Roost Site Selection of Dunlin (Calidris

ROOST SITE SELECTION OF DUNLIN (CALIDRIS

ALPINA) ON ARCATA BAY, HUMBOLDT BAY, HUMBOLDT COUNTY CALIFORNIA

Tia L. Adams

A Thesis

Presented to

The Faculty of Humboldt State University

In Partial Fulfillment

Of the Requirements for the Degree

Masters of Science

In Natural Resources: Wildlife

November, 2011 ABSTRACT

Roost Site Selection of dunlin (Calidris alpina) on Arcata Bay, Humboldt Bay, Humboldt County, California

Tia Adams

I examined the relationship between nonbreeding dunlin (Calidris alpina) roost use on Arcata Bay and energy balance (proximity hypothesis), predation, and predation risk. Individuals used roosts that were closer to their last foraging locations than roosts that were known to them and roosts that were available to the population. These findings are similar to findings on temperate and tropical bays as well as those derived from models. Predation danger and predation avoidance, two complementary factors included in the predation risk analysis, influenced dunlin roost use at high tide. Individuals were likely to abandon their roost during the observation period if the initial flock size was high. Conversely, they did not return to the roost the following day if the number of attacks sustained at the roost was low and if the final flock size from the previous day was low. Individuals consistently selected roosts that had higher initial flock sizes as well as higher numbers of attacks, successful attacks, and predators present. These results suggest that individuals are making daily decisions regarding their roost use and basing those decisions on the need to minimize energy as well as reduce their predation risk.

iii

ACKNOWLEGDMENTS

I thank my advisor Dr. M.A. Colwell and my committee members Drs. M. D.

Johnson and T. L. George for their invaluable time and assistance in this endeavor. I would like to thank Jesse Conklin for radio-marking all of the dunlin and collecting additional data and Mark Colwell and Jason Meyer for collecting additional field data during my project. I am very appreciative to Dr. Bill Bigg for always making time to answer my statistics questions. Funding was provided by the California Department of

Fish and Game’s Oil Spill Response Trust Fund through the Oiled Wildlife Care Network at the Wildlife Health Center, School of Veterinary Medicine, University of California,

Davis and The Arcata Marsh Project.

I am indebted to ABR, Inc for allowing me access to their Information

Technology (IT) and office space. I am especially indebted to the Bureau of Land

Management, Arcata Field Office and the Klamath Falls, Office of the U.S. Fish and

Wildlife Service for encouraging me throughout this process. I wish to thank my friends

Jon Plissner and Josh Rasmussen for reading lots of iterations of this project, being constant sources of encouragement, and endless amounts of information. I am forever grateful to Peter Sanzenbacher for his advice, time, patience, understanding, and encouragement; it was always a pleasure to pick your brain. To Roxy and Louie for enduring days without long walks, those days are finally gone. To my family without whom none of this would be possible, you are the most amazing people on the planet.

TABLE OF CONTENTS

ABSTRACT ...... iii ACKNOWLEGDMENTS ...... iv TABLE OF CONTENTS ...... v LIST OF TABLES ...... vi LIST OF FIGURES ...... vii LIST OF APPENDICES ...... viii INTRODUCTION ...... 1 MATERIALS AND METHODS ...... 6 Study Area ...... 6 Field methods, Data Summary and Analysis ...... 8 RESULTS ...... 15 DISCUSSION ...... 25 LITERATURE CITED ...... 31

LIST OF TABLES

Table Page 1 The coefficients of the logistic regression models comparing whether an individual abandoned (0) or utilized (1) its roost during its observation period...... 18

2 AIC values for the abandonment models: “abandoned or stayed at its roost during its observation period” and “abandoned or utilized its roost from the prior observation period.” FlockI represents initial flock size, while FlockF indicates final flock size, Pred is the total number of predators observed, AttacksT is the total number of attacks, AttacksS is the number of successful attacks, and Raptor indicates Presence or absence of a raptor predator...... 19

3 The coefficients of the logistic regression models comparing whether an individual abandoned (1) or utilized (0) its roost from the prior observation period...... 20

LIST OF FIGURES

Figure Page 1 Location of Arcata Bay in Humboldt County, California...... 7

2 Locations of roosts used by monitored dunlin wintering on Humboldt Bay, California, December 1, 2004 - March 16, 2005 (Conklin and Colwell 2007). Five most used roosts on Arcata Bay: Klopp Lake (53), Indianola (63), Mad River Slough (32), the Mad River Slough pastures (36), and Vance Road (81) (Colwell et al. 2003)...... 13

3 Foraging locations ● of wintering dunlin on Arcata Bay, Humboldt County, California, December 1, 2004 - March 16, 2005. Jacoby Creek, the most used last foraging location, is identified on the right...... 16

4 The initial flock size of roosts selected by radio-marked dunlin compared to random roosts in Arcata Bay relative to whether an individual abandoned or stayed at its roost. The differences in sample sizes are due to the days when multiple individuals were tracked. Bars indicate the mean number of attacks, while the whiskers are one Standard Error (SE)...... 21

5 The number of attacks by avian predators at roosts that were selected by focal dunlin as compared to random roosts on Arcata Bay relative to whether an individual abandoned or stayed at its roost. The differences in sample sizes are due to the days when multiple individuals were tracked. Bars indicate the mean number of attacks, while the whiskers are one Standard Error (SE)...... 22

6 The number of successful attacks at the roosts selected by radio-marked dunlins as compared to random roosts on Arcata Bay relative to whether an individual abandoned or stayed at its roost. The differences in sample sizes are due to the days when multiple individuals were tracked. Bars indicate the mean number of attacks, while the whiskers are one Standard Error (SE)...... 23

7 The final flock size of roosts selected by radio-marked dunlin compared to random roosts on Arcata Bay relative to whether an individual abandoned or stayed at its roost. The differences in sample sizes are due to the days when multiple individuals were tracked. Bars indicate the mean number of attacks, while the whiskers are one Standard Error (SE)...... 24

vii

LIST OF APPENDICES

Appendix Page A. Individual radio-marked dunlin and their successive diurnal roost use over their radio- life days during the winter study of 2004-2005 on Humboldt Bay, California. One through 38 indicate the number of radio days individuals were tracked...... 36

B. Number of roosts used in one observation period by an individual monitored, radio- marked dunlin their radio-life days during the winter of 2004-2005 on Humboldt Bay, California. Day one occurs on the first day of trapping. One through 38 indicate the number of radio days individuals were tracked...... 38

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INTRODUCTION

Roosts, locations where individuals concentrate when they are not feeding or breeding (Hockey 1985), are an integral facet of the nonbreeding ecology of shorebirds

(suborder: Charadrii) (Hale 1980). Roosts are especially important in coastal regions where high tides predictably inundate principal foraging habitats and force birds onto higher ground until the tides ebb and their principal foraging habitat becomes available again (Hale 1980, Colwell et al. 2003, Conklin and Colwell 2007). Individuals may spend a significant portion of their time resting, preening, supplemental feeding, or remaining vigilant while at roosts (Hale 1980, Burton et al. 1996, Conklin and Colwell

2007).

Two of the hypotheses associated with roost site selection are the proximity hypothesis and the predation risk or safe site hypothesis. The proximity hypothesis predicts that individuals will select roost sites that are closer to their last foraging locations. Energetics may be one factor prompting individuals to roost near their most recent foraging location. Conservation of energy may be especially critical during the nonbreeding season because of the additional stressors experienced during this period

(Hilton et al. 1999, Van Gils et al. 2006). In particular, individuals must meet their energetic costs while also contending with frequent winter storms, which may limit their foraging opportunities and require additional energy stores (Dugan et al. 1981, Castro et al. 1992, Carey and Dawson 1999). Studies on the movements of shorebirds in their nonbreeding habitats indicate that individuals use roosts close to their last foraging

1 locations in order to minimize their energy expenditure (Dias et al. 2006, Rogers et al.

2006a, Rogers et al. 2006b, Zharikov and Milton 2009).

The proximity hypothesis suggests that by using a roost site close to their last foraging location, individuals can reduce “commuting” times and energy expenditure while traveling to their roosts (Caccamise and Morrison 1988, Morrison and Caccamise

1990, Reiter and Curio 2001, Rogers et al. 2006a, Rogers et al. 2006b). The results of two studies by Morrison and Caccamise (1988, 1990) suggested that European starlings

(Sturnus vulgaris), American robins (Turdus migratorius), and common grackles

(Quiscalus quiscula) were more faithful to their major foraging locations, referred to as diurnal activity centers, than their roosts. However, they also roosted closer to their foraging locations. Great knots (Calidris tenuirostris) and red knots (C. canutus) selected diurnal roosts that are closer to their roost to avoid over-heating in a tropical environment as well as to minimize energy expenditure (Rogers et al. 2006a, Rogers et al. 2006b).

Studies of other animal groups (i.e. mammals) have also documented similar findings.

For instance, frugivorous bats in the Philippines switched their roosts in conjunction with a change in their foraging locations, suggesting that they were minimizing their travel costs (Reiter and Curio 2001).

An alternative, or complementary, hypothesis is the predation risk hypothesis, which proposes that individuals return to specific roosts because the population experiences fewer disturbances or has a lower predation risk at these sites (Hale 1980,

Switzer and Grether 2000, Rosa et al. 2006). In fact, the presence of individuals at a roost may advertise the safety of the site and attract more individuals, presumably further increasing the safety of the roost (Switzer and Grether 2000). A reduction in an

2 individual’s predation risk does not necessarily imply that they experience reduced predation events, as predators may more easily detect larger flocks of roosting individuals

(Hale 1980). Rather, Kus (1982) demonstrated that an increase in flock size resulted in a decrease in an individual’s predation risk. The likelihood of a large flock containing a sick or weak individual is greater, thereby decreasing each healthy individual’s predation risk (Kus 1982).

Predation strongly influences the demography, behavior, movements, and social structure of nonbreeding shorebirds. Raptors are a major source of mortality for wintering shorebirds (Page and Whitacre 1975, Cresswell and Whitfield 1994, Warnock and Gill 1996, Barbosa 1997). At Bolinas Lagoon in California, for example, Page and

Whitacre (1975) estimated that raptor predation resulted in mortality of 7.5-20.7 percent of wintering populations of dunlin (C. alpina), western sandpiper (C. mauri), sanderling

(C. alba), and least sandpiper (C. minutilla). The formation of roosts may function as an anti-predator mechanism and lessen the impact of predation (Dimond and Lazarus 1974,

Treisman 1975, Caraco et al. 1980, Hockey 1985, Kus 1985, Beauchamp 1999, Switzer and Grether 2000). Any adaptation that allows for increased vigilance with minimal individual effort is advantageous, since advanced detection and warning of a predator increases an individual’s survival. It also allows an individual to devote more attention to other necessary behaviors such as sleeping or preening (Dimond and Lazarus 1974, Page and Whitacre 1975, Kus 1982, Ydenberg and Prins 1984, Thompson and Thompson

1985).

Shorebird roost site fidelity has been demonstrated in several studies (Swennen

1984, Hockey 1985, Warnock and Takekawa 1996, Colwell et al. 2003, Rehfisch et al.

2003). However, one study on Humboldt Bay, California, demonstrated that while dunlin

(C. a. pacifica) had fidelity to the study area, there was considerable variation in the use of roosts within the bay. Individuals spent only a third of high tides at their primary roosts (Conklin and Colwell 2007, Conklin et al. 2008). Conklin et al. (2008) suggested that while environmental correlates such as time of day, precipitation, and wind influenced dunlin in their roost selection, they did not explain all of the variation in roost use. In addition, while dunlins were at their roosts, avian predators caused frequent movements from the roost, while human disturbance was comparatively unimportant

(Conklin et al. 2008).

Dunlin are highly gregarious in the non-breeding season, flocking with large numbers of other shorebirds (Warnock and Gill 1996). Maximum winter counts on

Humboldt Bay are in the tens of thousands (Colwell 1994). Mortality, for dunlin, appears to be highest in winter, and avian predators are a major source of winter shorebird mortality (Warnock and Gill 1996). The first autumn migrants arrive in late September, and individuals remain within Humboldt Bay until the last individuals depart for arctic breeding areas in early May (Harris 1996).

I studied roost use of wintering dunlin on Arcata Bay, focusing on relationships with energy balance (proximity hypothesis), predation, and predation risk. I tracked radio-marked individuals to discern their foraging locations and quantified the predation risk at the roosts they selected. Based on the two hypotheses described above (proximity hypothesis and predation risk hypothesis), I made two predictions. First, if dunlin minimize the energy costs of commuting between foraging locations and roosts on Arcata

Bay, then individuals will use roosts nearest their last foraging location. Second, if safety

4 from predation influences dunlin roost use on Arcata Bay, then individuals will vary duration of roost use (i.e., abandon a roost site today or in the near future ) in association with the perceived risk at the roost or change roosts in response to the variation in predation risk at surrounding roost sites .

MATERIALS AND METHODS

Study Area

I studied dunlin on Arcata Bay, in Humboldt Bay, Humboldt County, California

(Figure 1). Humboldt Bay is the second largest estuary between San Francisco Bay,

California [328 kilometers (km) to the south] and the Columbia River (612 km to the north; Western Hemisphere Shorebird Reserve Network 2009) and is recognized as a site of international importance to shorebirds by the Western Hemisphere Shorebird Reserve

Network (Harrington and Perry 1995). As many as 34 species of shorebird overwinter at this site, and total shorebird abundance can exceed 100,000 birds (Colwell 1994). The bay is influenced by mixed semi-diurnal tides. High tides inundate tidal mudflats and cause shorebirds to move onto roosts adjacent to the bay and in ephemeral wetland roosts

(Conklin and Colwell 2007). Habitat surrounding Arcata Bay consists primarily of saltmarsh, pasture, urban, and industrial areas.

Figure 1. Location of Arcata Bay in Humboldt County, California.

Field methods, Data Summary and Analysis

I conducted fieldwork daily from 2 December 2004 through 16 March 2005, the period of minimal migratory movement for dunlin at Humboldt Bay (Harris 1996). I conducted surveys at roosts identified during a one-year, high-tide, diurnal roost study conducted in 2002 (Colwell et al. 2003). Each day, I started surveys five hours before high-tide (based upon North Jetty/Humboldt Bay readings) to allow for adequate time to determine the foraging locations of dunlin. I began surveying for dunlin at roosts 30 minutes prior to high-tide and continued for 1 hour. Tides advanced daily by ~50 min resulting in several possible temporal tide combinations: 1) diurnal low tide followed by a diurnal high tide, 2) nocturnal low tide followed by a diurnal high tide, and 3) diurnal low followed by a nocturnal high. I did not monitor individuals when a nocturnal low tide was followed by a nocturnal high tide which occurred every two weeks for one night.

Conklin and Colwell (2007) showed that, at night, dunlin roosted in pastures and did not feed on the bay even though foraging mudflats were available. Throughout the study period, the high-tide roost locations of all radio-tagged individuals were recorded daily as part of a concurrent study (Conklin and Colwell 2007). An additional observer monitored the five most used roosts (Colwell et al. 2003) for predation risk.

Throughout the study, observers periodically captured dunlin and attached radio- transmitters with a 4-5 week battery life (see Appendix A) [Holohil Systems, Ltd., Carp,

Ontario; model BD-2, 1.15 gram (g); AVM Instrument Company, Ltd., Colfax,

California; model MP-2 radios, 1.43g] using a modified version of the figure-eight harness described by Sanzenbacher et al. (2000). Dunlin were fitted with a unique combination of colored leg bands. Capture and marking techniques were approved under

IACUC Section 5 Protocol 04/05.W.18.A. Each day I tracked a single radio-marked individual. I tracked individuals using a receiver (TRX-2000S; Wildlife Materials Inc.) and a hand-held 3-element Yagi antenna. I determined locations of birds with triangulations (51 percent of locations), three directional locations, or, when triangulations were not possible, biangulations (49 percent of all locations), two directional locations, of radio signals. Time between directional locations did not exceed

15 minutes thus minimizing the likelihood of a bird moving between bearings. I used the center of the triangulation area and where the biangulation lines crossed as the foraging locations and mapped these locations onto high resolution images of the bay and later transcribed these data into a geographic information system (GIS) (ESRI, Redlands,

California). Although no formal testing of the spatial accuracy of locations was conducted, in many cases the last foraging locations of dunlin coincided with the locations of the known last available foraging habitat available on Arcata Bay. This suggested that telemetry locations were accurate at a scale sufficient for the purpose of this study.

During each low tide-high tide interval, I continuously monitored one marked dunlin, selected randomly from those individuals known to be alive and with functioning transmitters. For the selected individual, I identified those foraging locations used during the rising tide and determined the roost used during the subsequent high tide. In a few cases (n = 3; 1.8 percent of surveys), I was not able to locate the randomly selected individual and instead monitored another marked bird encountered opportunistically while searching the bay.

To address the proximity hypothesis, I calculated the straight line distance in meters (m) between an individual’s last detected foraging area and its subsequent roost site using ArcGIS 9.3 (ESRI, Inc., Redlands, California). I averaged all the distances of individuals across all sampling days. These data were not normally distributed.

Therefore I log(10) transformed the data. I tested the hypothesis that individuals roosted close to their last foraging locations with a paired t-test (one-tailed). I compared the distance they actually traveled to the mean distance they could have traveled to: a) all roosts they used during the time when their radio was active (familiar roosts), and b) all roosts available to the population.

To evaluate risk posed by avian predators, I surveyed the focal individual’s high tide roost for one hour and recorded the presence of potential predators (Page and

Whitacre 1975) including peregrine falcon (Falco peregrinus), merlin (F. columbarius), northern harrier (Circus cyaneus), gulls (Larus spp.), American kestrel (F. sparverius), short-eared owl (Asio flammeus), prairie falcon (F. mexicanus), sharp-shinned hawk

(Accipiter striatus), and Cooper’s hawk (A. cooperii). Following Elphick (2000), I recorded the number of predators, their movements (i.e., flights near roosts), and the frequency of attacks, or predatory stoops, that occurred on a flock of shorebirds at their monitored roost. For each attack that I witnessed within the one hour observation period,

I noted details of the attack including the predator species, the targeted prey species, and the outcome of the attack (i.e., success/failure).

Coincident with my study, Conklin and Colwell (2008) determined that radio- marked dunlins co-occurred at roosts 15 percent of the time. Therefore, because more than one marked dunlin may have utilized a roost simultaneously, I recorded the presence

10 of all radio-marked dunlin at the roost, their responses and their flock’s responses to each of the observed predation events. At the beginning and end of the survey, I recorded flock size of all calidrid shorebirds (i.e., sandpipers [including dunlin]); referred to as

“initial” and “final” flock size respective. I noted the movements of flocks at roosts, including the time of arrival and departure of all marked birds. In addition, I estimated the size of all shorebird flocks at the roost. When a focal individual departed a roost prior to the end of a survey, I continued surveying the roost for the duration of the hour. I considered a focal individual to have abandoned its roost if it did not return to the roost prior to the end of the one hour high tide survey period. At the end of the survey, I attempted to determine the location of the focal individual(s) that had abandoned the roost.

I collated information associated with the roosts of radio-marked dunlin including: roost identification; initial and final flock sizes; presence and absence of a predator; total numbers of predators (avian or mammalian) present; number of attacks; and number of successful attacks. I used logistic regression to examine if the departure or presence of a marked dunlin during the one hour observation period was associated with the variables collected at the roost on that day. My full model included: roost ID; initial flock size; final flock size; total predators; number attacks; number successful

(attacks); presence or absence of raptors. For logistic regression analysis of roost abandonment during the observation period, I established dependent, categorical variables “roost abandonment” as a 0 and “remained at roost” as a 1. I used a drop-in deviance approach (Hosmer and Lemeshow 2000) to assess the importance of each variable to the full model and excluded uninformative variables to generate a reduced

11 model of variables that were significant. I compared the models using Akaike’s

Information Criterion (AIC) criterion. I then used a Receiver Operating Curve (ROC) analysis to evaluate the predictability of the best or final model; the higher the ROC, the better predictive value of the model.

I used the same full and reduced models approach to determine if “today’s” predation risk at a monitored roost influenced an individual’s presence at the same roost on the subsequent day. However, for this analysis, I established my dependent, categorical variables “roost abandonment” as a 1 and “remained at roost” as a 0.

To quantify predation risk at specific roosts, I examined predator activity at the five most frequently used roosts on Arcata Bay (Colwell et al. 2003) (Figure 2). An observer monitored these roosts and recorded the same information I collected at the roost occupied by the focal radio-marked individual. I used a Hotelling’s t-test to compare the monitored roost to the unselected roost in order to determine if the predation risk, indexed by 6 variables [initial flock size; final flock size; total predators; number of attacks; number of successful (attacks); and presence or absence of raptor predator] differed at the roost occupied by radio-marked individuals from other roosts around the bay. For each daily observation of a radio-marked individual’s, roost there was at least one observation of an unselected and monitored roost. I considered a roost to be unselected if the focal individual did not use it during the monitored high tide observation period. In most instances, however, there were shorebirds, including dunlin, utilizing the unselected roosts. In a few instances, other radio-marked individuals were present.

However, since they were not being monitored the roost was still considered unselected.

Figure 2. Locations of roosts used by monitored dunlin wintering on Humboldt Bay, California, December 1, 2004 - March 16, 2005 (Conklin and Colwell 2007). Five most used roosts on Arcata Bay: Klopp Lake (53), Indianola (63), Mad River Slough (32), the Mad River Slough pastures (36), and Vance Road (81) (Colwell et al. 2003).

I analyzed these data in two subsets: 1) all surveys when the individual(s) abandoned their roost, and 2) all surveys when the individual(s) stayed at the roost.

RESULTS

Radio transmitters were attached to 25 dunlins between 1 December 2004 and 23

February 2005. The operating life of transmitters ranged from 2 to 39 days. I relocated

19 individuals a minimum of one time with some detected up to 7 different days for a total of 157 observations (Appendix 1). I detected 1-6 radio-marked individuals at a roost per survey. Based on the 157 observations or surveys of radio-marked individuals, dunlin used 19 of the 86 roosts identified by Colwell et al. (2003) (Figure 2). Roosts were given unique identification numbers from 1-86. Marked dunlin used three roosts most frequently: Klopp Lake (#53) = 27 percent of observations; Vance Road (#81) = 23 percent of observations; and South Old Samoa Road (#36) = 16 percent of observations.

The number of roosts used by an individual during a high tide interval ranged from one to four roosts, although in most instances individuals occupied only one (38 percent) or two

(40 percent) roosts versus use of three (14 percent) or four roosts (less than 1 percent)

(Appendix B).

Individuals used various locations on the bay and in the pastures as their last foraging locations prior to a high tide (Figure 3). The tidal mudflat at the mouth of

Jacoby Creek, which is the last area inundated on a flooding tide, was the last foraging location for 32% of marked birds (Figure 3).

Individuals used roosts that were on average closer to their last foraging location than all the roosts they had used [mean distance = 3.2 ± 0.4 km (observed), 3.6 ± 0.09 km

(expected); df = 57, t = -6.175, one-tailed p <0.001) and all

Figure 3. Foraging locations ● of wintering dunlin on Arcata Bay, Humboldt County, California, December 1, 2004 - March 16, 2005. Jacoby Creek, the most used last foraging location, is identified on the right.

roosts available on Arcata Bay [mean distance = 3.2 ± 0.4 km (observed), 3.7 ± 0.05 km

(expected) ; df = 57, t = -7.74, one-tailed p <0.001; Figure 3].

The likelihood that a marked dunlin abandoned its roost during the one hour focal survey was greater when initial and final flock sizes were large and small, respectively.

The reduced model with only the variables initial and final flock size was the most parsimonious model (Table 1). This model had an ROC value of 0.83 indicating it was a highly predictive model (Table 2).

By contrast, subsequent-day roost use increased in association with an increase of attacks and a increase of final flock sizes (Table 3). The reduced logistic regression model, including total number of attacks and final flock size, had an ROC value of 0.76

(Table 2).

Radio marked individuals selected roosts that had higher initial flock sizes, greater numbers of attacks by raptors, and more successful attacks (Figure 4-6). Only when individuals abandoned their roosts was the final flock size lower than random roost sites around the bay (Figure 7). There was a significant difference in the initial and final flock sizes, total number of predators, number of attacks, successful attacks, presence or absence of a raptor predator for surveys where individuals abandoned their roosts, remained at their roosts (T² = 35.213, p ≤ 0.0009; T² = 31.518, p ≤ 0.004, respectively).

Individuals abandoned roosts that had lower final flock sizes, lower initial flock sizes, more predators, and more predator attacks as well as more successful attacks. Individuals remained at roosts that had higher initial and final flock sizes as well as more predators, more predator attacks, and more successful attacks.

Table 1. The coefficients of the logistic regression models comparing whether an individual abandoned (0) or utilized (1) its roost during its observation period.

Model/variables Coefficients 95% Confidence Interval z value Wald probability Odds Ratio

Full Model Intercept -0.54676 1.99959 -0.536 0.59201 0.57882 a Initial Flock Size -0.00027 0.00026 -2.032 0.04216 0.76338b a Final Flock Size 0.00088 0.00037 4.691 0.00000 2.4109 b

Number of Attacks -0.08561 0.37301 -0.450 0.65284 0.91796

Number of Successful Attacks -0.32090 0.63247 -0.994 0.32001 0.72550

Presence or absence of raptor predator 0.96357 1.40155 1.347 0.17782 2.62105

Roost ID 0.00964 0.02694 0.701 0.48316 1.00968

Total predators -0.12578 0.35341 -0.698 0.48545 0.88181

Reduced Model 3 variables Intercept -0.06575 0.96524 -0.134 0.89378 0.93636 a Initial Flock Size -0.00028 0.00023 -2.346 0.01900 0.75578b a Final Flock Size 0.00084 0.00030 5.341 0.00000 2.31637b

Presence or absence of raptor predator 0.01821 0.94843 0.038 0.96999 1.01837

Reduced Model 2 variables Intercept -0.05511 0.79043 -0.137 0.89130 0.94638 a Initial Flock Size -0.00028 0.00023 -2.358 0.01837 0.75578b a Final Flock Size 0.00084 0.00030 5.376 0.00000 2.31637b a minimum and maximum values do not overlap zero and thus indicate a significant result. b value adjusted to represent flock size increments of 1,000 (Hosmer and Lemeshow 2000).

Table 2. AIC values for the abandonment models: “abandoned or stayed at its roost during its observation period” and “abandoned or utilized its roost from the prior observation period.” FlockI represents initial flock size, while FlockF indicates final flock size, Pred is the total number of predators observed, AttacksT is the total number of attacks, AttacksS is the number of successful attacks, and Raptor indicates Presence or absence of a raptor predator.

Model #Parameters AIC ΔAIC AIC Weight ROC value

Roost abandoned during observation period FlockI + FlockF 2 188.3 0.00 0.73 0.83 FlockI + FlockF + Raptor 3 190.3 2.00 0.27

ID + FlockI + FlockF+ Pred + AttacksT + 7 206.6 18.28 0.00 AttacksS + Raptor

Roost abandoned or utilized in subsequent day observation period

FlockF + AttacksT 2 179.1 0.00 0.65 0.76

F T S Flock + Attacks + Attacks 3 180.3 1.28 0.35

ID + FlockI + FlockF+ Pred + AttacksT + 7 192.3 13.23 0.00 AttacksS + Raptor

Table 3. The coefficients of the logistic regression models comparing whether an individual abandoned (1) or utilized (0) its roost from the prior observation period.

Model/Variables Coefficients 95% Confidence Interval z value Wald probability Odds Ratio

Full Model a Intercept 2.61212 1.89145 2.707 0.00679 13.62784

Initial Flock Size 0.00013 0.00034 0.732 0.46393 1.138828b a Final Flock Size -0.00043 0.00029 -2.921 0.00349 0.650509b

Number of Attacks -0.74459 0.50558 -2.887 0.00390 0.474930

Number of Successful Attacks 0.58892 0.92867 1.243 0.21390 1.802040

Presence or absence of raptor predator 0.63003 1.31106 0.942 0.34626 1.877670

Roost ID -0.01539 0.02482 -1.215 0.22425 0.984730

Total predators 0.01747 0.32391 0.106 0.91581 1.017620

Reduced Model 3 variables a Intercept 2.30760 0.96311 4.696 0.00000 10.05031 a Final Flock Size -0.00036 0.00021 -3.312 0.00093 0.697676b a Number of Attacks -0.50518 0.31935 -3.100 0.00193 0.603390

Number of Successful Attacks 0.29603 0.77647 0.747 0.45493 1.344500

Reduced Model 2 variables a Intercept 2.26268 0.89415 4.960 0.00000 9.608770 a Final Flock Size -0.00034 0.00020 -3.382 0.00072 0.71177b a Number of Attacks -0.42158 0.23496 -3.517 0.00044 0.656010 a minimum and maximum values do not overlap zero and thus indicate a significant result. b value adjusted to represent flock size increments of 1,000 (Hosmer and Lemeshow 2000).

6000 n = 21 n = 36

5000 n = 19

4000

n = 31 3000 Focal Random

2000 Initial Flock Initial Estimation Size

1000

0 Abandon Stay

Figure 4. The initial flock size of roosts selected by radio-marked dunlin compared to random roosts in Arcata Bay relative to whether an individual abandoned or stayed at its roost. The differences in sample sizes are due to the days when multiple individuals were tracked. Bars indicate the mean number of attacks, while the whiskers are one Standard Error (SE).

6 n = 21

5 n = 36

Focal Random

3 Number of AttacksofNumber

2 n = 19 n = 31

0 Abandon Stay

Figure 5. The number of attacks by avian predators at roosts that were selected by focal dunlin as compared to random roosts on Arcata Bay relative to whether an individual abandoned or stayed at its roost. The differences in sample sizes are due to the days when multiple individuals were tracked. Bars indicate the mean number of attacks, while the whiskers are one Standard Error (SE).

6 n = 21

5 n = 36

Focal Random 3

2 n = 19 Number of SuccessfulofNumber Attacks n = 31

0 Abandon Stay

Figure 6. The number of successful attacks at the roosts selected by radio-marked dunlins as compared to random roosts on Arcata Bay relative to whether an individual abandoned or stayed at its roost. The differences in sample sizes are due to the days when multiple individuals were tracked. Bars indicate the mean number of attacks, while the whiskers are one Standard Error (SE).

6000 n = 36

5000 n = 19

4000

n = 21 n = 31 3000 Focal Random

2000 FinalFlock Estimation Size

1000

0 Abandon Stay

Figure 7. The final flock size of roosts selected by radio-marked dunlin compared to random roosts on Arcata Bay relative to whether an individual abandoned or stayed at its roost. The differences in sample sizes are due to the days when multiple individuals were tracked. Bars indicate the mean number of attacks, while the whiskers are one Standard Error (SE).

DISCUSSION

Movements of wintering dunlin on Arcata Bay provided support for both hypotheses that I examined in this study: proximity to foraging locations and the predation risk assessment. These results indicate that individual dunlin are selecting roosts in the nonbreeding season to reduce their energy expenditures in addition to selecting roosts that appear safer from a predation risk assessment. Specifically, individual dunlin used roosts closer to their last foraging location (within 2-3 km) compared to roosts they used on other occasions or roosts used by other dunlin during the study. Moreover, individuals selected roosts that had larger flock sizes than alternative available roosts, despite higher frequency of attacks by raptors.

The finding that dunlin roosted near their last foraging location is similar to other studies conducted on movements of shorebirds in their nonbreeding season. Previous studies demonstrated shorebirds of various species traveled distances (1-9 km one way) and indicated shorebirds attempt to minimize their energy expenditures by selecting roost that are closer to their last foraging locations (Van Gils et al. 2006, Dias et al. 2006,

Rogers et al. 2006b, and Zharikov and Milton 2009). For instance, Van Gils et al. (2006) examined factors influencing foraging site selection by red knots on the Dutch Wadden

Sea, a tidal system. They directly quantified an individual’s energetics by analyzing the amount of food at foraging locations, the energy stores of the individuals, and the proximity of their used roosts. Their results indicated that individuals were most influenced by travel costs rather than energy intake at sites.

Similarly, in a study on the Tagus Sea in Portugal, Dias et al. (2006) marked dunlin at high tide roosts and conducted counts of marked and unmarked dunlin at their roosts and on their low tide foraging areas. They found that marked dunlin showed extremely high fidelity to roost sites with greater than 99 percent of sightings of marked birds occurring at the roosts where they were captured and marked. Additionally, 80 percent of all marked individuals foraged within 5 km of their high tide roost. There was a significant negative correlation between the percentage of marked dunlin on foraging areas and the distance of the foraging areas from roosts (Dias et al. 2006). Their study differs from my study, in that I measured the distance traveled by individuals on a daily basis, whereas Dias et al. (2006) carried out counts on low tide foraging areas once a year and then measured the straight line distance to the known roosts on the Tagus Sea.

In addition, various studies have shown that shorebirds in tropical regions show similar results as individuals in temperate regions. However, this could be a result of an individual attempting to avoid over-heating. For example, great knot and red knot movements were influenced by the distance from high tide roost and foraging areas at

Roebuck Bay, a tropical bay in Australia (Rogers et al. 2006b). Individuals flew one way distances of 1-3 km to reach their closest diurnal roosts that met a suite of habitat characteristics including shallow water depth and increased distances from tall vegetation that obstructed views (Rogers et al. 2006b). Zharikov and Milton (2009) predicted roost site selection with published information of high tide shorebird roost use. Their results indicated that proximity to a large foraging area is the most common predictor of roost use. Though the distances traveled by dunlins on Arcata Bay (2-3 km) differ from the distances traveled by shorebirds on the Dutch Wadden Sea, they are comparable to the

26 distances flown by shorebirds on the Tagus Sea and Roebuck Bay. While these studies examined shorebirds on estuaries that varied greatly in size and temperature, as well as used different approaches to address the proximity hypothesis, their results indicate that shorebirds attempt to minimize their energy expenditures by selecting roosts that are near their last foraging locations. The consistency of these findings suggests that the minimization of energy expenditure is a widespread and common phenomenon within shorebird ecology.

Strategies to minimize energy expenditures in the nonbreeding season by shorebirds may be influenced by predation risk, which assumes that individual shorebirds expend additional energy in flight attempting to avoid predators. Roosting in open and exposed habitats may have evolved as a form of protection from predators (Hamilton

1971). Lank and Ydenberg (2003) determined that shorebird predation risk by raptors is a combination of three different but complementary factors: danger (e.g. number of predators present), escape performance (e.g. wing loading), and anti-predator behavior

(e.g. flock size, species composition, habitat selection). Previous studies have primarily focused on only one of the three factors at a time (Desholm et al. 1999, Hedenstrom and

Rosen 2001). For example, studies have examined the danger aspect by looking at the success rate of predators and their associated mortality rates on prey populations

(Roalkvam 1985, Buchanan et al. 1986, Buchanan et al. 1988, Buchanan 1996). In addition, a number of studies have investigated habitat selection and flocking behavior of shorebirds (both examples of anti-predator behaviors) as mechanisms to avoid predation

(Kus 1982, Metcalfe 1984, Dekker and Ydenberg 2003). Few studies have analyzed

27 more than one factor and its implications on roost abandonment in a single nonbreeding habitat.

I found that predation risk influenced the daily use of roosts by dunlin on Arcata

Bay. I measured predation risk by addressing two aspects of the above three factors. I directly accounted for predation danger by measuring the number of successful attacks that occurred on a roost during a given high tide period. And I also collected information on the presence or absence of predators (i.e., raptors) as well as the number of attacks the roost sustained. In addition, while my study did not examine escape performance, it examined anti-predator behavior by measuring the initial and final flock size of shorebirds that were utilizing the roost.

The results of my study suggest that the measurement of predation danger is of lesser consequence to individuals in their decision to immediately abandon their roost than other factors. Individuals are more likely to abandon their roost if the flock size from the beginning of the survey period is high, regardless of the number of attacks the roost sustained or number of predators present. Therefore, in an immediate sense, it is the anti-predator behavior that they use to determine their predation risk. Conversely, the use of the monitored roost by the same individuals on subsequent days is influenced by predation danger and anti-predator behaviors. Individuals chose to go to a different roost on subsequent days if the number of attacks a roost sustained during their survey period was low and if the final flock sizes at that roost were low. In addition, a comparison of the monitored roosts to the five most used roosts on Arcata Bay showed that radio- marked individuals consistently chose roosts that had higher beginning flock sizes

(Figure 4). However, they also had higher numbers of predator attacks as well as

28 successful attacks (Figure 5, 6). These comparisons could potentially be misleading as we selected the comparison roosts based on their frequency of use from previous studies

(Colwell et al. 2003, Conklin and Colwell 2007); therefore they should consistently contain higher proportions of the nonbreeding shorebird populations. However, since radio-marked dunlin utilized three of those five roosts over the field season, this should minimize the risk of misleading information (Figure 2).

Individuals could be taking advantage of their flocking behaviors to direct their energies to other important processes, such as thermoregulation and resting. The dilution effect states that individuals in a group will have an equal risk of being depredated if equally spaced from one another and the predator (Hamilton 1971, Cresswell et al. 2003).

Species that are at risk from the same predator can reduce their level of vigilance and energy expenditure by forming large groups (Pulliam 1973, Kus 1985, Downes and

Hoefer 2004). This concept is supported by the observation that vigilance rates were constant over all flock sizes of sandpipers except in small flocks where individuals scanned more frequently (Kus 1985). Regardless of flock size, however, a truly nonvigilant individual could be at risk for depredation (Cresswell et al. 2003).

Conklin et al. (2008) observed that dunlin on Humboldt Bay appeared to be more nervous in large flocks. Large flocks may be more easily detected by predators and therefore more attractive (Kus 1985) and sustain higher numbers of predation events.

Consistent with this theory, Kus (1985) observed that two sizes of flocks were attacked most often, very large flocks and flocks with 2-25 individuals. Kus’ (1985) and Conklin et al.’s (2008) observations suggest that there is a threshold in flock size where the presence of conspecifics ceases to be a benefit to an individual’s safety. While my study

29 does not indicate where this threshold lies, it does suggest that individual dunlin on

Arcata Bay make decisions regarding roost use based on their predation risk at those roosts. In addition, it also suggests that individuals compare their predation risk knowledge at their roosts to alternative roosts available on Arcata Bay.

Starvation and predation risk are important factors influencing the nonbreeding ecology of shorebirds (Ydenberg et al. 2010). The two hypotheses for roost use: distance from last foraging location and predation risk, are not mutually exclusive. The ability to maintain a favorable energy balance is required by wild birds to avoid mortality (Kersten and Piersma 1987). In addition to having higher-than-expected basal metabolic rates

(BMR), shorebirds have high daily energy expenditures (DEE) with their highest DEE occurring in mid-winter (Kersten and Piersma 1987). During winter, dunlins on Arcata

Bay often encounter inclement weather conditions with varying food sources. They are under pressure to balance their energy needs as well as avoid predation. To do so they selected roosts that are closer to their last foraging locations with higher flock sizes than average. Ultimately these decisions will reduce energetic expenditures and possibly reduce predation risk.

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Appendix A. Individual radio-marked dunlin and their successive diurnal roost use over their radio-life days during the winter study of 2004-2005 on Humboldt Bay, California. One through 38 indicate the number of radio days individuals were tracked.

Color band Radio Date marked 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 combination Frequency mRBB 149.579 12/1/2004 - 81 - - 75 D mRBG 149.149 12/1/2004 53 81 81 - - 78 - - 22 30 31 61 81 36 30 33 81 - 53 mYBB 148.327 12/16/2004 53 53 81 72 81 53 53 53 53 53 30 2 26 29 29 30 39 81 57 mBBG 148.610 12/16/2004 53 53 57 31 31 33 81 81 80 81 31 36 31 55 36 36 36 36 - mGBB 148.267 12/16/2004 53 61 53 53 53 53 53 53 53 53 53 53 81 55 36 36 16 D mRBO 148.297 12/17/2004 53 57 53 53 61 53 61 53 53 31 36 81 81 ------mRBP 149.299 12/28/2004 80 81 16 1 16 16 57 16 30 53 17 - 39 16 16 D mYBO 148.020 12/28/2004 31 80 73 - 73 78 - - 73 73 66 - 66 72 73 25 36 70 36 mBBO 148.101 12/29/2004 81 36 36 36 81 57 30 99 31 17 - 36 36 16 D mGBG 148.120 12/29/2004 55 30 30 39 36 - 30 99 31 16 36 38 17 17 36 25 24 22 30 mRGP 148.070 1/13/2005 25 31 36 36 - 99 36 53 47 31 47 61 - - 61 - 62 - 57 mRPP 148.250 1/14/2005 36 36 24 - 31 10 31 30 31 - 81 81 81 81 81 81 81 82 81 mBBR 149.649 1/25/2005 36 2 81 36 81 81 36 81 - - 55 81 36 30 36 31 31 30 - mGBP 148.288 1/25/2005 36 77 81 36 81 81 81 81 - - 81 81 81 36 30 80 81 81 81 mRBR 148.348 2/8/2005 36 55 61 61 61 - 57 74 - 64 73 61 53 61 61 61 53 53 - BRmY 148.269 2/8/2005 36 81 81 81 81 47 81 73 - 73 73 80 30 73 81 81 81 78 81 mYGO 149.019 2/8/2005 57 53 D mBBY 148.818 2/9/2005 81 31 31 - 30 54 - - 81 84 - - 81 - 81 81 99 81 - mGBR 149.669 2/10/2005 53 53 53 47 57 99 - 33 81 81 81 81 81 81 81 D mBPB 149.708 2/10/2005 53 31 47 47 81 57 - - 61 61 30 81 61 31 61 47 53 53 53 mBGO 148.378 2/23/2005 86 53 53 53 53 53 53 54 - 45 99 53 53 53 53 53 81 53 61

Bold numbers indicate roosts where either individuals where tracked for low-tide locations with subsequent roost surveys, where low-tide locations were made and roost locations were noted, or where roost surveys alone where conducted. D indicates radio death, bird mortality, or abandonment of study area. – indicates the individual was not located during the high-tide period.

Color band Radio Date 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 combination Frequency marked mRBB 149.579 12/1/2004 mRBG 149.149 12/1/2004 81 53 53 53 53 53 30 2 81 39 30 30 16 D mYBB 148.327 12/16/2004 - 53 53 36 - 38 36 17 28 36 36 36 99 - - 72 99 - 31 D mBBG 148.610 12/16/2004 - 31 31 D mGBB 148.267 12/16/2004 mRBO 148.297 12/17/2004 ------36 D mRBP 149.299 12/28/2004 mYBO 148.020 12/28/2004 70 - 72 72 73 73 81 73 81 80 77 80 36 81 81 57 D mBBO 148.101 12/29/2004 mGBG 148.120 12/29/2004 - 18 18 31 30 24 24 61 80 81 81 81 81 47 57 55 D mRGP 148.070 1/13/2005 55 - - 55 55 53 30 61 53 61 61 61 - 57 57 D mRPP 148.250 1/14/2005 - - 81 81 81 31 81 81 81 79 47 30 57 33 - 81 84 D mBBR 149.649 1/25/2005 30 81 47 - - 99 84 36 - 81 31 D mGBP 148.288 1/25/2005 D mRBR 148.348 2/8/2005 53 - 53 54 - 53 D BRmY 148.269 2/8/2005 72 78 70 78 - - - 72 73 73 77 53 D mYGO 149.019 2/8/2005 mBBY 148.818 2/9/2005 53 22 54 - 16 81 53 53 D mGBR 149.669 2/10/2005 mBPB 149.708 2/10/2005 53 54 - 99 53 53 53 D mBGO 148.378 2/23/2005 53 57 D

Appendix B. Number of roosts used in one observation period by an individual monitored, radio-marked dunlin their radio-life days during the winter of 2004-2005 on Humboldt Bay, California. Day one occurs on the first day of trapping. One through 38 indicate the number of radio days individuals were tracked.

Color band Radio Date marked 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 combination Frequency mRBB 149.579 12/1/2004 mRBG 149.149 12/1/2004 1a 1a 1a - - 1a - - 2a - 1a 2a 3 2 - 1a - - - mYBB 148.327 12/16/2004 - 1a - - - 2a ------2a - - - mBBG 148.610 12/16/2004 ------2a - - 3 - 1 - - 2 3 mGBB 148.267 12/16/2004 ------1a ------3 D mRBO 148.297 12/17/2004 - - - - - 1a ------mRBP 149.299 12/28/2004 ------2 D mYBO 148.020 12/28/2004 ------1 - 1 mBBO 148.101 12/29/2004 - - - 3 ------3 - - - D mGBG 148.120 12/29/2004 - - - 2 ------2d - - - 2 - 4 - 3d mRGP 148.070 1/13/2005 - 1 1 2 ------2 - - 3 - 1 - - mRPP 148.250 1/14/2005 - - - 4b - - - 2 - - - 2 - 2 2 - - - - mBBR 149.649 1/25/2005 ------2 - - - 1d - - - 1 - 3 - - mGBP 148.288 1/25/2005 - - - - 1 - - 1 - - - - 1 ------mRBR 148.348 2/8/2005 ------2 - - - 1a ------2 - BRmY 148.269 2/8/2005 - 2 - - - - - 2b - - - - - 1 - - - - 4 mYGO 149.019 2/8/2005 mBBY 148.818 2/9/2005 - - 1 ------mGBR 149.669 2/10/2005 - 2 - 4 ------1 - - D mBPB 149.708 2/10/2005 - 1a 2 ------1a - - - 2 3 3 - - 2 mBGO 148.378 2/23/2005 - 3 - 1b - 2 - - 4b - - 2c 2c - 3 - 4c 2 3c a indicates when only a roost survey was completed without the tidal foraging movement observations. b indicates when only tidal foraging movement observations occurred. c indicates when the same roosts were used multiple times in one survey period. d indicates roost number includes nocturnal roosts. D indicates radio death, bird mortality, or abandonment of study area. – indicates the individual was not located during the high-tide period.

Appendix B. Number of roosts used in on one observation period by an individual monitored, radio-marked dunlin their radio- life days during the winter of 2004-2005 on Humboldt Bay, California. Day one occurs on the first day of trapping . One through 38 indicate the number of radio days individuals were tracked (continued).

Color band Radio Date marked 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 combination Frequency mRBB 149.579 12/1/2004 mRBG 149.149 12/1/2004 - - - - 1a - 1a - - - - 1 - D mYBB 148.327 12/16/2004 ------1a - - 2 - - - - 1d - - - - D mBBG 148.610 12/16/2004 - - - D mGBB 148.267 12/16/2004 mRBO 148.297 12/17/2004 ------D mRBP 149.299 12/28/2004 mYBO 148.020 12/28/2004 - - - - 1* - - - - - 1 - 1 - 1 - D mBBO 148.101 12/29/2004 mGBG 148.120 12/29/2004 ------3 - - - - D mRGP 148.070 1/13/2005 - 1d ------4c - - - - 1 - D mRPP 148.250 1/14/2005 - - - - - 2 - - - - - 2 - - 2b - D mBBR 149.649 1/25/2005 ------2d - - - D mGBP 148.288 1/25/2005 D mRBR 148.348 2/8/2005 ------D BRmY 148.269 2/8/2005 2 - - 1b - - - - - 2 - 2 D mYGO 149.019 2/8/2005 mBBY 148.818 2/9/2005 - - - - - 2d - - D mGBR 149.669 2/10/2005 mBPB 149.708 2/10/2005 2 ------D mBGO 148.378 2/23/2005 1 2 D a indicates when only a roost survey was completed without the tidal foraging movement observations. b indicates when only tidal foraging movement observations occurred. c indicates when the same roosts were used multiple times in one survey period. d indicates roost number includes nocturnal roosts. D indicates radio death, bird mortality, or abandonment of study area. – indicates the individual was not located during the high-tide period.