Elkhorn Slough National Estuarine Research Reserve: Monitoring rookery of nesting herons, egrets and cormorants

The purpose of this monitoring is to track the number of tree-nesting ardeids and phalacrocoracids adjacent to the Elkhorn Slough estuary in central California, USA.  Three species nest together in large rookeries in trees on the banks of the Elkhorn Slough estuary: great blue herons, great egrets, and double-crested cormorants.  Elkhorn Slough National Estuarine Research Reserve teams have been counting nests since 1985 to the present (2025 at current data submission).

Methods
Teams of ESNERR staff members, volunteers, and associated students counted nests using binoculars and spotting scopes. The number of active nests belonging to each species was counted from one or multiple vantage points beneath the trees that were being used.  Nests of each species are fairly distinctive (cormorants have more eelgrass/leaves woven in, heron nests are the largest and full of twigs, etc.), and the surveys were conducted in late Spring when chicks were in or near the nests. The numbers reported are likely to be underestimates, especially for the smaller cormorant and egret nests, because some nests are on the back side of the trees relative to the observer vantage points. The exact locations of the rookery in different periods are provided in an associated file called Rookery_Metadata_Sites.csv

From 1985-2008, the rookery was located in a grove of Monterey pines on the main Elkhorn Slough Reserve, in an area bordered by the Oak Marsh Trail, the Rookery pond, and the staff residences. Herons colonized first, followed a few years later by egrets, and considerably later by cormorants.  Guano from the nesting birds killed the trees used.  So the rookery moved over time from close to the Rookery pond and Oak Marsh Trail further back towards the staff residences and Elkhorn Road.  By 2008, most pines were dead, and very few birds attempted to nest.

In 2009, all three species nested in a new location, the eucalypt grove on the Seal Bend portion of the Elkhorn Slough Reserve.  They continued to nest there up to the time this file was prepared (2025).  No nesting occurred in the pine site on the main Reserve during this period.

In 2020, a small number of egret nests were observed on oak trees on the Avila portion of the Elkhorn Slough Reserve, facing Parsons Slough.  Some herons joined in subsequent years.  They continued to nest there in small numbers up to the time this file was prepared (2025).

Nest counts occurred in the active rookery areas.  Elkhorn Slough estuary is immensely popular with birdwatchers, and there is relatively high confidence that no nests of any of these three species occurred elsewhere adjacent to the estuary.

Nest counts were made in late Spring each year and have been carried out from 1985-2025.  There are some early years with missing data (1988-1991; 1994-1995), but other than that, it is a continuous record.  More details on surveys across the years, including the location of the archived field datasheets that have more information, are in an associated file called Rookery_Metadata_Surveys.csv