DOI: 10.5061/dryad.h9w0vt4vw
title: Elkhorn Slough National Estuarine Research Reserve: Monitoring rookery of nesting herons, egrets and cormorants

description: 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).
description: 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
description: # Elkhorn Slough National Estuarine Research Reserve: Monitoring rookery
 of nesting herons, egrets and cormorants Dataset DOI:
 [10.5061/dryad.h9w0vt4vw](10.5061/dryad.h9w0vt4vw) ## Description of the
 data and file structure 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.
 ### Files and variables #### File: Rookery.csv **Description:** Nest count
 data ##### Variables * Year: Monitoring Year * Species: one of three focal
 species * Nests_Rookery_Pines: number of nests in the Rookery pine grove *
 Nests_Seal_Bend_Eucalypts: number of nests in the Seal Bend eucalypt grove
 * Nests_Avila_Oaks: number of nests in the Avila oak grove * Nests_Total:
 total Elkhorn Slough nests; sum of previous three columns NA represents
 missing data - sites/years where no surveys were conducted. #### File:
 Rookery_Metadata_Surveys.csv **Description:** Metadata on who conducted
 the surveys, when, where hard copies of datasheets are stored, etc. for
 each year ##### Variables * Year: monitoring year * Location_Surveyed:
 locations surveyed (only active nesting areas surveyed throroughly) *
 Team_Census_Date: date of survey * Team_Census_Participants: people
 involved in survey * Archived_Field_Datasheet_Location: where hard copy of
 datasheets are stored #### File: Rookery_Metadata_Sites.csv
 **Description:** metadata with detail on location of nesting sites #####
 Variables * Location_Name: name of nesting area * Ownership: land owner *
 Locaton_Description: text describing location * Surveyer_Location: where
 counts were taken * Latitude: decimal degrees * Longitude: decimal degrees
 * Years_Active_Rookery: years when this area was used for nesting of the
 focal species * Notes: additional comments