Several species warrant special consideration in tree care due to the nature of their nests. They include herons, egrets, and cormorants.
Heron and Egret Nests
Species include Great Blue Heron, Great Egret, Snowy Egret, Green Heron and Black-crowned Night Heron.
Heron rookeries or nesting colonies, are frequently located in areas isolated from human disturbance, such as riparian corridors, marshes, and groves of trees next to water or on islands. However, some herons have adapted minimally to human activity and may nest in trees near apartment and condo complexes, ports, and harbors that have large trees.
Herons are especially vulnerable to human disturbance and habitat destruction during pair formation and the breeding season (mid-November to September of the following year) when large numbers of birds are concentrated in a rather confined area. Herons are delicate and tend to abandon nests and entire colonies if disturbed during periods of pair forming (starting mid-November), nest construction (starting in January), or early egg laying (as early as January).
Herons continue to be sensitive to disturbance after hatching and up until the young fledge (up to late September). In some cases, colonies have even been deserted after destruction or alteration of their habitat during the non-nesting season. Even if herons relocate after deserting a colony, consequences of disturbance include fragmentation of breeding populations, total reproductive failure in colonies, reduced number of breeding pairs, and reduced reproductive output per pair. Ultimately this can affect the stability of the entire regional population (Bowman & Siderius, 1984).
Herons are unpredictable in their response to disruption of a colony, and the severity of the response does not always correspond to the magnitude of the disturbance. Seemingly innocuous activities can produce serious results. The most important factors to consider when evaluating these effects are the timing of the disturbance in relation to critical periods of the nesting season and the degree to which the birds are able to adjust to human activities. Herons are sensitive to humans and mammals moving around under their nesting trees.
When conducting surveys or inventories, take care to avoid walking into heronries, especially under nesting trees (indicated by the ring of white guano around the base of the tree). Should you find yourself within a heronry, leave quietly and quickly by the same route you entered.
Density of vegetation in and around the colony can influence the impact of disturbances. The removal of vegetation near a colony can open paths into the heronry that would not only enable intrusion by humans and predators, but would result in an increased number of exposed nests. Maintaining the vegetation, including trees and shrubs, around a colony provides alternate nest sites and a buffer against disturbance.
Tree trimming should generally not remove more than the minimum of foliage necessary for human health and safety, and should be done, where it is permitted, in a manner that does not unnecessarily discourage herons and egrets from returning to their altered (trimmed) habitat during the next breeding cycle.
HERON & EGRET BREEDING ACTIVITY:
Great Blue Heron: Mid-November to end of September Great Egret: Late February to end of August Snowy Egret: Early April to late July Green Heron: Early April to early August Black-crowned Night Heron: Mid-January to end of SeptemberCormorant Nests
The Double-crested Cormorant, another colonial nesting species, occupies similar habitat and nesting areas as herons. Cormorants also need the protection of buffer zones while nesting, but they appear to be less sensitive to human disturbance than herons.
CORMORANT BREEDING ACTIVITY:
Late March to mid-October (especially in Lake Merced)
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Information on breeding activity from San Francisco Breeding Bird Atlas and from breeding bird atlas records. Some of the information on herons and cormorants is adapted from “Special Management Practices for Herons,” courtesy of Audubon Canyon Ranch. Although the information was provided by biologists in Marin County it is relevant to these species in most locales. Thank you to Los Angeles Audubon and local arborists, biologists, and ornithologists who helped determine modified dates to fit conditions in San Francisco.