Distribution and Abundance of Colonial Herons and Egrets in California, 2009–2012

Abstract

As part of an 11-state inventory in the western United States organized by the U.S. Fish and Wildlife Service, we coordinated censuses of 15 species of breeding colonial waterbirds throughout California from 2009 to 2012. Here we describe the status of the five widespread species of colonial ardeids in California during that period, combining the results of surveys from the air, boats, and ground. Statewide, we estimate 5517 pairs of the Great Blue Heron (Ardea herodias) nesting at 399 sites, 7973 pairs of the Great Egret (Ardea alba) at 182 sites, 1888 pairs of the Snowy Egret (Egretta thula) at 79 sites, 2678 pairs of the Cattle Egret (Bubulcus ibis) at 20 sites, and 2443 pairs of the Black-crowned Night-Heron (Nycticorax nycticorax) at 104 sites. For four of these species, the numbers of colonies and breeding pairs were highest near the coast and in the Central Valley, much lower in the Great Basin, Cascade Ranges, Sierra Nevada, and southern deserts. By contrast, about twothirds of the statewide total of nesting pairs of the Cattle Egret was in the Imperial Valley. The Central Valley was particularly important to the two most numerous species, holding about three-quarters and one half of the state's nesting pairs of the Great Egret and Great Blue Heron, respectively. The survey period coincided with drought, which greatly reduced potential foraging habitat in many regions and may have restricted herons' distribution and depressed their abundance. In the lack of broad-scale surveys during a wet climatic period, no quantification of the effect of drought on herons is possible, though such data are available for other colonial waterbird species. Although the populations of the five herons appear to be stable or increasing, considerable uncertainty in the magnitude and direction of trends remains because of substantial year-to-year variation in numbers of nests and a lack of a robust broad-scale monitoring program suited to these species. Plans for longterm monitoring of ardeids and other colonial waterbirds must account for the large fluctuations in their distribution and abundance over short-term cycles of drought and flood, and factor in the expectation of greater environmental fluctuations with continuing climate change.Initiatives to promote the conservation of waterbirds throughout North America recognize the importance of inventorying and monitoring. Such work is crucial for determining conservation status, detecting population a Appendices 1 and 2 and Tables S1-S5 are available at archive.westernfieldornithologists. org/archive/V51/Shuford-et-al-herons.

Effects of drought on the abundance and distribution of non-breeding shorebirds in central California, USA

“…In 2012, annual shorebird abundance peaked in response to widespread flooding of shorebird habitats that was supplemented by seasonal precipitation before and during the survey period. Prior to winter 2012-2013, four of six previous winters had less than average precipitation and there was already a declining trend in the habitat availability and abundance of colonial breeding herons and egrets (family Ardeidae) in the Central Valley [52]. In 2016, which was the first year of average precipitation after the most extreme years of drought in 2014 and 2015, our findings suggested a potential time lag in the response of non-breeding shorebirds repopulating some flooded habitats [7,11].…”

Distribution and Abundance of Double-crested Cormorants Nesting in the Interior of California, 2009–2012

“…In 2012, we also surveyed the very limited potential habitat for nesting cormorants in the Klamath River country of northwestern California in Shasta, (mainly) western Siskiyou, and northern Humboldt counties. Descriptions of these study regions and the survey methods used in each are detailed in Shuford et al (2020); the methods and areas surveyed were very similar to those used in a comparable statewide survey for cormorants in the interior from 1997 to 1999 (Shuford 2010).…”

“…Observers generally followed the protocols of the Western Colonial Waterbird Survey (Jones 2008), whether from the ground, aircraft (visually or with photographs), or boat. Methods varied by species, colony site, and region, depending on the species' nesting habits and local conditions, such as variation in nest substrates, proximity of other nesting species, and accessibility of colony sites (Shuford 2014a, Shuford et al 2020. Aerial surveys were particularly valuable for photographing some large or remote cormorant colonies and for visually covering large areas, such as the Central Valley, that would have been impossible to survey adequately by other methods alone.…”

“…Patterns of annual precipitation varied before and through the study but included several years of drought preceding survey years in most regions of the state (Shuford et al 2020, which see for details of rainfall history by ecoregion). Of particular relevance to the cormorant, our surveys of northeastern and east-central California in 2009 followed 3 years of drought that left some terminal lakes, reservoirs, and wetlands dry or with atypically low k The discrepancy between the 6865 pairs reported in Shuford (2010) for the 1999 survey and the 7170 reported here for 1998-1999 reflects (a) a slight enlargement of the interior survey area in 2009-2012, to include Suisun Marsh, and a comparable retrospective enlargement of the 1999 survey area, leading to the addition of a count of 110 pairs at one site in Suisun Marsh in 1999 that was not included in the prior total, and (b) the inclusion of data on four colonies counted in 1998 but not 1999. water levels.…”

“…In California, Point Blue Conservation Science and its collaborators coordinated surveys of 15 primary species of colonial waterbirds (Shuford 2014a). Results of those surveys from 2009 to 2012, and comparisons to prior surveys, have been published for three species of terns, two of gulls (Shuford et al 2016, Doster andShuford 2018), and five of ardeids surveyed statewide (Shuford et al 2020). Here we report on the distribution, number of colonies, and number of pairs of Double-crested Cormorants nesting in the interior of California from 2009 to 2012, and compare our results to those of surveys in 1998 and 1999 (Shuford 2010).…”