SURVIVAL AND NATAL DISPERSAL OF JUVENILE SNOWY PLOVERS (CHARADRIUS ALEXANDRINUS) IN CENTRAL COASTAL CALIFORNIA

Stenzel, Lynne E

ABSTRACT.-

Juvenile survival and dispersal rates are important demographic parameters in predicting the viability of avian populations, but estimates are seldom available because mortality is usually confounded with permanent natal dispersal in analyses of live-encounter data. We used the Barker model for combined captures, recoveries, and resightings to estimate juvenile survival in fledgling Snowy Plover (Charadrius alexandrinus) for the 6.5-to-10.5-month period between fledging at 28 days and 1 April the following year, on the central California coast, for a 16-year period, 1984-1999. By using a large body of year-round sighting data from throughout the species' Pacific-coast range, we estimated true survival and quantified natal dispersal rates and distances. Juvenile survival estimates varied annually between 0.283 ± 0.028 (mean ± SE) and 0.575 ± 0.061 with no trend over the study, and paralleled higher adult survival in our most parsimonious models. In comparison, annual survival of banded chicks from hatching to fledging at age 28 days was 0.285-0.483 (... = 0.382 ± 0.014 SE) for those 16 years. Males were more likely to disperse from Monterey Bay for winter and females were more likely to disperse for breeding. Dispersal distances to breeding sites were usually within 10 km of natal sites (64%) and seldom >50 km (16%). The present study provides the first estimate of true survival for a juvenile shorebird and new information on survival and dispersal rates that will be useful for modeling Snowy Plover population viability. Studies of local winter residents, focused on predator pressure and weather conditions, could further advance our understanding of factors determining Snowy Plover survival. Received 17 December 2004, accepted 14 September 2006.

Key words: Barker model, Charadrius alexandrinus, demography, endangered-threatened species, fledging rate, philopatry, program MARK, shorebird, Snowy Plover.

Supervivencia y Dispersión Natal de Juveniles de Charadrius alexandrinus en la Costa Central de California

RESUMEN.- La supervivencia y la tasa de dispersión de los juveniles son parámetros demográficos importantes para la predicción de la viabilidad de las poblaciones. Sin embargo las estimaciones de estos parámetros casi nunca están disponibles debido a que en los análisis de datos de avistamiento, la mortalidad es confundida con procesos de dispersión natal permanente. Utilizamos el modelo de Barker con datos de capturas combinadas, recapturas y avistamientos repetidos en la costa central de California para estimar la supervivencia de juveniles de Charadrius alexandrinus para un periodo de 6.5 a 10.5 meses entre el emplumamiento a los 28 días y el primer día de abril del año siguiente, durante un periodo de 16 años (1984-1999). Utilizando una gran base de datos de avistamientos en el rango de distribución de esta especie a lo largo de la costa Pacífica, estimamos tanto las tasas de supervivencia verdaderas como las tasas y distancias de dispersión natal. Los estimados de supervivencia juvenil variaron anualmente entre 0.283 ± 0.028 (media ± EE) y 0.575 ± 0.061, sin un patrón determinado durante el periodo estudiado, pero fluctuando de modo paralelo a las tasas de supervivencia más altas de los adultos según nuestros modelos más parsimoniosos. Comparativamente, la tasa de supervivencia anual de los polluelos anillados, desde la eclosión hasta el emplumamiento a los 28 días, fue de 0.285-0.483 (= 0.382 ± 0.014 EE) para los 16 años de datos. Los machos presentaron una mayor probabilidad de dispersarse desde la Bahía Monterrey para invernar, mientras que las hembras presentaron mayor probabilidad de dispersarse para reproducirse. Generalmente, la distancia de dispersión a los sitios de cría no fue más de 10 km desde el lugar natal (64%) y raramente más de 50 km (16%). Este estudio presenta la primera estimación de supervivencia verdadera de juveniles de aves playeras así como información nueva sobre las tasas de supervivencia y dispersión, las cuales serán útiles para modelar Ia viabilidad de poblaciones de C. alexandrinus. Nuevos estudios sobre los individuos residentes de invierno enfocados en las presiones por parte de los depredadores y en las condiciones climáticas podrían contribuir aún más para el entendimiento de los factores que determinan la supervivencia de esta especie.

(ProQuest: ... denotes formulae omitted.)

UNBIASED AND PRECISE estimates of demographic parameters are essential for understanding the population dynamics of small or imperiled bird populations (Beissinger and Westphal 1998). Demographic parameters are usually age-structured and often difficult to estimate. Juvenile survival is particularly difficult to estimate, because natal dispersal co-occurs with juvenile mortality, thus confounding estimates of both parameters (Greenwood and Harvey 1982, Larson et al. 2000). These problems are significant for shorebirds, with some species demonstrating considerable vagility (Stenzel et al. 1994, Clarke et al. 1997). Information on juvenile survival rates is lacking for most shorebird species (Sandercock 2003).

The population of Snowy Plovers (Charadrius alexandrinus) nesting along the Pacific coast of the United States is designated as threatened by the U.S. Fish and Wildlife Service (1993). Habitat loss, prédation, and human disturbance are major threats to this population, which nests primarily on coastal beaches (Page and Stenzel 1981). The lengthy nesting season of Snowy Plovers extends from mid-March to mid-September (Page et al. 1995b), enabling pairs to renest after egg failure, females to nest again after eggs hatch, and males to renest after chicks fledge or perish (Warriner et al. 1986, Page et al. 1995b). Although both sexes share incubation, the female typically deserts the brood soon after chicks hatch, leaving the male to rear them alone. Many Snowy Plovers occupy the same coastal site year-round, but some disperse to other coastal sites for winter. Since 1993, there have been extensive management efforts to increase the size of the population. Although information on Snowy Plover reproductive success is available (Page et al. 1995b, Powell et al. 2002, Ruhlen et al. 2003, Neuman et al. 2004), much less is known about survival rates, especially for juveniles (Paton 1994, Sandercock et al. 2005).

Standard mark-recapture models for live encounter data, such as the Cormack-JollySeber (CJS) model, are based on encounter histories of individually marked animals captured or resighted in focal study areas. The CJS models estimate only apparent survival (f), because mortality is confounded with permanent emigration (Lebreton et al. 1992). Snowy Plovers in western North America were monitored year-round (Page et al. 1986, 1991, 1995a; PRBO unpubl. data) and could be detected throughout their range during survival intervals, providing additional data that cannot be included in encounter histories for standard CJS models. Consequently, we used a joint model for live encounters (on encounter occasions and in survival intervals) and dead recoveries developed by Barker (1999) to examine 16 years of data on Snowy Plover fledglings. This model permits estimation of true survival (S), site fidelity (F), and other demographic parameters. To date, for birds, Barker's (1999) model has been used primarily for waterfowl (e.g., Sedinger et al. 2002) but also for an oystercatcher (Sagar et al. 2002).

The objectives of this long-term field study were five-fold. The first was to estimate true juvenile survival for a shorebird of conservation concern. The second was to examine the relationship of juvenile, chick, and adult survival for evidence that juvenile survival was affected by factors in common with other age classes. The third was to examine the potential effects of predator control on juvenile survival. Management activities commencing in 1991 and fully underway by 1993 to protect Snowy Plover eggs and, to a lesser extent, chicks in the Monterey Bay area included nest exclosures and mammalian predator removal (Neuman et al. 2004). One of the main predators, red fox (Vulpes vulpes), is known to depredate adult shorebirds (Brunton 1986, L. Feeney unpubl. data). We examined the effect of these management efforts by looking for a trend in juvenile survival over the study or for a change after the commencement of these efforts. The fourth objective was to examine the potential effect of winter climate conditions on juvenile survival. Although winters in coastal California generally are mild, severe El Niño-La Niña events can disrupt the coastal environment for both the prey and avian predators of Snowy Plovers (Holmgren et al. 2001, PRBO unpubl. data). We were unable to directly examine the influence of avian predators on survival, because data on predator distribution and abundance are lacking. The last objective was to compare philopatry rates and natal dispersal distances between the sexes. Philopatry and natal dispersal are parameters shaping population dynamics that are often sex-biased and associated with a species' mating system (Greenwood 1980). For species like the Snowy Plover that employ a resource-defense mating system, the male, the sex that defends the resources, is hypothesized to be more philopatric than the female. Conversely, females are hypothesized to disperse more frequently or farther than males (Greenwood 1980, Clarke et al. 1997).

METHODS

Study area.- The study was conducted over 18 years (1984-2001) at Monterey Bay, on the central California coast. The study area included 40 km of contiguous sandy beach of Monterey Bay in Santa Cruz and Monterey counties (122°17'W, 37°6'N to 121°52'W, 36°36'N); retired salt ponds at Elkhorn Slough, Monterey County, 1 km inland of the center of the beach (121°47'W, 36°49'N); and five small creek mouths in northern Santa Cruz County. Individual Snowy Plovers moved regularly among these areas (hereafter "Monterey Bay area"). The nearest major nesting areas were 48 km to the north and 160 km to the south. Small beaches, 10-44 km north and 32-151 km south of Monterey Bay, were used irregularly by five or fewer pairs (Page and Stenzel 1981, PRBO unpubl. data; Fig. 1). In the study area, the distribution of breeders varied among years, with the largest concentrations near the Pajaro (121°49'W, 36°51'N) and Salinas (121°48'W, 36°45'N) river mouths and at the salt ponds. The greatest linear distance between potential breeding beaches in the Monterey Bay area was 65 km.

Data collection.- We attempted to find all Snowy Plover nests in the study area and banded chicks at hatch with individual color band combinations, mostly at or near nest sites. Unmarked parents attending chicks were also captured and marked with individual color band combinations. All birds received an aluminum federal band and one to three Darvic plastic color bands. For increased durability, all bands were wrapped with automobile pinstriping tape (color-matched to plastic bands) and the ends of plastic bands and tapes were heat soldered. Parents were monitored for broody behavior (alert posture, ground distraction displays, and aerial alarm displays). When these behaviors were not apparent, we searched for chicks and watched for broody behavior on subsequent visits to ascertain whether broods had been lost. Fledging was defined as the survival of chicks to 28 days, when they begin to fly and males are usually still in attendance. Males usually abandon or decrease their attendance of young shortly after fledging (Page et al. 1995b). We usually did not attempt to view the chicks until 28 days after hatch, because of concerns that we could be cueing predators to broods. Once young had fledged, we observed the male and brood for up to several hours to determine the identity of chicks that had fledged. Sex could be determined only for birds in alternate plumage, as early as December for juvenile males and March for juvenile females. Males were distinguished by a lack of brown feathers in the black forehead, ear covert, and foreneck markings, and females by some brown feathers in these areas. Males also exhibited bright rusty caps early in the breeding season, whereas female caps were pale brown (Page et al. 1995b).

We searched for pairs and nests beginning 1 March and monitored nests from mid-March to mid-August and chicks from mid-April to mid-September; we scanned local flocks for marked juveniles until 31 October. Detections of banded Snowy Plovers during the survival intervals came from four sources: (1) intensive year-round observations of banded birds in our study area, 1984-2001; (2) a volunteer-based program of site surveys, ongoing since 1979 (Page et al. 1986); (3) breeding-season sightings by Snowy Plover and Least Tern (Sterna albifrons) monitors; and (4) recoveries of dead banded Snowy Plovers. All color-band observations were screened for accuracy, and project personnel were sent to correct or verify implausible or unexpected sightings. Questionable sightings were not included in any analyses.

The volunteer-based surveys covered most California coastal locations used by Snowy Plovers outside our study area during the nonbreeding season (Page et al. 1986). We defined major sites as those holding >14 individuals on at least half the survival intervals in which they were checked. Although surveys were focused on the nonbreeding season, many sites were covered year-round. The authors supplemented this effort to ensure winter coverage of the most remote coastal sites between the study area and Point Conception. Survival-interval coverage from 1979 to 1986 is reported in Page et al. (1986). Subsequently, observers conducted 560-1,066 (median = 849) total surveys annually from 1986 to 2001, with 17,400-51,500 (median = 29,500) total birds checked for bands (including multiple checks of the same flocks); 60-84% (median = 69%) of the 95 major California sites were covered by observers annually. Overall, 14% of the surveys were in May or June, 35% in July-October, 40% in November-February, 8% in March, and 4% from 16-30 April. Because Snowy Plovers often move locally in winter (Page et al. 1986), some marked individuals were detected at more than one site. Other researchers conducted an annual comprehensive winter survey of Oregon beaches. We also obtained limited information from the Washington coast (where

During the breeding season, sightings of our banded birds outside the Monterey Bay area were obtained from other researchers monitoring Snowy Plovers at specific sites and from broad-scale surveys. Comprehensive coastal breeding surveys in May or June were conducted in Washington: 1994-2001; Oregon: 1984-2001; and California: 1989 (Page et al. 1991), 1991, 1995 (excluding San Francisco Bay), and 2000. In coastal California, north of Point Arena, comprehensive surveys were conducted from 1992 to 1994, but coverage was limited to fewer sites in other years. North of Monterey Bay to Point Arena, Marin County sites were surveyed or monitored, 1986-2001; coastal San Mateo County sites, 1987-1992 and 1994-2001; and San Francisco Bay sites, 1984, 1986-1989, 1991-1997, 1999, and 2001. South of Monterey Bay to Point Conception, most major nesting beaches were surveyed or monitored, 1986-1992 and 1994-2001. South of Point Conception to the Mexican Border, most major sites were surveyed or monitored, 1989-1992 and 1994-2001. Additionally, researchers monitoring Least Terns located some banded Snowy Plovers nesting in association with Least Terns in and south of Santa Barbara County. Comprehensive inland breeding surveys were made of Oregon sites in 1985-1990 and of California sites in 1988. Plovers also were checked for bands on breeding surveys in Baja California in 1991 and 1992 (Palacios et al. 1994).

Juvenile survival rate from fledging.- Survival of banded fledglings to the following April (hereafter "juvenile survival") was estimated from >66,000 live encounters and 35 dead recoveries from April 1984 to March 2001. Our "encounter occasion" was 1-15 April; by then, breeding was well underway in almost all years. Bearhop et al. (2003) identify methodological factors that can introduce heterogeneity into a mark-encounter sample and violate key model assumptions. For precocial shorebirds, survival and its key explanatory variables can be expected to vary among four periods: egg stage, chick stage, juvenile stage, and adult stage. Pooling survival estimates over two stages, and even within one stage (e.g., banding chicks at various ages), may introduce sample heterogeneity (Sandercock et al. 2005). To control for this source of heterogeneity, we started the survival interval for all juveniles at 28 days of age. Because chicks fledged from mid-May through mid-September, we controlled for the varying lengths of the first (juvenile) survival interval by including date of fledging as an individual covariate of the juvenile survival and first re-encounter parameters. To minimize heterogeneity caused by differing site fidelities, this sample is used only to estimate juvenile survivorship.

We considered individual and annual covariates of juvenile survival. Individual covariates were linear and quadratic terms for the date of fledging; both covariates were included to allow for midseason extrema in survival. Annual covariates were a trend variable, the overall prior fledging rate, predator management, and winter climate extremes. Because the progeny of Monterey Bay breeders may winter from Oregon to Baja California Sur, we examined the effect of broad-scale winter climate extremes on annual survival. We used the sum of the monthly multivariate El Niño-Southern Oscillation indices (MEIs) for September-March, 1984-2001 (available from the National Oceanographic and Atmospheric Administration [NOAA] Climate Diagnostics Center; see Acknowledgments) as a proxy variable for broad-scale climate conditions. We considered possible effects from La Niña events (low-negative MEIs, associated with low temperatures), El Niño events (high-positive MEIs, associated with high precipitation), or both extremes (high MEI absolute values).

We explored models and estimated demographic parameters using MARK, version 3.1 (White and Burnham 1999) and procedures suggested by Lebreton et al. (1992). To include information on live- and dead-marked Snowy Plovers between encounter occasions, we employed Barker's (1999) model. All models were created with design matrices, using the logit-link function. It was not possible to assess the fit of the global model, because the bootstrap method is not currently recommended because of bias in the ... estimate (White 2002, G. C. White pers. comm.). Nonetheless, the sensitivity of model selection to potential overdispersion was examined by adjusting ... in increments of 0.01 up to 10. To rank the candidate models, we used the Akaike Information Criterion (AIC) corrected for small sample size (AIC^sub c^, or QAIC^sub c^ for ... > 1). We identified models with the strongest support as those with normalized Akaike weights (W^sub i^; Table 1) >0.01 and ΔAIC^sub c^ (differences between AIC^sub c^ or QAIC^sub c^ of the model with lowest AIC^sub c^ or QAIC^sub c^ and the model under consideration) values

The Barker model estimates the following seven parameters: (1) S^sub i^ is the probability that an individual alive at i is alive at ? + 1 (survival to 1 April). All models considered were time-since-marking (two age-classes, S^sup 1^ and S^sup 2+^, and time-dependent, t). The survival interval for S^sup 1^ extended from age 28 days (fledging) to the following April. We also examined models in which survival was constrained to be additive for age class within years; to be a function of the observed proportion of chicks fledged that season (f), premanagement (1984-1992) or management (1993-2001) years (m), or the MEI (e or lel, its absolute value); or to follow an annual trend (T) over the study. We also modeled first-year survival with individual covariates for date of hatch as a linear trend (d) or as a quadratic function (d and d2 [d^sup 2^]). Specific survival terms for candidate models were ... with d and d.2 considered in the latter terms if warranted, based on results from fitting terms in the first three models. (2) p^sub i^ is the probability that an individual at risk of detection at (encounter occasion) i is detected at i. Because many year-old birds do not begin occupying territories and breeding as early as most older adults (PRBO unpubl. data), all models we considered were time-since-marking (two age-class, p^sup 1^ and p^sup 2+^, and time, t). Detection for first year of re-encounter was constrained to be additive for age class and date of hatch in some models examined. Specific detection terms considered in candidate models were ... and p^sup 1^ + p^sup 2+^, with d and d.2 considered in the latter terms if warranted, as above for S^sup 1^. (3) r^sub i^ is the the probability that an individual that dies in interval (i, i + 1) is found dead and the band reported. We had few recoveries and only considered r^sub c^, a constant recovery rate term. (4) R^sub i^ is the probability that an individual that survives the interval (i, 1 + 1) is sighted some time in interval (i, 1 + 1). For survivors, we considered the terms R^sub t^ and Rc. (5) R'^sub i^ is the probability that an individual that dies in interval (i, i + 1) without being found dead is sighted alive in (i, 1 + 1) before it dies. For nonsurvivors, we considered the terms R'^sub i^, R'^sub R^ (resighting of nonsurvivors constrained as a linear function of resighting of survivors), and R'^sub c^ . (6) F^sub i^ is the probability that an individual at risk of encounter at i and alive at i + 1 is at risk of encounter at i + 1 (study-site fidelity). We considered fidelity terms F^sub t^ and F^sub c^. And (7) F'^sub i^ is the probability that an individual not at risk of encounter at i is at risk of encounter at i + 1 (return of temporary emigrants). For returning emigrants, we considered only F'^sub c^.

Fledging rates and juvenile survival rates from hatch.- The annual chick fledging rates, f^sub i^, were calculated as the proportion of banded chicks that survived from hatch to 28 days. We estimated annual juvenile survival from hatch to the following April as the product of annual fledging rates and juvenile survival estimates, f^sub i^ S^sub i^; mean rates of fledging and survival from hatch were calculated by weighting years equally.

Natal dispersal.- We compared the proportions of each sex employing different first-year dispersal patterns. The four primary patterns were (1) remaining in the natal area for the first winter and breeding season, (2) remaining in the natal area for winter but dispersing to breed, (3) dispersing from the natal area for winter but returning to breed, and (4) dispersing from the natal area for winter and subsequent breeding season. Patterns for 16 Snowy Plovers of undetermined sex that emigrated for nesting were apportioned in relation to the ratio of males and females with the same patterns. "Corrected" dispersal patterns were derived by augmenting the observed Snowy Plover numbers by the estimated number that survived but were never detected after their first March; this was calculated as (true survival rate × total fledglings in the sample) minus the number detected after March. VVe added the estimate into the totals for the two dispersal patterns in which nesting was away from the Monterey Bay area, based on the ratio of identified males and females in those categories.

Natal dispersal distances.- Natal dispersal distances were calculated for birds nesting in the Monterey Bay area in their first breeding season or seen elsewhere in their first or a subsequent breeding season. We included all individuals with confirmed (observed attending eggs or chicks) or behavioral evidence of breeding and May and June sightings of birds unsupported by breeding evidence. Sightings at other times, without evidence of breeding, were excluded to avoid including birds engaged in pre- or postbreeding movements. Mean distances moved were used for Snowy Plovers nesting at more than one site. For juveniles absent from the Monterey Bay area in their first breeding season and found breeding elsewhere in a subsequent breeding season, we assumed breeding-site fidelity and used that location for distance calculations. All estimates are reported as means ± SE, unless otherwise noted.

RESULTS

Juvenile survival rate from fledging.- Tor the five models we identified with w^sub i^ > 0.01 and ΔAIC^sub c^ (or ΔQAIC^sub c^)

Overall estimated juvenile survival was ... = 0.463 ± 0.018 (Table 2); model averaging produced annual estimates between 0.283 ± 0.028 and 0.575 ± 0.061; three-quarters were between 0.422 and 0.552 (Fig. 2). Encounter-occasion detection estimates for juveniles (...) and survival interval sighting estimates for all birds (... and ...) were relatively high (≥0.56), but the recovery rate of dead Snowy Plovers was quite low (...

Fledging rate and juvenile survival from hatch.- We banded 161-434 chicks per year from 1984 to 1999 at Monterey Bay (... = 242 ± 20, n = 16 years, 3,873 total chicks). Of these, 0.285-0.483 survived to fledging at 28 days (... = 0.382 ± 0.014, 1,466 total fledglings). Only four chicks, initially recorded as not fledging, were later discovered alive. These rates differ slightly from those in Neuman et al. (2004), who reported only on the portion of the study area where predator management was concentrated. The fledging rate (f^sub i^) was particularly low from 1991 to 1993, just as predator management was becoming fully implemented in parts of the study area (Fig. 2). No long-term trend was observed in the fledging rate (nonparametric test for trend, Spearmann's D = 406, P = 0.118, n = 16). The proportion of 124-234 clutches annually producing >1 fledgling declined from 0.379 in 1984 to a low of 0.190 in 1991, then increased to 0.329-0.446 per year with predator management (x = 0.329 ± 0.019 for the 16 years considered). Overall, about one-third of the 2,813 clutches produced >1 fledgling. We rarely determined the sex of Snowy Plovers not surviving to their first breeding season, but of 642 fledglings seen after 31 March of the year following hatch, 48.6% were females, 48.6% males, and 2.8% were of unknown sex.

Juvenile survival is usually reported for the 12-month period from hatching to the following year. Our juvenile survival estimate was for the 6.5-10.5 month period following fledging. Mean annual survival from hatch to April of the first breeding season was 0.179 ± 0.010 for ∂

Natal dispersal.- We relocated 642 first-winter survivors, including 31 Snowy Plovers without breeding evidence that were never seen again after 6 May. The remainder included 297 females, 294 males, 16 Snowy Plovers of undetermined sex that survived long enough to be assigned to a first-year dispersal pattern, and 4 Snowy Plovers that did not fall into one of the four primary dispersal patterns. For three patterns, involving some residency in our study area, we believe that relative detection probabilities were similar for the sexes. Assuming that our detection of permanent emigrants was also similar between sexes, the proportions of females and males employing the four primary dispersal patterns differed (X^sup 2^ = 27.89, df = 3, P

The above proportions are probably biased toward birds spending some time in the Monterey Bay area because some permanent emigrants were not seen again after leaving. Given an overall juvenile survival rate of S^sup 1^ = 0.463, an estimated 679 ± 34 fledglings survived to the following April. Adding the estimated 37 additional birds (679-642) to those known to have bred away from Monterey Bay, the estimate of breeding and permanent emigrants increased by 12% for females and 17% for males (Table 3). Philopatry rates for Monterey Bay declined to 59% for females and to 74% for males, but the sexual difference was maintained (odds ratio = 2.0, 95% CI: 1.4 to 2.8; Table 3).

Natal dispersal distances.- We determined natal dispersal distances for 44 females and 30 males that spent their entire first breeding season away from Monterey Bay. Of these, 37 females and 29 males were found at breeding sites by their first breeding season; 28 of the females and 26 of the males also met our criteria for nesting by then.

The mean distance between the natal site and site of first breeding was greater for 238 females (median = 6.9 km, maximum = 790 km) than for 259 males (median = 4.2 km, maximum = 360 km; one-tailed Z-test, Zn = 1.890, P = 0.029; Fig. 3; Reed 1993). Overall, observed natal dispersal distances were usually within 10 km of natal sites (64%) but were occasionally >50 km (16%). Among the local fledglings recruiting into the study area, 35% of the females and 27% of the males bred within 1 km, and 73% of females and 76% of males within 10 km, of natal sites. Both sexes dispersed north and south to breed. All 74 first-year dispersers were found only at coastal sites, including San Francisco Bay.

DISCUSSION

SURVIVAL

Potential sources of variability in juvenile Snowy Plover survival. - During the present study, Snowy Plovers exhibited annual variability in juvenile survival rates from 0.283 to 0.575, with no long-term trend. The parallel relationship between juvenile and adult survival rates suggests that important annual mortality factors for the two age classes were similar, but either these factors had a greater effect on juveniles or juveniles experienced an additional period of elevated mortality, possibly directly after fledging. There was no evidence that juvenile survival was affected by mammalian predator management midway through the study or that it was correlated to chick survival to fledging. Because predator management was focused on protecting eggs and unfledged chicks, a positive effect would be expected if the predators also were causing postfledging mortality. Conversely, a negative effect might be expected if management successfully increased fledgling numbers and if mortality after fledging were density-dependent (Côté and Sutherland 1997). The success of mammalian predator management at increasing the fledging rate was modest in our study area (Neuman et al. 2004), and there was no evidence that mammals were affecting postfledging survival. The lack of correlation between annual fledging and juvenile survival rates suggests that factors affecting survival during chick rearing differed from those during the subsequent fall and winter.

We did not find an effect of broad-scale climate extremes on juvenile survival. Mortality may be more strongly correlated with local (e.g., weather) than broad-scale (e.g., winter climate extremes) conditions, but it was not possible to determine dispersal patterns for most juveniles that did not survive their first winter and, thereby, account for local factors in our models. We believe that there could have been severe weather-related mortality, as reported for other shorebirds (Baillie 1980, Davidson and Clark 1985), but effects of climate on survival were masked because other factors, such as avian prédation, also were important. For example, in 1987, one of the years of lowest survival (Fig. 2), 30 pairs of Snowy Plovers moved from beaches to nearby riverine islands where they produced 26-29 fledglings. Subsequently, after a Northern Harrier (Circus cyaneus) began hunting over the islands, 16 of 23 banded fledglings were never seen again and feather remains characteristic of harrier kills were located. Such postfledging events may introduce variability to annual juvenile survival that could mask the effects of other factors, such as winter climate extremes. We suspect that avian predators may be important determinants of Snowy Plover survival, but we lacked adequate data on prédation pressure to examine this factor. Studies of winter residents focused on local predator pressure and weather conditions may further our understanding of the factors determining Snowy Plover survival. Because it takes a year to establish a nonbreeding residency pattern, adults may provide better subjects for such studies.

The time of fledging and, therefore, the length of the first survival interval were not related to survival, which suggests that mortality was not occurring at a steady rate after fledging. Prewinter mortality may have occurred soon after fledging and before fledglings had developed competent flight, such as described above. Weather-related winter mortality also may be episodic. For example, of 170 marked Snowy Plovers with established winter residency patterns between Monterey Bay and Point Reyes, 19% were last seen in November or December 1990, coincident with a week-long period of freezing temperatures in December (PRBO unpubl. data).

We could not control for sex in our analysis of juvenile survival, because it was unknown at hatching and fledging. Sexual bias in detection would be a factor primarily during the breeding season when sexual roles differ (Warriner et al. 1986); however, our overall estimate of adult detection was 1.0 (p^sup 2+^; Table 2). Therefore, we believe that sexual bias in detecting birds was negligible. Székely et al. (2004) report a hatchling sex ratio not different from 1:1 for Kentish Plovers (C. a. alexandrinus) in Turkey. If the hatchling sex ratio for our study was also 1:1, because of the 1:1 sex ratio for 624 fledglings seen after their first March, we found no evidence to suggest that juvenile survival (from hatch) differed between the sexes.

Overall juvenile survival.- Using Barker's (1999) model with extensive range-wide coverage, our estimated overall juvenile survival rate of S = 0.463 for the Snowy Plover is the first estimate of true survival for any shorebird. Previous survival estimates for plovers from mark-encounter models vary according to the estimation technique used and the birds' ages at the beginning of the survival interval. Although our estimate is considerably higher than a mark-encounter survival estimate reported for fledgling Kentish Plovers in Turkey, it is similar to mark-encounter survival estimates for two other plover species. Apparent firstwinter survival (f) estimates are 0.15 for Kentish Plover fledglings in Turkey (Sandercock et al. 2005), 0.47 for Mountain Plovers (C. montanus) in Montana (Dinsmore et al. 2003), and 0.48 for Piping Plovers (C. melodus) on the Atlantic coast (Melvin and Gibbs 1996), but none of these estimates distinguishes permanent emigration from mortality. Sandercock et al. (2005) report that their apparent first-winter survival estimate fell 40%, to 0.09, when they pooled four age groups of chick and fledgling Kentish Plovers. The effect of such pooling may account in part for the lower apparent survival rates reported by Paton (1994) for Snowy Plovers at Great Salt Lake (0.385) and Larson et al. (2000) for Piping Plovers in central North America (0.318).

Barker's (1999) model was suitable for this study, because it allowed the use of a large body of data from outside encounter occasions and outside our study area. Of 642 first-winter survivors detected, 79% were seen in the study area on at least one encounter occasion after original capture, 9% in the study area only during survival intervals, and 11% only outside the study area during survival intervals. Thus, we could estimate survival from all information on 99% of all birds known to have survived, compared with only 79% that would have been used in a CJS model. Further, it permitted use of a brief encounter occasion (4% of the survival interval length) without sacrificing most individual detections.

DISPERSAL

In a review of vertebrate dispersal studies, Greenwood (1980) hypothesized that the class of mating system- resource-defense versus mate-defense- should predict the direction of sexual bias in philopatry or dispersal. The Snowy Plovers we studied exhibited malebiased philopatry and female-biased natal dispersal, a pattern characterizing other avian species in which males defend resources rather than females (Greenwood 1980, Clarke et al. 1997). Other researchers reported male-biased philopatry (or female-biased natal dispersal) in five studies of resource-defense shorebird species, but failed to find a sexual bias in philopatry or dispersal in six other studies (Clarke et al. 1997 and citations therein, Robinson and Oring 1997, Kruk et al. 1998, FIynn et al. 1999). Reed and Oring (1993) and Jackson (1994) suggest that the lack of sexual bias reported for some shorebirds may be an artifact of study methods, particularly relatively small study plots within large areas of suitable breeding habitat. If limited area of search was a contributing factor to the lack of observed sexual bias in philopatry in the former studies, the pattern of male-biased philopatry reported by our study and 5 of 11 studies of other shorebirds may be more typical of shorebirds employing a resource-defense mating system than previously indicated.

We found that the probability that a Snowy Plover will breed in a location is inversely related to the distance from its natal site, which is consistent with other shorebirds. Most birds (64%) settled 50 km, from natal sites. Although search effort may have created some downward bias in dispersal distances outside the study area, even within the Monterey Bay area ~75% of the juveniles settled 250 km from natal sites. They suggested that the ephemeral nature of Little Ringed Plover breeding habitat imposes dispersal on that species; we believe that this is also true for Snowy Plovers nesting in coastal strand habitat.

If dispersal is regulated by locally fluctuating habitat quality, population density, or distribution of suitable habitat, as suggested for other birds (Reed and Oring 1993, Paradis et al. 1998, Altwegg et al. 2003), natal dispersal rates of Snowy Plovers could vary temporally and spatially. We did not examine these variables for Snowy Plovers because we lacked data on breeding-habitat extent for most years, though it clearly varied considerably. Population viability analysis for the Snowy Plover would benefit from improved information on patterns of natal dispersal and factors affecting this demographic parameter.

ACKNOWLEDGMENTS

We are particularly grateful to F. Bidstrup for many hours of field work, coordinating and keeping volunteer observers interested and informed about their sightings, and handling the contributed data. Over the course of the study, >100 volunteers generously contributed thousands of hours of field work. We owe special thanks to D. Dixon, F. Hanson, L. Henkel, L. O'Neil, P. Persons, M. Stern, D. Tobkin, B. Weed, and K. Wilson for their many observations of marked birds. E. Hutchinson, M. Reed, B. Sandercock, N. Warnock, and anonymous readers greatly improved this manuscript with thoughtful reviews. R. Barker and G. White provided helpful suggestions and comments on use of the model. Work in the Monterey Bay area was possible due to the cooperation of the Salinas River National Wildlife Refuge (under the Don Edwards San Francisco Bay NWR Complex), California Department of Parks and Recreation, and California Department of Fish and Game. This study was conducted under U.S. Fish and Wildlife Recovery Permit #TE807078. This paper is contribution #1079 of PRBO Conservation Science. For the monthly multivariate El Niño-Southern Oscillation indices for September-March, 1984-2001 (available from the NOAA Climate Diagnostics Center), see www.cdc.noaa.gov.

LITERATURE CITED

ALTWEGG, R., A. ROULIN, M. KESTENHOLTZ, AND L. JENNI. 2003. Variation and covariation in survival, dispersal, and population size in Barn Owls Tyto alba. Journal of Animal Ecology 72:391-399.

BAILLIE, S. 1980. The effect of the hard winter of 1978/79 on the wader populations of the Ythan Estuary. Wader Study Group Bulletin 28:16-17.

BARKER, R. J. 1999. Joint analysis of markrecapture, resighting and ring-recovery data with age-dependence and markingeffect. Bird Study 46 (Supplement):82-91.

BEARHOP, S., R. M. WARD, AND P. R. EVANS. 2003. Long-term survival rates in colour-ringed shorebirds- Practical considerations in the application of mark-recapture models. Bird Study 50:271-279.

BEISSINGER, S. R., AND M. 1. WESTPHAL. 1998. ON the use of demographic models of population viability in endangered species management. Journal of Wildlife Management 62:821-841.

BRUNTON, D. H. 1986. Fatal antipredator behavior of a Killdeer. Wilson Bulletin 98:605-607.

CLARKE, A. L., B.-E. SAETHER, and E. ROSKAFT. 1997. Sex biases in avian dispersal: A reappraisal. Oikos 79:429^138.

CôTé, I. M., AND W. J. SUTHERLAND. 1997. The effectiveness of removing predators to protect bird populations. Conservation Biology 11:395-405.

CRAMP, S., AND K. E. L. SIMMONS, EDS. 1983. Handbook of the Birds of Europe, the Middle East and North Africa: The Birds of the Western Palearctic, vol. Ill: Waders to Gulls. Oxford University Press, New York.

DAVIDSON, N. C, and N. A. CLARK. 1985. The effects of severe weather in January and February 1985 on waders in Britain. Wader Study Group Bulletin 44:10-16.

DINSMORE, S. }., G. C. WHITE, AND F. L. KNOPF. 2003. Annual survival and population estimates of Mountain Plovers in southern Phillips County, Montana. Ecological Applications 13:1013-1026.

FLYNN, L., E. NOL, AND Y. ZHARIKOV. 1999. Philopatry, nest-site tenacity, and mate fidelity of Semipalmated Plovers. Journal of Avian Biology 30:47-55.

GREENWOOD, P. J. 1980. Mating systems, philopatry and dispersal in birds and mammals. Animal Behaviour 28:1140-1162.

GREENWOOD, P. J., and P. H. HARVEY. 1982. The natal and breeding dispersal of birds. Annual Review of Ecology and Systematics 13:1-21.

HAIG, S. M., and L. W. ORING. 1988. Distribution and dispersal in the Piping Plover. Auk 105: 630-638.

HOLMGREN, M., M. SCHEFFER, E. EZCURRA, J. R. GUTIÉRREZ, AND G. M. J. MOHREN. 2001. El Niño effects on the dynamics of terrestrial ecosystems. Trends in Ecology and Evolution 16:89-94.

JACKSON, D. B. 1994. Breeding dispersal and site-fidelity in three monogamous wader species in the Western Isles, U.K. Ibis 136: 463-473.

KRUK, M., M. A. W. NOORDERVLIET, and W. J. TER KEURS. 1998. Natal philopatry in the Blacktailed Godwit Limosa limosa L. and its possible implications for conservation. Ringing and Migration 19:13-16.

LARSON, M. A., M. R. RYAN, and B. G. ROOT. 2000. Piping Plover survival in the Great Plains: An updated analysis. Journal of Field Ornithology 71:721-729.

LEBRETON, J.-D., K. P. BURNHAM, J. CLOBERT, AND D. R. ANDERSON. 1992. Modeling survival and testing biological hypotheses using marked animals: A unified approach with case studies. Ecological Monographs 62:67-118.

MELVIN, S. M., AND J. P. GIBBS. 1996. Viability analysis for the Atlantic coast population of Piping Plovers. Pages 175-186 in Piping Plover (Charadrius melodus) Atlantic Coast Population Revised Recovery Plan (A. Hecht, D. Avrin, S. Melvin, J. Nicholls, C Raithel, and K. Terwilliger, Eds.). U.S. Fish and Wildlife Service, Hadley, Massachusetts.

NEUMAN, K. K., G. W. PAGE, L. E. STENZEL, J. C. WARRINER, AND J. S. WARRINER. 2004. Effect of mammalian predator management on Snowy Plover breeding success. Waterbirds 27:257-263.

PAGE, G. W., F. C BIDSTRUP, R. J. RAMER, AND L. E. STENZEL. 1986. Distribution of wintering Snowy Plovers in California and adjacent states. Western Birds 17:145-170.

PAGE, G. W., E. PALACIOS, L. ALFARO, S. GONZALEZ, L. E. STENZEL, AND M. JUNGERS. 1997. Numbers of wintering shorebirds in coastal wetlands of Baja California, Mexico. Journal of Field Ornithology 68:562-574.

PAGE, G. W., AND L. E. STENZEL. 1981. The breeding status of the Snowy Plover in California. Western Birds 12:1^0.

PAGE, G. W., L. E. STENZEL, W. D. Shuford, and C. R. Bruce. 1991. Distribution and abundance of the Snowy Plover on its western North American breeding grounds. Journal of Field Ornithology 62:245-255.

PAGE, G. W., M. A. STERN, AND P. W. C. PATON. 1995a. Differences in wintering areas of Snowy Plovers from inland breeding sites in western North America. Condor 97:258-262.

PAGE, G. W., J. S. WARRINER, J. C. WARRINER, AND P. W. C. PATON. 1995b. Snowy Plover (Charadrius alexandrinus). In The Birds of North America, no. 154 (A. Poole and F. Gill, Eds.). Academy of Natural Sciences, Philadelphia, and American Ornithologists' Union, Washington, D.C.

PALACIOS, E., L. ALFARO, AND G. W. PAGE. 1994. Distribution and abundance of breeding Snowy Plovers on the Pacific coast of Baja California. Journal of Field Ornithology 65: 490-497.

PARADIS, E., S. R. BAILLIE, W. J. SUTHERLAND, AND R. D. GREGORY. 1998. Patterns of natal and breeding dispersal in birds. Journal of Animal Ecology 67:518-536.

PATON, P. W. C. 1994. Survival estimates for Snowy Plovers breeding at Great Salt Lake, Utah. Condor 96:1106-1109.

POWELL, A. N., C. L. FRITZ, B. L. PETERSON, AND J. M. TERP. 2002. Status of breeding and wintering Snowy Plovers in San Diego County, California, 1994-1999. Journal of Field Ornithology 73:156-165.

REED, J. M. 1993. A parametric method for comparing dispersal distances. Condor 95: 716-718.

REED, J. M., AND L. W. ORING. 1993. Philopatry, site fidelity, dispersal, and survival of Spotted Sandpipers. Auk 110:541-551.

ROBINSON, J. A., AND L. W. ORING. 1997. Natal and breeding dispersal in American Avocets. Auk 114:416-430.

RUHLEN, T. D., S. ABBOTT, L. E. STENZEL, AND G. W. PAGE. 2003. Evidence that human disturbance reduces Snowy Plover chick survival. Journal of Field Ornithology 74:300-304.

SAGAR, P. M., R. J. BARKER, AND D. GEDDES. 2002. Survival of breeding Finsch's Oystercatchers (Haematopus finschi) on farmland in Canterbury, New Nealand. Notornis 49:233-240.

SANDERCOCK, B. K. 2003. Estimation of survival rates for wader populations: A review of mark-recapture methods. Wader Study Group Bulletin 100:163-174.

SANDERCOCK, B. K., T SZÉKELY, and A. KOSZTOLáNYI. 2005. The effects of age and sex on the apparent survival of Kentish Plovers breeding in southern Turkey. Condor 107:583-596.

SEDINGER, J. S., N. D. CHELGREN, M. S. LINDBERG, T. OBRITCHKEWITCH, M. T. KIRK, P. MARTIN, B. A. ANDERSON, AND D. H. WARD. 2002. Life-history implications of large-scale spatial variation in adult survival of Black Brant (Branta bernicla nigricans). Auk 119:510-515.

STENZEL, L. E., J. C. WARRINER, J. S. WARRINER, K. S. WILSON, F. C. BIDSTRUP, AND G. W. PAGE. 1994. Long-distance breeding dispersal of Snowy Plovers in western North America. Journal of Animal Ecology 63:887-902.

SZÉKELY, T., I. C. CUTHILL, S. YEZERINAC, R. GRIFFITHS, AND J. KIS. 2004. Brood sex ratio in the Kentish Plover. Behavioral Ecology 15:58-62.

THOMPSON, P. S., D. BAINES, J. C. COULSON, AND G. LONGRIGG. 1994. Age at first breeding, philopatry and breeding site-fidelity in the Lapwing Vanellus vanellus. Ibis 136:474-484.

U.S. FISH AND WILDLIFE SERVICE. 1993. Endangered and threatened wildlife and plants; determination of threatened status for the Pacific coast population of the western Snowy Plover; final rule. Federal Register 58:12864-12874.

WARRINER, J. S., J. C. WARRINER, G. W. PAGE, AND L. E. STENZEL. 1986. Mating system and reproductive success of a small population of polygamous Snowy Plovers. Wilson Bulletin 98:15-37.

WHITE, G. C. 2002. Discussion comments on the use of auxiliary variables in capture-recapture modelling: An overview. Journal of Applied Statistics 29:103-106.

WHITE, G. C, AND K. P. BURNHAM. 1999. Program MARK: Survival estimation from populations of marked animals. Bird Study (Supplement) 46:120-139.

Associate Editor: B. K. Sandercock

LYNNE E. STENZEL,1 GARY W. PAGE, JANE C. WARRINER, JOHN S. WARRINER,

DOUGLAS E. GEORGE, CARLETON R. EYSTER, BERNADETTE A. RAMER,

AND KRISTINA K. NEUMAN

PRBO Conservation Science, 3820 Cypress Drive Suite 11, Petaluma, California 94954, USA

1 Present address: PRBO Conservation Science Wetlands Center, P.O. Box 69, Bolinas, California 94924, USA. E-mail: lstenzel@>prbo.org

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