Distribution and abundance of colonial herons and egrets in California, 2009-2012


Shuford, W. D., J. P. Kelly, T. E. Condeso, D. S. Cooper, K. C. Molina, D. Jongsomjit,

Publication Date:
Aug 2020

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 nyctico- rax) 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 two-thirds 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 long- term 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 trends, assessing habitat health, and evaluating whether management and environmental change are affecting waterbirds (Kushlan et al. 2002). To help fulfill this need, from 2009 to 2012 the U.S. Fish and Wildlife Service coordinated the Western Colonial Waterbird Survey, a broad-scale inventory of 19 species of colonial waterbirds in 11 western states (www.fws.gov/mountain- prairie/migbirds/species/birds/western_colonial/index.html). Goals were to document the species composition, size, and location of colonies; estimate minimum regional population sizes for each breeding species; produce an atlas of colonies; and establish a baseline for development of a long-term monitoring program for colonial waterbirds in the West. To date, a lack of adequate data on population sizes and trends of waterbirds has hampered efforts to set population and habitat goals for conservation of these birds in California (Shuford 2014b, Shuford and Dybala 2017).To ensure adequate coverage of the vast expanse of the West, regional experts organized and implemented the surveys at the state level. In California, Point Blue Conservation Science coordinated surveys of 15 primary species of colonial waterbirds with the extensive aid of many collaborating organizations and individuals (Shuford 2014a). Results of those surveys, with comparisons to prior surveys in the late 1990s, have been published for three tern species and two gull species (Shuford et al. 2016, Doster and Shuford 2018). Here we report on the distribution, number of colonies, and estimated number of pairs nesting in California from 2009 to 2012 for five species of colonial ardeids: the Great Blue Heron (Ardea herodias), Great Egret
(Ardea alba), Snowy Egret (Egretta thula), Cattle Egret (Bubulcus ibis), and Black-crowned Night-Heron (Nycticorax nycticorax). Early assessments of herons’ status in California were based on anecdotal information, and more recent efforts to quantify the abundance of the state’s nesting ardeids have substantial limitations (Belluomini 1978, Schlorff 1982). Nevertheless, we compare the 2009–2012 results to these prior assessments and other broad-scale and regional surveys or monitoring programs extending over multiple years and interpret results on the basis of environmental conditions during the 2009–2012 survey period. We discuss current and future threats to colonial waterbirds in California, and outline in development of a strategy for monitoring birds throughout the western United States.

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Journal Article

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Shuford, W. D., J. P. Kelly, T. E. Condeso, D. S. Cooper, K. C. Molina, D. Jongsomjit. 2020. Distribution and abundance of colonial herons and egrets in California, 2009 - 2012. Western Birds 51:190 - 220, 2020; doi 10.21199/WB51.3.2.