Local Annual Survival and Seasonal Residency Rates of Semipalmated Sandpipers (Calidris Pusilla) in Puerto RICO

By Rice, Susan M Collazo, Jaime A; Alldredge, Mathew W; Harrington, Brian A; Lewis, Allen R

ABSTRACT.- We report seasonal residency and local annual survival rates of migratory Semipalmated Sandpipers (Calidris pusilla) at the Cabo Rojo salt flats, Puerto Rico. Residency rate (daily probability of remaining on the flats) was 0.991 +- 0.001 (x +- SE), yielding a mean length of stay of 110 days. This finding supports the inclusion of the Caribbean as part of the species’ winter range. Average estimated percentage of fat was low but increased throughout the season, which suggests that birds replenish some spent fat reserves and strive for energetic maintenance. Local annual survival rate was 0.62 +- 0.04, within the range of values reported for breeding populations at Manitoba and Alaska (0.53-0.76). The similarity was not unexpected because estimates were obtained annually but at opposite sites of their annual migratory movements. Birds captured at the salt flats appeared to be a mix of birds from various parts of the breeding range, judging from morphology (culmen’s coefficient of variation = 9.1, n = 106). This suggested that origin (breeding area) of birds and their proportion in the data should be ascertained and accounted for in analyses to glean the full conservation implications of winter-based annual survival estimates. Those data are needed to unravel the possibility that individuals of distinct populations are affected by differential mortality factors across different migratory routes. Mean length of stay strongly suggested that habitat quality at the salt flats was high. Rainfall and tidal flow combine to increase food availability during fall. The salt flats dry up gradually toward late January, at the onset of the dry season. Semipalmated Sandpipers may move west to other Greater Antilles or south to sites such as coastal Surinam until the onset of spring migration. They are not an oversummering species at the salt flats. Conservation efforts in the Caribbean region require understanding the dynamics of this species throughout winter to protect essential habitat. Received 7 December 2005, accepted 30 December 2006.

Key words: apparent survival, Calidris pusilla, Caribbean, mark- recapture, migration, Semipalmated Sandpiper, stopover, winter distribution.

Tasas de Supervivencia Anual Local y de Residencia Estacional de Calidris pusilla en Puerto Rico

RESUMEN.-Reportamos la tasa estacional de residencia y de supervivencia anual local para los playeros migratorios Calidris pusilla en las Salinas de Cabo Rojo, Puerto Rico. Las tasas de residencia (probabilidad diaria de permanecer en las salinas) fueron de 0.991 +- 0.001 (x +- EE), resultando en una estadia promedio de 110 dias. Los hallazgos apoyan la inclusion del Caribe como parte del ambito geografico invernal de la especie. El promedio porcentual estimado de grasa fue bajo pero aumento a traves de la temporada, sugiriendo que los playeros reabastecen parte de sus reservas de grasa y tratan de mantenerse energeticamente. La tasa de supervivencia anual local fue de 0.62 +- 0.04, la cual esta dentro del rango de valores reportados para poblaciones reproductivas en Manitoba y Alaska (0.53-0.76). La semejanza no fue sorpresiva porque los estimados fueron obtenidos anualmente, pero en areas opuestas durante sus movimientos migratorios anuales. Con base en la morfologia, los playeros capturados en las salinas fueron una mezcla de aves de varias partes del ambito geografico reproductivo (coeficiente de variacion del culmen = 9.1, n = 106). Esto sugiere que el origen (area reproductiva) de los playeros y sus respectivas proporciones en los datos deben ser determinados y tornados en cuenta en los analisis, para asi establecer a cabalidad las implicaciones para la conservacion de los estimados invernales anuales de supervivencia. Estos datos son necesarios para elucidar la posibilidad de que individuos de distintas poblaciones sean afectados por distintos factores de mortandad a lo largo de sus rutas migratorias. La estadia promedio sugirio que la calidad del habitat en las salinas era alta. La precipitacion y el flujo intermareal se combinan para aumentar la disponibilidad de alimento durante el otono. Las salinas se secan gradualmente hacia finales de enero, el comienzo de la epoca de sequia. Es posible que estos playeros puedan desplazarce hacia el oeste a las Antillas Mayores o hacia el sur a lugares como la costa de Surinam para invernar hasta que comience la migracion de primavera. Los playeros no pasan el verano en las salinas. Los esfuerzos de conservacion en el Caribe precisan de un entendimiento sobre la dinamica de esta especie durante el invierno para proteger el habitat esencial.

MYERS ET AL. (1987) emphasized the importance of identifying and protecting the geographic network of wetlands used annually by migrant shorebirds. This endeavor is one of the cornerstones of shorebird conservation efforts (Brown et al. 2000), aided greatly by programs such as the International Shorebird Survey (Howe et al. 1989). Evans and Pienkowski (1984) and Myers et al. (1985) also underscored the need to understand migratory shorebird population dynamics. Unfortunately, examples of studies in North America reporting reliable estimates of residency and annual survival rates are few (e.g., Sandercock and Gratto-Trevor 1997, Warnock et al. 1997, Warnock and Bishop 1998, Farmer and Wiens 1999, Sandercock et al. 2000, Fernandez et al. 2003, Ydenberg et al. 2004), and nonexistent for the Caribbean. Lack of data makes it difficult to identify when and where shorebirds are most vulnerable to mortality and, therefore, limits our ability to test hypotheses about the evolution of shorebird migratory strategies (Myers et al. 1985; see also Webster et al. 2002). It also limits our ability to formulate effective conservation strategies. For example, estimates of residency rates (i.e., mean length of stay), population size, and prey density are necessary for estimating area requirements of migratory shorebirds at stopover sites (Loesch et al. 2000).

The Cabo Rojo salt flats in southwestern Puerto Rico support the highest number of migratory shorebirds on the island and are of regional importance in the eastern Caribbean (Collazo et al. 1995). Of the 28 species recorded at the salt flats, small calidridine sandpipers, primarily Semipalmated Sandpipers (Calidris pusilla) and Western Sandpipers (C. mauri), constitute 60-70% of the fall- migrant shorebirds. Here, we report seasonal residency and local annual survival rates for Semipalmated Sandpipers, parameters needed to better understand their nonbreeding ecology and the functional value of the salt flats. Residency rates are defined as the daily probability that a banded bird remains at the salt flats. Local annual survival rates (synonymous to apparent survival) are defined as the annual probability that a bird banded at the salt flats will survive and return to the salt flats. We estimated the percentage of body fat at capture to describe how this body-condition index changed throughout the season. We tested whether this index influenced residency rates and the likelihood of surviving and returning to the salt flats. Our aim was to gain insights on the quality and significance of the Cabo Rojo salt flats to migrating Semipalmated Sandpipers (Skagen and Knopf 1994, Pfister et al. 1998, Loesch et al. 2000).

METHODS

The study area comprised the Cabo Rojo National Wildlife Refuge (U.S. Fish and Wildlife Service) and nearby sites under jurisdiction of the Puerto Rico Department of Natural and Environmental Resources (Refugio de Aves and sector Pitahaya). These sites are located in southwest Puerto Rico (17[degrees]56′N and 67[degrees]06′W; Fig. 1). The salt flats consist primarily of two large, shallow lagoons covering ~445 ha. The lagoons are hypersaline and used as storage ponds prior to channeling water to evaporating basins for salt extraction. Refugio de Aves (2.5 km long) and sector Pitahaya (3.2 km long) consist of salt flats, saline lagoons, and mangrove swamps.

Using mist nets, we captured 578 Semipalmated Sandpipers at the Cabo Rojo salt flats during fall from 1989 to 1992. Each bird was individually color-marked using UV-stable bands and flags. Birds were also banded with a federal metal band. Birds were banded over several days within the following periods: 30 September to 9 November 1989 (n = 150), 30 September to 14 November 1990 (n = 141), 5 October to 11 November 1991 (n = 129), and 29 August to 1 November 1992 (n = 158). We aged birds by plumage characteristics (hatch- year or after-hatch-year; Prater et al. 1977), recorded mass to the nearest gram, measured the bill from its tip to the feather margin on the forehead (exposed oilmen, nearest 0.1 mm), and measured wing chord from the carpal joint to the wing tip (nearest 1.0 mm).

We calculated the estimated percentage of fat (EPF) using the equation EPF = 100 ([WT - FFW]/ WT), where WT is the total body mass and FFW is fat-free mass (Page and Middleton 1972, Dunn et al. 1988). Fat-free weights were calculated using the equation FFW = – 9.0513 + 0.3134 (wing length) (n = 53; Page and Middleton 1972). Multiyear data were pooled within one of three time periods: early season (last week of August to first week of September), midseason (first week of October), and late season (last week of Ortober to first week of November). We compared mean EPF among time periods using one-way analysis of variance (ANOVA) in JMP, version 3 (SAS Institute, Cary, North Carolina). Data met assumptions of homogeneity of variance (Levene’s test). We ascertained the probable origin (breeding area) of adult birds captured at the salt flats to aid with interpretation of local annual survival estimates. This was done by calculating the coeffirient of variation (CV) of oilmen measurements and determining where it fit along the range of CVs generated by sex and breeding zones (Harrington and Morrison 1979). Residency rates were estimated from resight data collected from 29 August 1992 to 9 January 1993. We divided this period into 18 sampling occasions or 17 intervals. The length of each interval was 7 days, except for the first and the last two intervals, which were 5.5, 6.5, and 20 days, respectively. Data were collected from sunrise until 1000 hours Atlantic Time on every sampling occasion. We used the “recaptures only” model in MARK (White and Burnham 1999) to estimate residency rates and specified the unequal interval lengths in days. We examined 10 models of residency and recapture rates, which included the following 4 basic models and 6 more with EPF at capture as an individual covariate: (1) constant residency and recapture rates, (2) time-specific residency and constant recapture rates, (3) constant residency and timespecific recapture rates, and (4) time-specific residency and recapture rates. Residency and recapture rates for the last interval were not estimable using model (3).

We used the median c procedure in MARK to estimate the variance inflation fador for the global model. We used Akaike’s Information Criterion (AIC) modified to account for small sample size (AIC^sub c^) to seled the most parsimonious model (Burnham and Anderson 2002). To account for unequal interval-length between capture periods, daily residency probabilities were calculated. The daily probability is the nth root of the period’s residency probability, where n is the number of days in the interval. Shorebird mean length of stay (MLS), expressed in days, was estimated using the mean-life- expectancy equation derived by Brownie et al. (1985): MLS = -1 / In (daily residency probability).

Local annual survival rates were estimated for adult (n = 215) and juvenile (n = 319) Semipalmated Sandpipers on the basis of resighting data colleded during fall 1990 (26 September-17 November), 1991 (27 September-10 November), and 1992 (29 August-20 December). We used a “time-since-marking” model structure under the “recaptures only” data type in MARK (White and Burnham 1999). We constructed 16 models to evaluate (1) variation in local survival and recapture rates associated with time (year) and age-class (juvenile or adult at first capture) and (2) the relationship between body condition and local annual survival rates. These included the following eight basic models and eight variants that incorporated time specific survival by age-class and EPF as a covariate: (1) constant local survival and recapture rates; (2) time- specific local survival and constant recapture rates; (3) constant local survival and time-specific recapture rates; (4) time-specific local survival and recapture rates; (5) constant local survival and recapture rates, with an age effect on survival; (6) time-specific local survival and constant recapture rates, with an age effect on survival; (7) constant local survival and timespecific recapture rates, with an age effect on survival; and (8) time-specific local survival and recapture rates, with an age effect on survival.

We modeled local survival as a function of EPF to determine whether it influenced the likelihood of surviving and returning to the salt flats annually. Because the EPF content was known at the time of first capture, we considered models where it affected survival only for the first year after capture. Estimated percentage of fat content was modeled as an additive effect; that is, it had a constant effect on local survival. We used the median c procedure in MARK to estimate the variance inflation factor and AIC to select the most parsimonious model (Burnham and Anderson 2002).

We tested for transients before running the models, because their presence, particularly a high proportion, would cause first-year survival rates to be biased low. Transients are adult birds that are captured in the study area but are not residents, so are not likely to return to the study area and be available for recapture (resighting). If transients are present in the marked population, it is hypothesized that adult firstyear survival after marking will be lower than in subsequent years. We used a “time-sincemarking” model structure to identify differences in survival between the first year after marking and subsequent years. We considered several competing models addressing the transient hypothesis (e.g., constant survival, timespecific, EPF effect) but report only results for the top model, because plausible alternatives (DeltaQAIC^sub c^ < 2) yielded similar estimates for the parameters of interest.

Careful consideration of four model assumptions is important in interpreting results. First, we assumed that every bird had the same probability of being resighted in sampling period i and that every marked bird had the same probability of surviving from sampling period i to i + 1, assuming that it was alive and present in the population at the time the survey was conducted. second, we assumed that emigration (i.e., departure) was permanent. We believe that this assumption was met, because coverage of the study areas was frequent and surveys included nearly all available shorebird habitat at the three sites (Fig. 1), reducing chances that color bands were overlooked (e.g., temporary emigration). Third, we assumed that marks were not lost and that all marked birds were correctly reported. Last, we assumed that all individuals were correctly aged. Parameter estimates are reported as mean +- SE.

RESULTS

Mean EPF for early-season Semipalmated Sandpipers was 1.7 +- 1.9 (n – 28); for midseason birds, 12.3 +- 0.9 (n = 185); and for late- season birds, 9.9 +- 1.1 (n = 324). Differences among periods were significant (F = 4.58, df = 2 and 534; P = 0.01). Estimated percentage of fat was significantly higher for birds during mid- and late season than for birds captured early in the season (Tukey- Kramer HSD; P < 0.05). The proportions of adults in the samples were 0.82 (early), 0.23 (mid), and 0.43 (late season). The CV of adult oilmen measurements was 9.1. Average oilmen length was 18.9 +-1.1 mm (n = 106).

No evidence of overdispersion was found for seasonal-residency- rate data, median 6 = 1. Seasonal residency rates measured over 17 intervals were best described by a model with constant residency and time-specific capture probabilities and EPF as a covariate (lowest AIC^sub c^; Table 1). Small differences in DeltaAIC^sub c^ suggested that a model without EPF was a reasonable alternative. Daily mean residency probability based on the selected model was 0.991 +- 0.001. Daily capture probabilities ranged from 0.24 +- 0.10 to 0.79 +- 0.04. The extrapolated mean length of stay was 110 days (95% confidence interval [CI]: 90 to 142 days). There was a negative trend between EPF and residency rates (beta = -0.02, 95% CI: -0.04 to 0.003). The relationship, however, was not significant, because the beta-parameter 95% CIs included 0.

Survival for first year after capture and for subsequent years was best described by a model that allowed for first-year survival of adults to be different from survival in subsequent years but constant over time (QAIC^sub c^ weight = 0.31). Estimates were obtained over four annual intervals. The variance inflation factor was c = 1.6. Adult survival was 0.73 +- 0.08 for the first year and 0.59 +- 0.04 for subsequent years. Recapture rate was 0.57 +- 0.05. These results do not support the hypothesized presence of transients in the marked population (i.e., adult first-year survival was higher than adult survival in subsequent years).

Local annual survival was best described by a model with constant survival and recapture rates (lowest QAIC^sub c^; Table 2). Local survival rate was 0.62 +- 0.04, and recapture rate was 0.59 +- 0.05. Small differences in DeltaQAIC^sub c^ indicate that a model with constant survival for adults (0.65 +- 0.04) and juveniles (0.55 +- 0.06), constant recapture rates (0.60 +- 0.05), and EPF as a covariate was a reasonable alternative. There was a positive trend between EPF and local annual survival (beta = 0.42, 95% CI: -0.004 to 0.84). The next six models had some support as alternative models (DeltaQAIC^sub c^ < 2), yielding constant survival estimates between 0.61 and 0.62, and time-specific survival estimates between 0.63 and 0.64 (adults) and 0.53 and 0.55 (juveniles).

DISCUSSION

Semipalmated Sandpipers spent an average of 110 days at the Cabo Rojo salt flats in Puerto Rico during the fall. This was in sharp contrast to mean lengths of stay recorded at northern-latitude stopover sites during southbound migration. In North Dakota and New Brunswick, Lank (1983) reported length of stay for Semipalmated Sandpipers ranging from 15 to 20 days. In Maine, length of stay ranged from 11 to 14 days (Dunn et al. 1988). The latter estimates of length of stay were based on “last seen minus first seen” and, thus, were biased low when compared with mark-recapture estimates (Dunn et al. 1988, Farmer and Wiens 1999, Lehnen and Krementz 2005). Notwithstanding that possibility, our findings strongly suggested that, rather than a southbound stopover area, the salt flats are part of the wintering range for Semipalmated Sandpipers. Our inference is consistent with reports of winter population distributions of the species that include the Caribbean (Morrison 1984, Morrison and Ross 1989). Local annual survival rates of Semipalmated Sandpipers (0.62 +- 0.04) were within the range of values obtained at breeding grounds in Manitoba (0.53-0.76) and Alaska (0.66; Sandercock and Gratto-Trevor 1997, Sandercock et al. 2000, Gratto-Trevor and Vacek 2001). Our estimate of survival was obtained at the opposite end of the species’ migratory cycle; hence, the similarity among estimates was not surprising. Age-specific survival estimates, though variable, were consistent with the expectation that adult survival should be higher than that of juveniles (Evans 1991). Our findings provide a better basis for understanding the population dynamics of Semipalmated Sandpipers throughout their annual cycle and underscore the need for and importance of knowledge about population origin and representation (proportion) to realize the full conservation value of winter-based survival estimates (Myers et al. 1985, Webster et al. 2002). The high CV of oilmen measurements (9.08) suggested that birds (both sexes) captured at the Cabo Rojo salt flats came from different zones across their breeding range (Harrington and Morrison 1979: 94). Lower CVs are associated with samples of one sex from one breeding zone (3-4%) or two sexes from one breeding zone (5-6%). It is possible, then, that differential mortality factors across different migratory routes could go undetected (Sillett and Holmes 2002), making it more difficult to discern whether particular segments of the population are facing problems (e.g., declining eastern populations; Morrison and Hicklin 2001). Molecular approaches and stable-isotope signatures of feathers are promising tools for discerning population origin and migratory connectivity (e.g., Haig et al. 1997, Webster et al. 2002), though morphometries might still be used as a first, exploratory step (e.g., Nebel et al. 2002) or in conjunction with molecular approaches as a practical means to estimate representation in samples. Such assessments might also help define appropriate sampling scales that would minimize the likelihood of obtaining biased-low survival estimates, the site- fidelity component of local (apparent) survival rates. We acknowledge that although we sampled all major wetland areas in southwestern Puerto Rico, birds marked in the Cabo Rojo salt flats could have wintered elsewhere on the island or on a neighboring island (e.g., Dominican Republic) in any given year during our study.

The range of average EPF at capture in the present study (2-12%) was markedly lower than the range of values (>20%) recorded at a northern locale during southbound migration (Pfister et al. 1998). The discrepancy was most apparent for birds captured early in the season (<2%). Light fat loads are not unusual at wintering grounds, a condition that also appears to confer survival advantages via improved predator avoidance (Warnock and Bishop 1998, Van Der Veen and Sivars 2000, Gentle and Gosler 2001). We speculate that the low fat values early in the season were consistent with birds that have completed long migration flights (Morrison 1984) and that subsequently, higher values later in the season were indicative of birds replenishing spent reserves and striving to achieve maintenance energy budgets. Thus, it was not surprising that the EPF content at capture was a poor predictor of length of stay and of the probability of surviving and returning to the Cabo Rojo salt flats.

The length of stay of Semipalmated Sandpipers at the Cabo Rojo salt flats strongly suggested that habitat quality for foraging was high (Farmer and Wiens 1999). Tidal flow and rainfall combine during fall to promote high prey densities (Grear and Collazo 1999, Tripp and Collazo 2003). Habitat quality drops with the onset of the dry season (around December-January; Wunderle et al. 1989, Collazo et al. 1995). Semipalmated Sandpipers and other shorebird species using the salt flats depart, likely moving west to other wetlands within the Greater Antilles or south to another major migratory terminus, such as Surinam, during February and March (Morrison 1984, Thomas 1987; see Collazo et al. 2002 for timing of migration in the southeastern United States). Semipalmated Sandpipers are not an oversummering species at the Cabo Rojo salt flats (Collazo et al. 1995). It is possible that as shorebirds move to such locales, the likelihood of detecting a positive and significant relationship between estimates of body fat and return is greater. Birds probably meet part of their premigration energetic requirements in such areas prior to northbound movements, these locales serving as functional equivalents to areas such as the one studied by Pfister et al. (1998) for southbound migrants.

Collazo et al. (1995) established the numeric importance of the Cabo Rojo salt flats in Puerto Rico for Semipalmated Sandpipers. Here, we defined their functional importance-that is, the salt flats constitute part of the species’ winter range. The demographic parameter estimates reported in this work were in agreement with the seasonal habitat quality and stable population numbers documented at the salt flats for more than eight years (Collazo et al. 1995, Tripp and Collazo 2003). Future research efforts should continue quantifying the dynamics and vital parameters of small calidridine sandpipers across the Caribbean, as well as identifying wetlands used throughout winter and early spring to ensure their conservation.

ACKNOWLEDGMENTS

We thank T. Carlo, J. Colon, M. Kasprzyk, and the students of the Manomet Observatory for Conservation Field Biology Training Program for their assistance with data collection, and J. Oland and C. Delannoy for their suggestions and support throughout the project. We are grateful to J. Hines, J. Nichols, and K. Pollock for assistance with data analyses and interpretation. Manomet Observatory for Conservation Sdences, U.S. Fish and Wildlife Service and the North Carolina Fish and Wildlife Cooperative Research Unit provided support for this project. S. Dinsmore, D. Lank, J. Lyons, B. Sandercock, N. Warnock, and two anonymous reviewers provided useful comments that improved the manuscript.

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Associate Editor: B. K. Sandercock

SUSAN M. RICE,1’4 JAIME A. COLLAZO,2,5 MATHEW W. ALLDREDGE,2 BRIAN A. HARRINGTON,3 AND ALLEN R. LEWIS4

1U.S. Fish and Wildlife Service, Eastern Shore of Virginia National Wildlife Refuge, 5003 Hallett Circle, Cape Charles,

Virginia 23310, USA;

2U.S. Geological Survey, North Carolina Cooperative Fish and Wildlife Research Unit, North Carolina State University,

Raleigh, North Carolina 27695, USA;

3Manomet Center for Conservation Sciences, Manomet, Massachusetts 02345, USA; and

4Department of Biology, University of Puerto Rico, Mayaguez Campus, Mayaguez 00681, Puerto Rico

5Address correspondence to this author. Present address: Department of Zoology, Campus Box 7617, North Carolina State University, Raleigh, North Carolina 27695, USA. E-mail: jaime_collazo@ncsu.edu

Copyright American Ornithologists’ Union Oct 2007

(c) 2007 Auk, The. Provided by ProQuest Information and Learning. All rights Reserved.