Tidal Marsh Herons and Egrets

John Kelly (Audubon Canyon Ranch); Caitlin Robinson-Nilsen (San Francisco Bay Bird Observatory)

Status: Tidal marsh herons and egrets are stable to decreasing.

San Francisco Bay’s herons and egrets depend on large trees, dense types of vegetation, and man-made structures surrounded by tidal marsh, tidal mudflats, and non-tidal wetlands for nesting in spring and summer and for feeding year-round. Important feeding sites also include creeks and ponds.
View a map of the locations of all known egret and heron colonies in the San Francisco Bay Area: www.egret.org/googleearthheronries.


Great Blue Herons (blue) and Great Egrets (red) reveal dynamic but generally stable populations.

Black-crowned Night-Herons (blue) and Snowy Egrets (red) show dramatic variation in nesting abundances; however, recent trends (since 2005) suggest regional declines.

Great Blue Heron

Great Blue Heron

Great Egret

Great Egret

Snowy Egret

Snowy Egret

Black-crowned Night-Heron

Black-crowned Night-Heron

The number of nests of herons and egrets throughout the San Francisco Bay show dramatic variation from year to year with a potential decrease in the last 5–10 years. Large between-year declines are related to heavy rainfall, which can reduce the survival of young birds before they are old enough to breed.


Great Egret feeding half-grown chicks


Nest colony on Sherman Island in the North Bay

Threats

Primary threat: Loss or disturbance of colony nesting sites from damage to nest trees or construction activities (noise) that scare birds away from nesting sites.

Loss of wetland feeding areas close to the nesting colony (within 1–6 miles). Greater distance between nest and feeding areas reduces the chance of survival for their young.

Degradation of wetland feeding areas and associated declines in prey (fish, small mammals, invertebrates).

Nest predation by native or non-native pests, such as raccoons, feral cats, or ravens.

More intense winter storms, as predicted with climate change, lowers the survival of young egrets and herons.

Actions

Planning, Management, Restoration

Protect and restore tidal marsh and tidal flat habitat within 1-6 miles of nesting sites. This is the most urgent action needed to protect or sustain heron and egret nesting populations in San Francisco Bay.

Provide year-round protection to colony nesting sites. They are frequently destroyed when trees or other habitat features are removed or damaged during the non-breeding season (fall and winter). Such protection depends on local action, recognizing that heron and egret use of surrounding areas depends on the year-round protection of colony sites.

Create 200-meter buffer zones of no human activity around nesting areas during the nesting season (January–August).

Protect and restore wetland areas surrounding colony sites.

Create and protect clumps of native trees at distances of 5 miles or greater from existing colonies, preferably near open water.

Scientists

Improve models of heron and egret habitat sensitivity as potential biological indicators of wetland condition, and identify factors that can determine the linkage between colonies and surrounding habitat.


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