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Habitat Use and Reproductive Success of Western Snowy Plovers at New Nesting Areas Created For California Least Terns

Abby N. Powell and Christine L. Collier


Abstract: The Pacific coast population of western snowy plovers (Charadrius alexandrinus nivosus) was listed as threatened under the Endangered Species Act (ESA) in 1993 and its decline is primarily attributed to habitat loss. In southern California, snowy plovers typically nest in association with federally endangered California least terns (Sterna antillarum browni). Since least terns were afforded protection under the ESA, the creation of nesting habitat from dredged materials has been a popular component of habitat restoration to partially compensate for wetland loss in this region. We had a unique opportunity to monitor habitat use and reproductive success at newly created habitats associated with the restoration of Batiquitos Lagoon, San Diego County, California from 1994 to 1998. We also compared hatch and fledge rates and habitat characteristics of snowy plovers nesting at new nesting areas at Batiquitos Lagoon to a nearby natural beach and a dredged-material area created in the 1970s. The number of nesting attempts by snowy plovers increased from 5 in 1994 to a high of 38 in 1997, and plovers nested on 4 of the 5 created areas. Fledge rates at Batiquitos Lagoon varied annually and declined after the initial colonization in 1995. Fledge rate in 1995 was higher at the newly created area than at the older dredged-material and natural beach areas in any other year. Nests on the created areas at Batiquitos Lagoon were surrounded by less vegetative cover, less debris, and shorter vegetation than nests at the older dredged-material and natural beach areas. Nonbreeding snowy plovers used created habitats within the lagoon, and more plovers used the lagoon and its adjacent beach during fall than winter. Predation pressure and habitat quality were important factors determining use and reproductive success on created areas at Batiquitos Lagoon.

Key words: Batiquitos Lagoon, California least tern, Charadrius alexandrinus nivosus, created habitat, reproductive success, site fidelity, southern California, Sterna antillarum browni, threatened and endangered species, western snowy plover.


Table of Contents


Introduction

Over the past 20 years, changes in coastal beach habitat along the Pacific coast, including heavy recreational use, urbanization, and invasion of exotic grasses, have resulted in loss of suitable breeding habitat for snowy plovers (Federal Register 1993, Page et al. 1995). The Pacific coast population of western snowy plovers has also declined during this period and was listed as federally threatened in 1993 (Federal Register 1993). In southern California, snowy plovers often nest in association with California least terns. Snowy plovers and least terns both nest on wide, sparsely vegetated beaches, dredge spoils, salt pans, and levees around salt evaporation ponds (Wilson-Jacobs and Dorsey 1985, Page et al. 1995). California least terns were listed as federally endangered in 1970 and most nesting sites have been protected since the early 1980s (U.S. Fish and Wildlife Service 1985). Creation of nesting areas for least terns is a common component of wetland restoration plans in this region. Created nesting habitats usually consist of dredge spoils composed of coarse substrates; native vegetation typically is sparse at first. Least tern populations have grown rapidly since the 1980s and this increase has been attributed to protection of breeding areas from human disturbance and predators, and to some extent, creation of new nesting areas (Powell 1998). In contrast, populations of western snowy plovers have declined, even in regions where California least tern populations have increased (Page et al. 1991, Powell 1998). In southern California, for example, snowy plovers no longer breed within Los Angeles County, and only 1 nesting area remains in Orange County. As a result, San Diego County supports most of the snowy plover breeding population in the region. Currently there are approximately 10 sites remaining in San Diego County, each supporting an estimated 1-75 pairs of snowy plovers annually.

Several coastal lagoons in San Diego County historically supported breeding snowy plovers. Due to habitat loss and alteration of water flows, however, many of these sites have been degraded severely and now only sporadically support breeding pairs. One of these sites, Batiquitos Lagoon, was tidally influenced in the 1800s, but sedimentation from changes in surrounding land use reduced its tidal prism. Water input was restricted to rainfall and stormwater runoff when flow from the ocean was partially obstructed by a highway (circa 1912) and railroad (California State Coastal Conservancy 1989). Historically, small numbers of snowy plovers and least terns nested on the salt pan around the perimeter of the lagoon's marsh (Page and Stenzel 1981, U.S. Fish and Wildlife Service 1985). By the 1990s, plans were made to restore Batiquitos Lagoon to a tidal marsh and to enhance least tern habitat by creating dredge-spoils for nesting as mitigation for wetland loss elsewhere in the region. Restoration took place from 1994 to 1996, and in January 1997 the lagoon mouth was opened to the Pacific Ocean.

Due to the expense of restoration projects in southern California, opportunities to document colonization of newly created areas have been rare. No documentation is available regarding the use and reproductive success of snowy plovers nesting at restored lagoons and created nesting areas for least terns. Least tern studies were required as part of mitigation monitoring at Batiquitos Lagoon, but we took this unique opportunity to determine whether the restoration project was beneficial to snowy plovers. From 1994 before restoration through 1998, after the lagoon was made tidal, we monitored use of the site by breeding and wintering snowy plovers. The objectives of our study were to determine whether snowy plovers used new areas created for least terns, quantify reproductive success, quantify site fidelity to new breeding and wintering areas, and document use of the lagoon and its adjacent beach by migrating and wintering snowy plovers. In addition, we evaluated created areas at Batiquitos Lagoon for their potential to maintain or increase the San Diego County's snowy plover population. To do this, we compared hatch and fledge rates from 1994 to 1998 of snowy plovers nesting at Batiquitos Lagoon to plovers nesting at a dredge-spoil area created in the 1970s and a natural beach area within San Diego County. We also compared habitat characteristics of snowy plover nest sites at Batiquitos Lagoon to the older dredge-spoil and natural beach nesting areas to evaluate habitat quality of the new created areas.

Study Area

There were 9 major snowy plover nesting areas in San Diego County, California, ranging from Camp Pendleton Marine Corps Base (MCB) to Tijuana Slough National Wildlife Refuge (NWR), north to south. The 9 areas combined supported approximately 89-197 snowy plover nests between 1994 and 1998 (A. N. Powell, U.S. Geological Survey, unpublished data).

Newly created nesting areas were established at Batiquitos Lagoon (33° 5´ N, 117° 17´ W) in northern San Diego County. The long, narrow lagoon is approximately 240 ha and extends east from the Pacific Ocean. South Carlsbad State Beach is directly west of the lagoon and is bisected by the lagoon's mouth. Before the enhancement project began, a narrow strip of salt pan habitat surrounded the northern and eastern perimeters of the lagoon and brackish marsh occupied the southern perimeter. Two least tern nesting areas were created in 1994 (W1, 0.8 ha; E1, 7.7 ha) and 2 in 1995 (W2, 1.6 ha; E2, 2.4 ha). These 4 areas were connected to the lagoon edge and were mostly devoid of vegetation. The substrate consisted of medium to coarse-grain sand and shell fragments from dredged and trucked materials. An additional area created in 1995 (E3, 1.2 ha) was an island in the east lagoon consisting of finer, siltier dredge material. The extent of potential breeding sites within the lagoon changed each year as the restoration effort progressed. Weedy vegetation quickly invaded E3 and became denser in subsequent years.

An older nesting areas was created on D Street Fill within the Sweetwater Marsh NWR (32° 36´ N, 117° 07´ W) on the eastern shore of San Diego Bay. This area is approximately 45 km south of Batiquitos Lagoon and was created in the mid-1970s after major dredging activities changed the configuration of Sweetwater Marsh. The substrate at D Street Fill was a shallow layer of coarse sand and broken shells over finer dredge-spoil material. Weedy vegetation invaded this area annually, and efforts by the U.S. Fish and Wildlife Service were made each spring to clear it by disking before least terns arrived. Depending on rainfall patterns, however, annual weeds often became dense by June. Least terns and snowy plovers may have nested at this area since it was created, but California least terns were not monitored there until 1984.

The closest area to Batiquitos Lagoon that supported nesting snowy plovers was a natural beach area 18 km north at Camp Pendleton MCB (33° 14´ N, 117° 25´ W). Camp Pendleton typically supported half of all nesting pairs of snowy plovers within San Diego County during our study (A. N. Powell, U.S. Geological Survey, unpublished data). Although some plovers nested on salt pan, we chose to include only natural beach at Camp Pendleton for our comparisons of reproductive success and habitat characteristics because it was most representative of plover nesting habitat in the region (A. N. Powell, U.S. Geological Survey, unpublished data). Also, salt pan was subject to flooding from periodic high tides and rainfall, which influenced reproductive success within this habitat type. The natural beach site at Camp Pendleton was located around the mouth of the Santa Margarita River. The area consisted of stretches of sandy beach and low dunes covered with plants such as bur-sage (Ambrosia chamissonis), sand verbena (Abronia umbellata), and sea rocket (Cakile maritima).

Methods

All 5 created areas within Batiquitos Lagoon were surveyed for nesting activity by snowy plovers throughout the breeding season (1 March-31 August) from 1994 to 1998. We also monitored snowy plover nests at Camp Pendleton MCB and D Street Fill during this period. One to 3 observer(s) systematically searched potential breeding habitat on foot to look for nest scrapes. We marked nests with numbered tongue depressors inserted vertically into the substrate 1 m west of each nest. Nests were monitored 1-5 days per week to determine clutch size, outcome, and hatching rate. Observations of predators (including tracks and scat) at all sites were recorded.

In southern California, female plovers may have up to 3 successful, consecutive nests with different mates. Males, however, are unable to renest quickly because they care for broods up to 47 days after hatching (Warriner et al. 1986). Therefore, for each nest, we expressed hatch rate as average proportion of eggs hatched per number of eggs laid, and fledge rate was proportion of chicks that survived until fledging per number of hatched chicks. Fledglings (young birds capable of flight) sometimes remained with the male parent even after he initiated a new nest. We defined nest success as number of fledglings produced per nest attempt because many of the adults early in the study were unbanded, prohibiting calculation of reproductive success based on average fledglings produced per male or female. This underestimates reproductive success per individual because both sexes may have multiple nests and females renest rapidly after nest failure or hatching. However, nest success is an index of productivity.

To compare and evaluate productivity among the created and natural beach nesting areas over time, we used logistic regression to model the probability of success for hatching and fledging (chicks that survived 30 days after hatch) among years and nesting areas. Snowy plovers in coastal California have an average incubation period of 28 days and fledge at an average of age 30 days (Warriner et al. 1986). Plover chicks leave the nest within hours of hatching and can move long distances from the nest site, therefore, causes of egg loss may be different than causes of chick loss. Thus, we fitted separate logistic regression models to indicate whether there were differences in hatch and fledge rates between study areas. We created indicator variables for year and area and for interactions between year and area. The main effect for year and area were forced into the model and stepwise selection was used to choose the most significant interaction (α = 0.05) because the original model showed quasi-complete separation of our data (Agresti 1996). Finally, we used Hosmer and Lemeshow goodness-of-fit tests (χ²) to assess how well the models fit our data (SAS Institute 1997).

To determine site fidelity and immigration, we trapped adult snowy plovers on the nest with open-bottomed Potter's traps (Powell and Cuthbert 1993, Schemnitz 1994) or noose mats placed around the nest (Bub 1991) at all breeding areas within San Diego County 1994-98. Snowy plovers were given unique combinations with standard aluminum bands (size 1P) covered with colored tape and 3 colored-plastic bands (A. C. Hughes, Hampton Hill, Middlesex, United Kingdom; split-ring, size XCL). Two bands per leg were placed below the joint on the tarsometatarsus. Automotive pin-striping tape was used to prolong the life of plastic bands and to create additional color combinations when used over aluminum bands. We used a portable electronic-ignition, propane-powered soldering iron to seal seams of colored-plastic bands and to adhere pin-striping tape over plastic and aluminum bands. Chicks were uniquely color-banded at the nest at hatching. Determination of fledging was based on observations of banded juveniles, capable of flight, by the end of the breeding season. In a few cases, unbanded fledglings consistently observed in the presence of a banded parent were used in determination of fledge rate. We received information from Point Reyes Bird Observatory on observations of our color-banded birds sighted in other areas of California and Mexico.

We measured habitat variables at individual nests, 1995-1996, within a 1-m² quadrat centered over the nest. Habitat measurements were not taken until nests had hatched or failed. Within each quadrat we estimated percent foliar cover of vegetation and percent of the substrate covered by debris (nonliving vegetation and inorganic materials). We identified living plants within each quadrat to species (Munz 1974). We determined plant height by measuring the length and calculating a mean of the tallest 3 plants within each quarter of the quadrat. Habitat variables were also recorded for 1 non-nest quadrat placed at a random distance and compass bearing within 50 m of each nest. We compared paired random and nest samples using Wilcoxon Signed Rank (Z) tests. We found that habitat data did not differ between years (vegetative cover, P = 0.256; debris, P = 0.758; plant height, P = 0.514) so data from 1995 and 1996 were pooled in the analyses. Habitat variables from nests sampled at Batiquitos Lagoon, D Street Fill, and the natural beach area (Camp Pendleton) were compared using Kruskal-Wallis (Chi squared) tests. We then compared each area using Wilcoxon 2-sample tests if differences among areas were significant.

In addition to breeding studies, we conducted biweekly surveys of wintering and migrating snowy plovers at Batiquitos Lagoon and its adjacent beach (South Carlsbad State Beach) from September-March, 1995-98. One observer slowly walked or drove along the beaches or lagoon and periodically stopped to count plovers through binoculars or spotting scopes. Winter surveys were timed to coincide with low and high tides, determined from standard computerized charts for the nearest tide recording station. We recorded location, total number of plovers, and band combinations of marked birds. To examine use of the lagoon and its adjacent beach we first used linear regression to determine whether tidal amplitude influenced numbers of plovers observed during winter surveys. To evaluate differences in number of plovers seen at Batiquitos Lagoon and its adjacent beach during fall and winter we used 2-way analysis of variance with area and season as main effects. We pooled years in the analysis because we found no differences in numbers of plovers among years (P = 0.570).

Results

Nesting at Batiquitos Lagoon

We found 5 snowy plover nests at Batiquitos Lagoon in 1994 before construction for the enhancement project. All were located on the salt pan around the northern perimeter of the lagoon. After the winter of 1994-95, water levels in the lagoon were kept high to facilitate dredging, which made the traditional salt pan habitat very narrow. In addition, salt marsh vegetation was planted on part of the old tern and plover nesting area along the northeastern perimeter. Although some salt pan habitat remained, snowy plovers did not nest at the traditional areas after 1994. In 1995, all snowy plover nests (n = 15) were located on E1. Nests were found on the 3 eastern sites in 1996 (n = 39; E1 = 25, E2 = 3, E3 = 11), 1997 (n = 38; E1 = 18, E2 = 4, E3 = 16), and 1998 (n = 26; E1 = 4, E2 = 4, E3 = 16). Densities of nests ranged from 0.5 to 3.2 nests/ha at E1 and 1.3 to 1.7 nests/ha at E2. The use of E3 by snowy plovers increased from approximately 3 pairs in 1994 to 13 pairs in 1998, a density of 13.3 nests/ha. By 1998, snowy plovers were found nesting at 4 of the 5 sites; plover nests were never found at W1. Two plover nests were found on W2 in 1998, but it was not apparent why.

Clutch Size and Reproductive Success

Clutch size (2.84 ± 0.04; Mean of X ± SE) at Batiquitos Lagoon did not differ among years (P = 0.088, n = 118). All known predation on plover eggs was attributed to common ravens (Corvus corax) based on tracks around nests. In 1994, 2 of the 5 nests at the traditional site were lost to unknown predators. In subsequent years, raven predation accounted for the loss of 2, 1, and 4 nests, respectively. In 1998 unknown predators took 4 nests. Several nests were abandoned during 1995-97 (n = 1, 4, 5), and no nests were abandoned in 1998.

The Hosmer and Lemeshow goodness-of-fit test indicated that the logistic regression model for hatch rate could not be rejected as an appropriate model (χ² = 5.504, df = 8, P = 0.703). An area by year interaction (P = 0.031) indicated that in 1994 the probability of hatch success was lowest at Batiquitos Lagoon, but for the rest of the years the natural beach area at Camp Pendleton had the lowest hatch rates (P ≥ 0.001; Fig. 1). Although hatch rates were greatest at D Street Fill in all years, it had the fewest numbers of nests each year, except 1994 (Table 1). The goodness-of-fit test for fledge rate also indicated that the model could not be rejected as appropriate (χ² = 2.376, df = 8, P = 0.967). In 1994, 1997, and 1998, fledge rates were greatest at Camp Pendleton and lowest at D Street Fill (P = 0.002). In 1995, Batiquitos Lagoon had higher fledge rates (P ≥ 0.001) than the other areas (Fig. 1). Although Batiquitos Lagoon also had high fledge rates in 1996, only the natural beach was different from the other two areas (P = 0.002). Nest success exceeded 1 fledgling/nest at Batiquitos Lagoon in 1995; no other area approached this level of productivity in any year (Table 1). Nest success at D Street Fill and the natural beach at Camp Pendleton ranged from approximately 0.3 to 0.7 fledglings/nest.

fig 1
Fig. 1. Hatch and fledge rates predicted by logistic regression models for snowy plovers at newly created dredge-spoil areas at Batiquitos Lagoon, an old dredge-spoil at D Street Fill, and a natural beach area at Camp Pendleton MCB, San Diego County, California, 1994-98.

Table 1. Nest success (number of fledglings produced per nest) for snowy plovers at new created (Batiquitos Lagoon), old created (D Street Fill), and natural beach (Camp Pendleton) areas in San Diego County, California, 1994-98.
Year Batiquitos Lagoon D Street Fill Natural Beach Site
na Mean of X SE n Mean of X SE n Mean of X SE
1994 5 0.40 0.45 8 0.50 0.27 24 0.46 0.10
1995 15 1.40 0.30 11 0.27 0.21 36 0.58 0.09
1996 39 0.51 0.16 7 0.57 0.38 49 0.37 0.07
1997 38 0.29 0.13 10 0.50 0.27 45 0.51 0.09
1998 26 0.27 0.19 2 0.50 0 43 0.65 0.08
a n = number of nests

Causes of chick mortality were difficult to determine. Potential predators on chicks, based on our observations at Batiquitos Lagoon, included domestic and feral cats (Felis catus), American crow (Corvus brachyrynchos), common raven, American kestrel (Falco sparverius), red-tailed hawk (Buteo jamaicensis), and great-horned owl (Bubo virginianus). At D Street Fill, peregrine falcons (Falco peregrinus) preyed upon 2 snowy plover fledglings. Kestrels, loggerhead shrikes (Lanius ludovicianus), and feral dogs (Canus domesticus) were also seen at D Street Fill.

Site Fidelity at Batiquitos Lagoon

Although we attempted to band as many adults as possible each year, unbanded plovers were present at Batiquitos Lagoon each breeding season. Adult male plovers banded at Batiquitos Lagoon were site faithful; 72% for all years combined returned the subsequent breeding season and beyond. The single male we banded in 1994 at the traditional nesting area nested at newly created areas in 1995 and 1996. Males usually did not mate with the same females each year. Immigration of adult males we banded at other nesting areas to Batiquitos Lagoon included 2 from Camp Pendleton (19 km north) and 1 from Tijuana Slough NWR (62 km south). Females were also site faithful; 62% for all years combined returned to breed the next year and beyond. One female banded in 1994 at the traditional nesting area nested at created nesting areas 1995-98. Based on banded birds, more females (n = 18) than males (n = 8) immigrated to Batiquitos Lagoon from other areas in San Diego County.

First-year (post-fledging) survival of snowy plovers in San Diego County (1994-97) was estimated as 43% (Lynne Stenzel and Gary Page, Point Reyes Bird Observatory, personal communication). Nineteen percent of banded fledglings from Batiquitos Lagoon returned to breed, and males were twice as likely to return than females. Five plovers that hatched at Batiquitos were known to have emigrated to other breeding sites within San Diego County. There were no known cases of juveniles from Batiquitos Lagoon breeding at sites outside our study area.

Nest Habitat Characteristics

Quadrats containing snowy plover nests at all areas had more vegetative cover (nests = 9.2 ± 1.2, random = 4.8 ± 1.2%, P ≤ 0.001) and debris (nests = 7.9 ± 1.4, random = 4.1 ± 1.1%, P ≤ 0.001), and taller plants (nests = 39.8 ± 4.2, random = 23.3 ± 3.1 cm, P ≤ 0.001) than paired random quadrats.

Total cover of debris and vegetation was lower at Batiquitos Lagoon than either D Street Fill (P ≤ 0.001) or the natural beach area (P ≤ 0.001). The 3 areas differed from each other by cover type. Percent foliar cover was lowest at Batiquitos and greatest at D Street Fill, and debris cover was greater at the natural beach than at either the new or old created area. For the 2 created areas, Batiquitos Lagoon had less debris cover than D Street Fill. Mean plant height around nests was lowest at Batiquitos Lagoon, while D Street Fill was more similar to the natural beach.

At Batiquitos Lagoon, E1 was virtually devoid of vegetation and debris, except along its perimeter where little nesting occurred. Nests on the E3 island had low vegetative cover of Halberd-leaf saltbush (Atriplex patula) and saltgrass (Distichlis spicata) and very little debris. Plant species composition consisted of weedy invasives at D Street Fill: burdock (Arctium minus), beach primrose (Camissonia cheiranthifolia), Jerusalem oak (Chenopodium botrys), coast wooly-heads (Nemacaulis denudata), Russian thistle (Salsola australis), and filaree (Erodium spp.). Most of the vegetative cover at the natural beach consisted primarily of native dune plants such as sand verbena, bur-sage, beach primrose, Watson's saltbush (Atriplex watsonii), and helioptrope (Heliotropium curvassavicum) with some cover by exotics (sea rocket; Hottentot fig, Carpobrotus edulis).

Winter Use of Batiquitos Lagoon

We did not detect a relationship between number of snowy plovers and tidal amplitude at the beach (South Carlsbad State Beach) adjacent to the lagoon (r² = 0.002, P = 0.841) or within the lagoon (r² = 0.076, P = 0.319). Snowy plovers were not seen at any of the created nesting areas within Batiquitos Lagoon during the winters of 1995-96 or 1996-97. More plovers were seen at South Carlsbad State Beach (18.3 ± 2.9) than within Batiquitos Lagoon (10.1 ± 3.4; P = 0.045), and use of both sites was greater in fall (Sep-Nov) than winter (Dec-Feb; P = 0.029). There was no interaction between site and season (P = 0.871). After tidal restoration, plovers were observed during winter at E1 and E2. Of the 71 plovers banded at Batiquitos Lagoon from 1994-97, 25 were seen during winter in southern California. Most of the birds overwintered in areas close to Batiquitos Lagoon, including Camp Pendleton, South Carlsbad State Beach, and San Elijo Lagoon.

Discussion

The availability of snowy plover habitat along coastal beaches was historically subject to constant change due to patterns of sand deposition and erosion from water flows and wind. Anthropogenic changes in southern California, such as river channelization, have altered hydrological patterns with the net result of reducing suitable nesting habitat for plovers and terns (Powell 1998). In recent years, events such as El Niño have eroded existing beaches and further reduced amounts of available nesting habitat. Snowy plovers disperse among and between breeding sites along the coast of California and are able to take advantage of new sites as they become available (Stenzel et al. 1994). Before the restoration of Batiquitos Lagoon, availability of nesting habitat for snowy plovers and least terns annually fluctuated, depending on winter and spring rainfall; in years with high freshwater input most of the salt pan habitat was flooded. Least terns had low reproductive rates at this site, primarily due to high rates of predation and disturbance by humans and their pets (Fancher 1992, Caffrey 1994). Productivity of snowy plovers was not known, but we assumed it was low, similar to least terns. Although the traditional nesting area at Batiquitos was reduced in size after 1994, some salt pan habitat remained along the perimeter of the lagoon, but no nests were found there in subsequent years. The additional pairs of snowy plovers that nested at Batiquitos Lagoon in 1995 came from other areas and were likely to return to nest in subsequent years.

Reproductive success for snowy plovers is affected by predation rates, weather, and human disturbance (Page et al. 1983, 1985; Warriner et al. 1986; Flemming et al. 1988; Melvin et al. 1994). At Batiquitos Lagoon, fledge rates and nest success declined after peaking the first year the created sites were used, suggesting that the area attracted nesting plovers but ultimately became a habitat sink (Pulliam 1988). We suspect the extremely high rate of chick survival at Batiquitos Lagoon in 1995 was because predators had not yet discovered these new nesting areas. As more birds, including least terns, used the created nesting areas, more predators such as corvids and owls were observed and predation rates became similar to other plover nesting areas in southern California. Unfortunately, there are no historical data on number of nests and reproductive success of snowy plovers at D Street Fill immediately after its creation. However, use by California least terns at D Street Fill has fluctuated dramatically since 1978, ranging from no breeding (1981, 1990) to 47 breeding pairs in 1978 (Fancher 1992). The pattern of high productivity and low predation rates at newly colonized areas is an advantage of temporal changes in habitat availability due to creation and loss of sand bars and beaches through natural processes. These processes, however, have been disrupted by changes in land use and snowy plovers no longer have many opportunities to exploit new habitats.

The same natural processes that create new beach habitats also deposit debris and establish native beach vegetation. Most newly created nesting areas at Batiquitos Lagoon, with the exception of E3, were devoid of almost all vegetation and other cover, and thus increased the vulnerability of plover eggs and chicks to predators. In general, plovers select nest sites that allow incubating adults good visibility, yet afford them some camouflage from predators (Page et al. 1985, Powell and Cuthbert 1992). Snowy plovers selected nest sites with low amounts of cover, but nests were preferentially located in greater cover than nearby random sites. Although their eggs are relatively cryptic, snowy plovers often nest adjacent to objects and select nest sites that are broken up by small amounts of vegetation or debris. Page et al. (1985) showed that snowy plover nests located under an object were more likely to survive than nests that were exposed. Visual cues used by predators to locate nests may be disrupted at natural coastal breeding areas that have more cover than newly created dredge-spoil nesting areas.

Cover is important to plover chicks because they are cryptic and often react to predators by running into cover and crouching. Snowy plover chicks had no place to hide at some sites at Batiquitos Lagoon because vegetation and debris were absent. In contrast, E3 quickly acquired weedy vegetation, possibly because of a different substrate than other created sites. This nesting area was different in size and appearance from any others within San Diego County. In addition to greater amounts of cover, other species present at E3 may have provided protection through their defensive behaviors such as alarm calls (early warning), mobbing, and dive bombing (driving predators away). The E3 site supported nesting black skimmer (Rynchops niger), killdeer (Charadrius vociferus), and American avocet (Recurvirostra americana). Densities of black skimmers on E3 ranged from 22.5-25.0 nests/ha (Whelchel et al. 1996) and California least tern densities ranged from 62.5-86.7 nests/ha (K. Keane, Keane Biological Consulting, personal communication). The high density of other species on E3 may have created a dilution effect, decreasing the likelihood that a predator would take a snowy plover. Despite the higher cover and presence of more aggressive species, plover chick survival on the island decreased annually. Two factors may have been involved in the reduction of reproductive success on E3. First, lowered reproductive success of snowy plovers has been linked to high nesting density, and densities on the island increased each year (Page et al. 1983, 1985). Second, the small size of the island, coupled with high densities of other breeding birds, may have reduced the amount of suitable brood-rearing and foraging areas for plovers (Wilson-Jacobs and Dorsey 1985).

The older nesting area at D Street Fill provided cover similar to natural beach sites within San Diego County. In contrast to the natural beach areas, however, cover composition was almost entirely weedy vegetation and very little debris. Vegetation removal at D Street Fill occurred annually prior to the arrival of least terns in April; although the amount of area cleared, the intensity of disking, and timing of site preparation varied from year to year. The amount of area cleared and the regrowth of weeds may have influenced numbers of plovers using the site in a given year. Unlike Batiquitos Lagoon, alternative nesting areas were located within a short distance from D Street Fill across San Diego Bay. Plovers readily move to alternate sites within a breeding season if predation pressure is high (Stenzel et al. 1994). In addition, short-distance dispersal to alternate sites within a breeding season by female plovers provides opportunities for new mates. We observed movement of snowy plovers within and between years from D Street Fill to sites around San Diego Bay when predation pressure was high or the amount of quality nesting habitat was low (A. N. Powell, U.S. Geological Survey, unpublished data).

In addition to nesting habitat, created nesting areas can be important to snowy plovers during migration and overwintering. Snowy plovers used the beach adjacent to Batiquitos Lagoon during the winter for foraging and resting. Tidal flats were exposed during low tides after the lagoon mouth was opened to the ocean in 1997. Incoming tides provided opportunity for movement of marine invertebrates and other food sources into the lagoon. The combination of new foraging and loafing areas increased suitable habitat for winter use by snowy plovers.

Management Implications

Several management issues must be considered when developing mitigation plans that include habitat enhancement for snowy plovers. First, evaluation of created nesting areas should not be based solely on presence of nesting snowy plovers, but needs to track productivity over time. Snowy plovers may be attracted to created areas, but low reproductive rates there may result in population sinks. Site fidelity, regardless of reproductive success, may amplify these effects and result in lower annual productivity for the region as a whole. Historically, natural cycles of temporal variation in habitat availability may have allowed years of high reproductive success for snowy plovers before predators keyed in on new nesting areas. Creation of "temporary" habitats by clearing invasive vegetation in different nesting areas in different years may come closer to mimicking cyclical habitat availability and release local populations from high predation pressure.

It is critical that resource managers recognize the impact of predation on survival of snowy plovers, especially chicks. Actions to reduce predation pressure include creating sites large enough for snowy plovers to nest at lower densities, and managing the amounts of vegetation and debris in order to ensure adequate cover for nesting plovers and hiding places for chicks. Areas surrounding snowy plovers nests at natural beaches in southern California had approximately 25% total cover; 10% foliar cover (<60 cm in height) and 15% debris cover. Managers may enhance habitat at created areas that are devoid of vegetation and debris by adding patches of sticks, small rocks, and dried kelp to mimic amounts and patterns of cover on natural beaches. Conversely, plovers will not be attracted to areas with excess cover, therefore, weedy vegetation must be controlled annually. There is some evidence that sites covered with coarser substrates, such as shell, may reduce amounts of vegetative cover and enhance nesting success of seabirds on dredge-spoil islands (Mallach and Leberg 1999). We found that the area at Batiquitos Lagoon created with coarse substrates had little foliar cover, while areas with finer substrates were invaded by weedy plants indicative of disturbed areas (e.g., Halberd-leaf saltbush, filaree).

Finally, loss of suitable habitat for wintering and migrating snowy plovers has most likely contributed to the overall decline in coastal populations (Page et al. 1986). Use of created nesting areas at Batiquitos Lagoon by nonbreeding plovers increased after its mouth was dredged and became tidal. Areas within the lagoon used by snowy plovers during winter were not disturbed by people and their pets as was South Carlsbad State Beach. Plans to create new nesting sites for snowy plovers need to consider use during nonbreeding seasons to enhance overall survival of snowy plovers in coastal California.


Acknowledgments

Funding for the work on Camp Pendleton MCB was provided by the U.S. Marine Corps. We thank B. L. Peterson, J. M. Terp, and M. A. Tucker for their field work. K. Keane provided information on the construction activities at Batiquitos Lagoon and California least terns at that site. We are grateful to L. K. Duncan for assistance in statistical analysis, and to C. Caffrey, R. E. Kirby, E. M. Kirsch, K. Keane, and G. D. Willson for their helpful comments on this manuscript.


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This resource is based on the following source (Northern Prairie Publication 1073):

Powell, Abby N., and Christine L. Collier.  2000.  Habitat use and reproductive success of western snowy plovers at new nesting areas created for California least terns.  Journal of Wildlife Management 64(1):24-33.

This resource should be cited as:

Powell, Abby N., and Christine L. Collier.  2000.  Habitat use and reproductive success of western snowy plovers at new nesting areas created for California least terns. Journal of Wildlife Management 64(1):24-33.  Jamestown, ND: Northern Prairie Wildlife Research Center Online.  http://www.npwrc.usgs.gov/resource/birds/wsplover/index.htm (Version 16MAY2000).


Abby N. Powell, U.S. Geological Survey, Western Ecological Research Center, Department of Biology, San Diego State University, San Diego, CA 92182, USA.  Present address: U.S. Geological Survey, Northern Prairie Wildlife Research Center, Arkansas Field Station, Department of Biological Sciences, University of Arkansas, Fayetteville, AR 72701, USA.  E-mail: Abby_Powell@usgs.gov

Christine L. Collier, U.S. Geological Survey, Western Ecological Research Center, Department of Biology, San Diego State University, San Diego, CA 92182, USA.


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