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The American Ornithologists' Union POPULATION REGULATION Severe winters might reduce northern popula- tions when foraging sites freeze (Blus and Henny 1981). Populations in Florida decimated by hurricanes, but recovered to former levels (Powell et al. 1989). Evidence for regulation during the breeding season weak or non - existent. Nest sites not limiting in general; number of breeding pairs in colonies positively related to the area of nearby wetlands (Bayer and McMahon 1981, Gibbs et al. 1987), suggesting food supply a critical limitation, perhaps not surprising in a species so strongly territorial. CONSERVATION AND MANAGEMENT EFFECTS OF HUMAN ACTIVITY Contaminants. Eggshell thickness correlated negatively with DDE (Blus et al. 1980, Laporte 1982, Fleming et al. 1984) but not PCB (Vermeer and Reynolds 1972, Ohlendorf et al. 1979, Fleming et al. 1984, but see Blus et al. 1980), although PCB levels were high in a colony that failed in Quebec (Laporte 1982). Mean eggshell thickness has declined compared to pre-1947 levels (Anderson and Hickey 1972): 5.3% to 13.1% (12 colonies, Laporte 1982; 2 colonies, Blus et al. 1980;13 colonies, Bayer 1982; 4 colonies, Fleming et al. 1984). Eggshells broken during incubation an average of 14% to 17% thinner than pre-1947 levels (Faber et al. 1972, Bayer 1982). Wide variation in intraclutch eggshell thickness (Fleming et al. 1984). No clear evidence of adverse effects on reproductive success (Blus et al. 1980, Fleming et al. 1984), but measures of success often too crude to detect subtle differences (see Demography and Populations: measures of breeding activity). Induction of liver enzymes (Bellward et al. 1990), reduced growth and development of young (Hart et a1.1991), and possible reproductive failure (Elliott et al. 1988, 1989) associated with colonies highly contaminated with dioxins (but see Moul 1990). High con- centrations of dieldrin (Ohlendorf et al. 1981) and endrin (Ohlendorf et al. 1979) suspected to be lethal to Great Blue Herons. Disturbance. Nest and colony abandonments increase with increased visits by humans (Drapeau et al. 1984) and with road building and logging activity within 0.5 km (Werschkul et al. 1976). Some colonies splinter and attempt to settle nearby following abandonment (Parker 1980). Response to disturbance can vary between sites and time of breeding season (Vos et a1.1985). Early in season, herons flush easily from nests with slightest disturbance; after eggs, they fly reluctantly and ROBERT W. BUTLER 13 Figure 6. Great Blue Heron at rest. Drawing by AIIan Brooks; US Fish & Wildlife Service art collection, archives of the Academy of Natural Sciences (Stewart Library), Philadelphia. return quickly to nests; few flush when chicks in nest. Birds habituate to non - threatening repeated activities (Anderson 1978, Parker 1980, Vos et al. 1985). Colonies are probably dynamic in areas of high disturbance with individuals and entire colonies relocating between years. Most studies recommend a minimum 300 m buffer zone from the periphery of colonies in which no human activity should take place during courtship and nesting seasons, with the exception of scientific study (reviewed by Butler 1992); however, the most easily disturbed herons left nests in a colony in British Columbia when Butler (1992) approached on foot within 200 m early in the season. Other colonies were less easily disturbed early in the season. This species might have suffered more from loss of wetlands than from overt human activities (English 1978, Rosenberg et al. 1991) and benefitted from open fish ponds and hatcheries. Populations suffered from shooting in past but have not retracted from former range. Attitudes have changed; conspicuousness of this bird that led to its hunting early in the century may now act The Academy of Natural Sciences of Philadelphia