Herons are particularly sensitive to disturbance while nesting. Scientists suggest as a general rule that there should be no development within 300 m of the edge of a heron colony and no disturbance in or near colonies from March to August.
In general, herons select nesting sites away from human activity, in quiet clusters of trees. If humans disturb heronries during breeding season, several studies show, the reproductive rate of the colony can drop or adult herons may move the entire colony. Bald Eagles may also attack heron chicks and cause a colony to move.
Many herons are sensitive to human activities near their nests. The sensitivity is most apparent early in the nesting season when herons are building nests and laying eggs. As a rule, general day-to-day activity by humans that reside near colonies does not interfere with heron nesting activities. It is novel sounds that frighten herons from nests and lead to abandonment. Sudden blasts of horns or dynamite and starting of chain saws are known to frighten herons from nests. Colonies will sometimes abandon if these activities persist.
At one time, biologists counted colonies made up of 100 or 200 nests. Recently, because of urbanization and habitat loss, colonies have been breaking up into smaller units of 30 or 40 nests, said Nanette Seto, wildlife biologist at Nisqually Refuge. Sometimes the colonies fail to fledge any chicks at all. The colony at Nisqually has been a complete failure of late.
Evidence based on the Canadian Wildlife Service’s Breeding Bird Surveys (BBS) conducted between 1969 and 1996 indicates that the Great Blue Herons of the coast of British Columbia (includes Queen Charlotte Islands and Vancouver Island) are declining at a rate of 3.5 per cent per year. As part of this decline, few herons now breed on the Sunshine Coast although they were numerous in the 1980’s. Although not as dramatically as the Sunshine Coast, the numbers on Central Vancouver Island have also dropped in recent years.
Yet many wildlife experts think that if the highly adaptable great blue has an Achilles' heel, it is probably the shrinking number of places suitable for its nesting colonies. It is not uncommon for herons to pull out of a colony for no apparent reason. They seek out nesting areas that are isolated from intrusion by human and mammalian predators.
Although the great blue heron has become a symbol of wetland conservation, in recent years it has had to face new challenges as a consequence of rapid urbanization of its environment. "They don't like predators or people nearby." And if they are disturbed by one or the other early in the breeding season, they may abandon their nests for the year--or forever.
As humans encroach on prime heron nesting sites, such disturbances are on the rise. Although heron populations are stable or increasing in most of their range, biologists sound a note of caution. "Great blues are not something we have to worry about now, but in 30 years, maybe," says R. Michael Erwin, a biologist with the U.S. Geological Survey's Patuxent Wildlife Research Center in Maryland.
Great blue herons nest together in colonies, otherwise known as a heronry, and are sensitive to the effects of human disturbances. In one study of nesting behavior on the Oregon coast, heronries (with 15 to 161 active nests per heronry) were found in the tops of red alder, western hemlock, and sitka spruce. The study showed that there were far more active nest sites in isolated areas than areas adjacent to logging (Werschkul 1976) indicating a preference for areas away from human activity. Great blue herons often nest in the tops of trees in addition to rock ledges, sea cliffs, and the ground (Palmer 1962).
It is now evident that this species is experiencing a decline in Illinois. 25 colonies censused each year from 1974-76 indicate a 39% decline with a progressive loss of 12-18% per year (nesting birds).
Great Blue Herons are sensitive to exposure to environmental contaminants and pollutants that can lead to breeding failure. They also can be very sensitive to human disturbance at colonies. Loss of good nesting habitat near foraging areas may decrease reproductive output. (Cannings et al. In prep.).
And last year, seven of the 10 largest blue heron colonies in Puget Sound abandoned their usual nesting grounds and produced no chicks -- including the Black River forest colony. Because of the widespread abandonment, the state Department of Fish and Wildlife is now reviewing the status of the great blue heron. At this time, the state lists the birds a "priority species" and compels local government to preserve critical areas for them. With the Black River Riparian Forest, Renton has done that. But the question remains, has the city done enough, Norman said. "My whole point is, if we want to keep blue herons at a certain level in Puget Sound, we need to begin making some plans," he said. "And that means closely examining development issues like this one."
The California Department of Fish and Game has discovered that the heron population is getting smaller. It is a "species of concern", meaning that it could become endangered if this trend does not change.
The loss of herons is mostly due to habitat disturbance. People are constructing buildings in places near heronries. Herons are very sensitive to human presence around their nests and will abandon eggs and young if they are disturbed.
A. herodias is negatively affected by human disturbance, particularly during the early stages of the breeding cycle. Repeated human intrusion into nesting areas often results in nest failure, with abandonment of eggs or chicks (Carlson and McLean 1996). Great blue herons thus tend to choose nest sites in undisturbed wetland areas around lakes or rivers, well removed from roads (Gibbs and Kinkel 1997) and other signs of human activity. In one Ohio study, foot traffic was observed to be the most detrimental disruption to breeding great blue herons, sometimes leading to nest abandonment. Nesting birds were shown to tolerate routine mechanical disturbance, even if loud, as long as no humans were nearby (Carlson and McLean 1996). Nest success increased when nest sites were chosen near barriers such as fencing, ditches or moats that excluded human intrusion. Nests located in areas with this type of buffer zone, even if small in overall area, had higher nest success and fledging rates than those located near disturbed areas.
Nesting colonies are often abandoned as a result of human disturbance (Markham and Brechtel 1979). Human disturbance and loss of nesting and foraging sites have been important factors contributing to declines of some great blue heron populations (Thompson 1979, Kelsall and Simpson 1980, McCrimmon 1981).
Although this species is prevalent in Canada, much of the great blue heron's wetland and wooded island habitat in southern Canada continues to be degraded, drained and cleared. Consequently, this species is on the Audubon Society Blue List.
- Butler, R.W. 1995. The patient predator: foraging and population ecology of the Great Blue Heron (Ardea herodias) in British Columbia. Canadian Wildlife Service Occasional Paper No. 86. 44 pp.
- Butler, Robert W. 1997. COSEWIC Status Report on the PACIFIC GREAT BLUE HERON, Ardea herodias fannini. Committee on the Status of Endangered Wildlife in Canada. 18 pp.
Human activity near heron colonies poses the largest threat to this subspecies. The number of fledglings raised in heron colonies with frequent disturbance is significantly lower than at colonies with no disturbance, since disturbed adult herons leave eggs and nestlings unguarded and vulnerable to predation by Bald Eagles and other birds. The quiet forested areas near foraging habitats that are preferred by herons for nesting have become increasingly scarce in southern British Columbia, as a result of human encroachment.
Herons are particularly sensitive to disturbance while nesting. Scientists suggest as a general rule that there should be no development within 1000 ft of the edge of a heron colony and no disturbance in or near colonies from March to August.
Human disturbance is probably the major factor influencing nesting and foraging activities of the great blue heron. Though herons may become habituated to some human activities (Grubb 1979, Kelsall and Simpson 1980), heron nesting colonies are often abandoned due to human disturbance (Markham and Brechtel 1979). Colonies are typically located far from human structures in areas with abundant natural buffers (forest and wetland) and a low road density (Watts and Bradshaw 1994, Gibbs and Kinkel 1997). Major threats to great blue herons include housing and industrial development, water recreation, road construction, and any other activities that result in habitat degradation (Simpson and Kelsall 1979, Ryder et al. 1980, Popotnik and Giuliano 2000).
The largest colony is located just over the border in Point Roberts, Washington. This colony has over 400 active nests. While Great Blue Heron are a common sight, the coastal subspecies is on the British Columbia "blue" list, and is nationally listed as "vulnerable". Like most breeding birds, herons are easily disturbed. When viewing at a colony, keep your distance (at least 100 m) and use binoculars or scopes. Do not venture under the nesting trees or into surrounding habitats.
In 1990 it was estimated that 350 pairs of great egrets were breeding in the San Francisco Bay along with 160 pairs of great blue herons.
Some local populations of Great Blue Herons have declined in North America (see review by Butler 1992) and the species is considered "sensitive and vulnerable" in British Columbia (B.C. Ministry of Environment 1990). Many colony-sites have been abandoned following disturbance by humans (Werschkul et al. 1976, Kersall and Simpson 1979) and predators, especially the Bald Eagle (Haliaeetus leucocephalus; Norman et al. 1990). Contaminants might also reduce the nesting success of some herons (Elliott et al. 1989).
Management studies recommend "buffer zones" from human disturbance around colonies during the heron breeding season (e.g. Buckley and Buckley 1976, Parker 1980, Short and Cooper 1985, Koonz and Rakowski 1985, Vos et al. 1985). However, I know of no studies that have led to recommendations on protection of foraging and roost sites.