do
<br />not occupy gathering grounds in Texas and
<br />Michigan, and some roost at colony -site year -
<br />round (Cottrille and Cottrille 1958, Mock 1976).
<br />Function of gathering ground unclear.
<br />Little published information on dates of pair
<br />formation. Earliest adults return to colony -sites as
<br />follows: California, Dec (Brandman 1976); Texas,
<br />Jan (Mock 1976); British Columbia, mid -Jan on the
<br />coast, late Mar interior sites (Butler et al. 1986,
<br />RWB, Cannings et al. 1987); Pennsylvania, mid -
<br />Feb (E. Bruckner pers. comm.); Alberta, late Mar
<br />(Vermeer 1969). Herons . in some colonies apt'to
<br />depart from colony -sites with little provocation at
<br />this time;: may be inhibited from displaying by
<br />high winds and low temperatures (Palmer 1962).
<br />Courtship to egg laying early Jan to mid -Mar in
<br />California (Brandman 1976), mid -Feb to early Apr
<br />in British Columbia (RWB). Great White Heron
<br />(occidentalis group) breed year -round but most
<br />begin between Sep and Feb (Powell 1983).
<br />First brood per season. Figure 3. First eggs: late
<br />Feb in Oregon (Henny and Bethers 1971) and n.
<br />California (Pratt 1970, Pratt and Winkler 1985);
<br />early Mar in s. California (Brandman 1976); 3rd
<br />week Mar in Idaho (Collazo 1981); 1st week Apr in
<br />s. British Columbia (Butler 1989); mid-Apr in Nova
<br />Scotia (McAloney 1973); late Apr in Alberta
<br />(Vermeer 1969).
<br />The Birds of North America, No. 25, 1992
<br />Imo Primaries
<br />Molt Body
<br />Breeding Young
<br />E
<br />Mgration 7:11
<br />Figure 3.
<br />Annual cycle of
<br />breeding, molt, and
<br />migration of Great
<br />Blue Herons In
<br />British Columbia.
<br />Coastal populations
<br />are non-migratory.
<br />Thick lines equal
<br />peak activity, thin
<br />lines off peak.
<br />A. Poole, P. Stettenheim, and F. Gill, Editors
<br />Second brood per season. Unclear how many
<br />late -laid Butches are first or second attempts. Two
<br />pairs renested after fledging first brood, 5 pairs
<br />after death of nestlings in California (Brandman
<br />1976). Most choose new mates after first attempt
<br />fails (Simpson 1984). Insufficient time to raise 2
<br />broods at northern latitudes.
<br />NEST SITE
<br />Nest as single pairs but mostly in colonies (Fig.
<br />4). Mean distance flown from colony to principal
<br />feeding sites 2.3 to 6.5 km (Dowd and Flake 1985b,
<br />Thompson 1978, Parris 1979, Butler 1991). Two
<br />radio - equipped breeding adults travelled 61.8 and
<br />103.7 km in 20 to 25 h (Parris 1979). Number of
<br />nests in British Columbia (Butler 1991), Oregon
<br />(Werschkul et al. 1977, Bayer and McMahon 1981),
<br />and Maine (Gibbs et al. 1987, Gibbs 1991) positively
<br />related to area of nearby foraging habitat. Location
<br />of colony sites best explained by distribution of
<br />foraging habitats (Butler 1991, Gibbs 1991). Site
<br />selection also predator - driven; like most other
<br />herons, this species generally selects nest sites
<br />difficult for mammalian predators to reach, e.g.,
<br />islands, trees in swamps, high branches, etc.
<br />Recovery of beaver (Castor.canadensis)populations
<br />in ne. U.S. and s. Canada may have benefited this
<br />species by providing a patchwork of large and
<br />small swamps and wet meadows — additional
<br />nesting and foraging areas (e.g., N.Y. State; Andrle
<br />and Carroll 1988).
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