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Western Snowy Plovers and California Least Terns

Western Snowy Plover


Western snowy plovers are small shorebirds that breed along the Pacific coast of the United States and northern Mexico as well as at interior sites in several western states (U.S. Fish and Wildlife Service 1993). The Pacific coast population was recently listed as threatened (U.S. Fish and Wildlife Service 1992, 1993). This population nests in Washington, Oregon, California, and Baja California, Mexico, and is associated with coastal wetlands and coastal dune habitat (Palacios and Alfaro 1994; Page et al. 1995). As much as half of the Pacific coast population may breed in Mexico (Palacios and Alfaro 1994). This population winters along the coasts of southern Oregon, California, and Baja California, Mexico (Page et al. 1995). Some snowy plovers that nest along the coast of California do not migrate in winter but remain on their breeding grounds (Stenzel et al. 1994; Powell et al. 1995; Fig. 1).

Fig. 1. Western snowy plover nest, with two newly hatched chicks.
Courtesy A. Powell, USGS

The decline and loss of western snowy plover populations along the Pacific coast have been attributed to habitat loss and disturbance caused by urbanization. At northern sites, the invasion of nonindigenous beach grasses has reduced available breeding habitat, including dunes with scant vegetation, dredge-spoil islands, natural salt panne, and salt evaporation pond levees. The greatest loss of plover habitat has occurred along the southern California coast (U.S. Fish and Wildlife Service 1993). In southern California, many of the plover's nesting sites are associated with breeding colonies of California least terns.


Causes of low reproductive success in western snowy plovers include loss and degradation of breeding habitat, inclement weather, human disturbance, and increased numbers of predators associated with urban areas, including domestic and feral dogs, feral cats, red foxes, American kestrels, common ravens, American crows, striped skunks, Virginia opossums, and raccoons. Predators may take adults, chicks, or eggs. Plovers are highly susceptible to human disturbance and, if disturbed sufficiently, may abandon their nests. In addition, eggs have been lost from being trampled and run over by vehicles. At one site in coastal California, humans were directly responsible for the loss of at least 14% of nests over a 6-year period (Warriner et al. 1986). Chicks that become separated from adults through human disturbance or predators may die of exposure. Annual reproductive success for coastal snowy plovers in California has ranged from 0.8-0.9 fledglings per female near Monterey Bay to 0.8-1.1 fledglings per female in San Diego County (Warriner et al. 1986; Powell et al. 1995). Predation rates and levels of human disturbance are probably higher in central and northern California because plovers nesting in southern California benefit from site protection and predator management associated with California least tern colonies.


Western snowy plovers have high breeding-site fidelity, but some movement occurs between sites within and between years (Stenzel et al. 1994; Page et al. 1995; Powell et al. 1995). In addition, there is site fidelity associated with wintering areas (Page et al. 1995; A. Powell, U.S. Geological Survey, San Diego, California, unpublished data). Although some plovers return to their natal site to breed, there are few data on natal site fidelity. Little is known about the genetic makeup of snowy plover populations; however, banding studies indicate little mixing occurs among breeding sites in southern California (Powell et al. 1995).


Regular, standardized monitoring of western snowy plovers along the Pacific coast has not been conducted on an annual basis. However, a 20% reduction in population size was reported from surveys between the late 1970's and late 1980's, and winter numbers obtained from Christmas Bird Counts along the California coast declined significantly between 1962 and 1984 (Page et al. 1995). Other evidence of population decline has come from the documentation of the loss of breeding sites. Before 1970 snowy plovers nested at 53 sites along the California coast; the number of sites available has since been reduced by 62%. Currently, about 78% of the California breeding population is supported at only eight sites (U. S. Fish and Wildlife Service 1993). The breeding range along California's coast has been significantly interrupted by the loss of all historical breeding sites in Los Angeles County and most of Orange County. Loss of habitat in these areas has been attributed to high levels of recreational beach use and the raking of beach sand (for removal of debris) on a regular basis. Only one site in Orange County has supported a few nesting pairs in recent years (Powell et al. 1995).


Breeding populations of western snowy plovers in California continue to decline despite relatively high reproductive success at selected sites (Fig. 2). Numbers of snowy plovers surveyed in California during the middle of the breeding season in 1989, 1991, and 1995 were 1,139, 1,180, and 967, respectively (G. Page, Point Reyes Bird Observatory, Stinson Beach, California, unpublished data). Western snowy plovers have only been afforded protection by the Endangered Species Act for a short time, and populations appear to be steady or in decline.

Fig. 2. Populations of breeding western snowy plovers during statewide surveys in California in 1989, 1991, and 1995. Sites are listed from north (top) to south (bottom). Data compiled by Gary Page, Point Reyes Bird Observatory, Stinson Beach, California.

Management plans for snowy plovers need to include designation of critical habitat, protection of nesting areas from recreational use during the species' breeding season, increased monitoring of populations and their reproductive success, predator management, and education. There is some evidence of higher reproductive success for snowy plovers nesting in areas protected as California least tern breeding habitat, probably because of the limited recreational use and the predator management in these locations. However, many of the largest snowy plover breeding areas in California do not overlap with least tern colonies.


California Least Tern


California least terns (Fig. 3) are migratory and spend the breeding season, from April through August, along the central and southern California coast, as well as along northern Baja California, Mexico. Historically, the breeding range stretched from Monterey County, California, to Cabo San Lucas, Baja California Sur, Mexico (Atwood and Minsky 1983). California least terns nest in colonies on sandy beaches that are usually associated with river mouths or estuaries. Nesting habitat has been degraded by high levels of human disturbance in sandy dune areas as well as by the effects of urbanization, including industrial, recreational, and residential development of the shoreline. Least terns, however, have successfully used created sites for nesting, including areas on dredge-spoil islands, open areas adjacent to airport runways, and industrial ports. Like snowy plovers, least terns are ground-nesting birds. They feed themselves and their chicks with small fish such as anchovies and topsmelts, captured from nearshore waters, estuaries, river mouths, and bays (Massey 1974; Atwood and Minsky 1983).

Fig. 3. California least tern and chick.
Courtesy A. Powell, USGS

Low rates of reproductive success of California least terns have been linked to several factors. El Niño events, which cause nearshore water temperatures to rise, have depressed food availability for terns, which may in turn reduce tern productivity. The lowest annual production ever recorded for California least terns occurred after the 1982-1983 El Niño event, when fish populations off the shores of southern California plummeted (Massey et al. 1992). In addition to their vulnerability to catastrophic events, least tern colony sites in California have become restricted to fewer and smaller areas that are often surrounded by highly developed settings, leaving tern colonies susceptible to human disturbance as well as to intense predation. Predators associated with urban landscapes, such as domestic and feral dogs and cats, red foxes, American kestrels, American crows, common ravens, coyotes, raccoons, striped skunks, and Virginia opossums, eat least tern adults, chicks, and eggs.


Contaminants bioaccumulated in fish eaten by least terns may be another contributing factor to least terns' low reproductive success. Preliminary research on contaminants shows elevated levels of PCB's in California least tern eggs collected from sites around San Francisco Bay (Hothem and Zador 1995).


Although California least terns can and do nest again after losing eggs or chicks, some adults may abandon further breeding attempts that season (Fancher 1992). Least terns are fairly faithful to breeding sites and return year after year regardless of past nesting success. In addition, there is some evidence that least terns tend to return to their natal sites to breed (Atwood and Massey 1988). This may have major conservation implications because the average expected breeding life of California least terns is estimated at more than 9 years (Massey et al. 1992). Least terns breed after their second year, and first-time breeders are more likely to nest later in the breeding season (Massey and Atwood 1981).


Between 1978 and 1994, approximately 50 sites in California supported nesting least terns (Fancher 1992; Caffrey 1995). Fewer sites have been used in recent years; for example, only 36 sites were used in 1994 (Caffrey 1995). Furthermore, most California least terns nest at only a few select sites. In 1994, 76% of the population nested at nine sites, all in southernmost coastal California. Four of the nine sites (in Los Angeles, Orange, and San Diego counties) supported 48% of the breeding pairs (Caffrey 1995). In 1970, when California least terns were listed as endangered by the federal government and California, their population in California was estimated at 600 breeding pairs (Fancher 1992).


By 1994 the population had increased to an estimated 2,792 pairs (Fig. 4), which represents more than a fourfold increase (Caffrey 1995). Although the increase in the breeding population has not been consistent from year to year, long-term trends have shown steady population growth (Fig. 4). Tern population growth has been sustained even though ratios of fledglings to adults have fluctuated between colony sites and years (Massey et al. 1992; Caffrey 1995). Population growth rates have increased, especially since the mid-1980's, when active management for least terns was initiated.

Fig. 4. Populations of breeding California least terns in California. Data compiled by the U.S. Fish and Wildlife Service and the California Department of Fish and Game.

Management of California least tern colonies has included intensive monitoring of nesting colonies, site preparation to reduce vegetative cover, protection of sites by means of reduced access to humans, and predator management. Although individual nesting sites may not be used every year, and reproductive success varies among sites and years, the population of least terns in California continues to grow. Historical breeding sites should be preserved and managed for least terns because their adaptability to new or different sites depends on past reproductive success, predation pressure, and food supplies.

Abby Powell
U.S. Geological Survey
Biological Resources Division
Northern Prairie Wildlife Research Center
Department of Biological Sciences
University of Arkansas
Fayetteville, AR 72701