SPACE USE OF AMERICAN (RECURVIROSTRA AMERICANA) IN

SOUTH SAN FRANCISCO BAY, CALIFORNIA

by

Scott A. Demers

A Thesis

Presented to

The Faculty of Humboldt State University

In Partial Fulfillment

Of the Requirements for the Degree

Master of Science

In Natural Resources: Wildlife

August, 2007

ABSTRACT

Space Use of American Avocets (Recurvirostra americana) in South San Francisco Bay, California

Scott A. Demers

Examining the spatial ecology of waterbirds is essential for the conservation

and management of wetland areas. I studied the space use (movements and home ranges) of radio-marked female American avocets (Recurvirostra americana) in

South San Francisco Bay, California. Direct visual observations were used to determine the breeding stage (pre-incubation, incubation, brood-rearing, post- breeding) of marked avocets.

Linear movements, home ranges, core areas, and average distances of avocets from nests varied significantly among stages. Post-breeding space use metrics were the greatest, followed by pre-incubation, incubation, and brood-rearing. The majority of nest locations (93%) were within the pre-incubation core area boundaries, whereas 36% of nests were within post-breeding core areas. Distance between future nest sites and daily location decreased significantly with number of days prior to incubation.

The differences in home ranges, core areas, and movements between the pre- incubation and post-breeding stages suggest space use of female avocets may be influenced by breeding activities. Average distance from nest during the pre- incubation stage suggests that avocets may be prospecting future nest sites up to three

iii weeks prior to nesting. In a highly managed estuary like the San Francisco Bay,

natural resources managers can utilize spatial data and implement conservation

strategies that maximize the potential for continued survival of American avocets and other breeding waterbirds.

iv ACKNOWLEDGMENTS

This research was funded by the CalFed Bay-Delta Program’s Ecosystem

Restoration Project. Thanks to Dr. John Takekawa, Dr. Josh Ackerman, and all the members to the CalFed mercury project team that graciously allowed graduate students to be a part of the project. Thanks to committee members Dr. Rick Golightly and Dr. Matt Johnson for sharing their input and experience. I owe special thanks to

Dr. Mark Colwell for advising me through thick and thin. I would like to thank Jill

Bluso, Stacy Moskal, Brooke Hill, Joe Northrup, Kristen Dybla, Lindsay Dembosz,

Emily Eppinger, River Gates, Angela Rex, Mychal Truwe, and Ross Wilming for hard work and dedication to this project. I also thank Clyde Morris and the staff at the Don Edwards San Francisco Bay National Wildlife Refuge for logistical support.

v TABLE OF CONTENTS

Page

ABSTRACT ………..…………………………………………………………….. iii

ACKNOWLEDGMENTS ………………………………………………………… v

LIST OF TABLES ………...……………………………………………………… vii

LIST OF FIGURES …………..………………………………………………….. viii

LIST OF APPENDICES ……………...…………………………………………... ix

INTRODUCTION ……………...……………………………………………….... 1

STUDY AREA …………………………………...………………………………. 3

METHODS ……………………...………………………………………………... 5

Capture and Radio Telemetry ……...……………………………………... 5

Data Summary and Analysis …………...…….…………………………… 7

RESULTS …………………..…………………………………………………….. 10

Linear Movements and Home Range …………..………………………… 10

Behavior and Flock Size ……………………..…………………………… 17

DISCUSSION ……………………………………..……………………………… 19

Conservation Implications ………..……………… ……………………… 21

LITERATURE CITED …………………..……………………………………….. 24

APPENDIX A ………………..…………………………………………………… 29

APPENDIX B ………………..…………………………………………………… 30

APPENDIX C …………..………………………………………………………… 31

vi LIST OF TABLES

Table Page

1 Recorded behaviors of radio-marked American avocets in in South San Francisco Bay, California, USA. Behaviors were recorded opportunistically when individuals could be visually located .…………………………………………………………….….. 18 .

vii LIST OF FIGURES

Figure Page

1 Map of South San Francisco Bay, California, showing dominant habitat types and capture locations in 2005 and 2006. Avocets were captured in two salt ponds (Ponds A8 and A16) and a tidal flat (Coyote Creek Lagoon) ...……………………………………...……..…...... 4

2 Mean (± SE) linear distance between consecutive locations of American avocets in South San Francisco Bay, California, USA, 2005-06 ……..….… 11

3 Mean (± SE) home range size and linear movements of four American avocets sampled in each breeding stage in South San Francisco Bay, California, USA, 2005-06. Standardized scores were calculated based on individual mean values across the four stages .....………………..….…. 12

4 Mean (± SE) home range and core area size of American avocets in South San Francisco Bay, California, USA, 2005-06 …………………...… 13

5 Mean (± SE) distances from nest location, in each breeding stage, of American avocets in South San Francisco Bay, California, USA, 2005-06 ..………………………………………………………………….. 15

6 Relationship between average (± SE) distances from nest and days prior to incubations during the pre-incubation stage for American avocets in South San Francisco Bay, California, USA, 2005-06 ...…………………… 16

viii LIST OF APPENDICES

Appendix Page A The distance and time between consecutive locations, in each breeding stage, of American avocets in San Francisco Bay, California, USA. The fitted lines indicate there is no correlation in the time between locations and the distances moved (Pre-incubation, r2 = 0.01, P = 0.02; Incubation, r2 = 0.00, P = 0.892; Brood-rearing, r2 = 0.01, P = 0.420, Post-breeding, r2 = 0.0150, P = 0.031). All locations >1 hour apart were used for analyses ...... ……………………………...……………….29

B Average distances between consecutive locations of American avocets in each breeding stage, San Francisco Bay, California, USA. To test for temporal autocorrelation in linear movement data, locations less than 1, 2, 4, 8, and 16 hours apart were consecutively removed from analysis. These removals did not significantly alter average distance calculations; therefore all locations > 1 hour apart were used for analyses ………………...……... 30

C Average home range sizes of four American avocets in South San Francisco Bay, California, USA. Locations were randomly selected and removed to test for the minimum number of locations necessary to estimate home range size in each stage. The number of locations was visually determined to be 10 …………...……………………………………...….… 31

ix

INTRODUCTION

Investigating factors that influence space use of avian species is necessary to understand how they utilize habitats, when those areas are important to their survival, and when populations may be at risk. Recent studies have demonstrated that breeding stages can influence space use of avian species. Roth et al. (2004) found that common ravens

(Corvus corax) remained closer to the nest during the incubation and nestling stages than in the pre-incubation period, and core areas were centered around nest sites and food sources. Home ranges and movements of golden eagles (Aquila chrysaetos) were larger during the non-breeding than breeding season (Marzluff et al. 1997). Willets (Tringa semiplamata) and killdeer (Charadrius vociferus) remained closer to their nesting areas while breeding than in the post-breeding stage (Haig et al. 2002, Plissner et al. 2000a).

Killdeer also moved greater distances during the non-breeding stages than during the nesting stage (Plissner et al. 2000a).

However, previous studies of breeding stage and space use in shorebirds (Plissner et al. 2000a, Haig et al. 2002) have not separated brood-rearing from the post-breeding stage, leading to the assumption that there is no difference in space use between these two stages. Since shorebird species care for precocial chicks after hatching (Lack 1968), space use during the brood-rearing period may be different and should be treated as a distinct stage. To our knowledge, no study has examined shorebird space use within four breeding stages (pre-incubation, incubation, brood-rearing, and post-breeding), likely due to the logistical difficulties of capturing non-breeding and obtaining visual

1 2 confirmation of each stage. Space use data in each breeding stage may be utilized by resource managers, as contaminant exposure and other population risks may vary with stage.

Wetland-dependent species, like the American avocet (Recurvirostra americana), are often considered priorities for conservation and management due to habitat degradation and loss (Frayer et al. 1983, Dahl 1990, Catallo 1993, Plissner et al. 2000a).

San Francisco Bay estuary, California is an important region for wintering and breeding avocets (Rintoul et al. 2003). However, avocet populations in this estuary may be at risk due to the presence of environmental contaminants, increases in California gull (Larus californicus) populations, and conversion of habitat. Understanding the space use requirements of American avocets may be beneficial for natural resources managers to minimize and mitigate these threats. Therefore, this study examined how space use varies across four stages of the annual cycle: pre-incubation, incubation, brood-rearing, and post-breeding.

STUDY AREA

San Francisco Bay, located in central California, is one of the largest estuaries on

the Pacific coast of North America (Conomos 1979, Monroe & Kelly 1992). The study was conducted in the South San Francisco Bay (Fig. 1), which is defined as the region

south of the San Mateo Bridge. South San Francisco Bay includes tidal flats and sloughs,

tidal marshes, salt evaporation ponds, and other wetlands (sewage and water treatment ponds, small seasonal wetlands). Avocets were captured in two salt ponds (Ponds A8 and

A16) and a tidal flat (Coyote Creek Lagoon; Fig. 1). American avocet populations in the

San Francisco Bay have been estimated at 23,200 in winter and approximately 4,300 in spring (Stenzel et al. 2002). South Bay spring avocet numbers have been estimated at

2800 (Rintoul et al. 2003).

3 4

Figure 1. Map of South San Francisco Bay, California, showing dominant habitat types and American avocet capture locations in 2005 and 2006. Avocets were captured in two salt ponds (Ponds A8 and A16) and a tidal flat (Coyote Creek Lagoon).

METHODS

Capture and Radio Telemetry

Between 15 March and 30 March 2005 and 16 February and 16 March 2006,

American Avocets were captured with rocket-nets (Dill and Thornsberry 1950, Hill and

Frederick 1997) and net launchers (Coda Enterprises, Mesa, AZ, USA) at two salt ponds

(A8 and A16) and a tidal lagoon (Coyote Creek Lagoon: Fig. 1; Humboldt State

University Institutional Care and Use Committee # 04/05.W.59.A). Female avocets were chosen for radio-marking as part of a larger study examining contaminant levels in San Francisco Bay (Ackerman et al. 2007a). The sex of avocets was determined from dimorphic bill characteristics (Hamilton 1975). Female avocets were fitted with radio-transmitters (model A2470, Advanced Telemetry Systems, Isanti, MN) and UV- resistant color bands (DarvicTM, A. C. Hughes LTD, Hampton Hill, United Kingdom).

Transmitters were attached to U. S. Geological Survey leg bands with epoxy, and placed on each ’s tibiotarsus as described by Plissner et al. (2000b) and Haig et al. (2002).

Total mass of the transmitter and bands was < 4 g, or approximately 2% of the bird’s mass (mean = 340 g).

Avocets were located with truck-mounted null-peak telemetry systems (AVM

Instrument Company, Livermore, CA). To locate birds, systematic vehicular routes were implemented throughout the study area, with equal effort to locate each bird to eliminate bias in tracking effort (Haig et al. 2002). Locations were obtained during morning (3 h after sunrise), day (3 h after sunrise to 3 h before sunset), evening (3 h before sunset), and

5 6 night (all hours after sunset and before sunrise) intervals, as avocets are active both day and night (Dodd and Colwell 1998, Johnson et al. 2003, Kostecke and Smith 2003).

Avocet locations were obtained with at least two azimuths from telemetry- mounted trucks with known Universal Transverse Mercator (UTM) coordinates.

Locations were estimated with LOAS 3.0.1 (Ecological Software Solutions, Urnash,

Switzerland), unless recorded directly on maps from visual re-sightings (28%, 471 of

1667 locations). Locations were recorded as UTM coordinates (datum = NAD 83). The azimuths were measured within 15 minutes of each other to minimize error due to movement of the bird. To minimize autocorrelation of spatial data, consecutive location determinations were at least 1 h apart and most (92%, 1535 of 1667) observations were separated by >3 h (Haig et al. 2002). Error polygons (an estimate of spatial error) were created by the LOAS program for each location using the best biangulation method and an azimuth error of two degrees. Since the maximum transmitter range was 1 km, location error from signal bounce was minimal. In a study with similar transmitters and truck systems in the San Francisco Bay, average telemetry location error was reported at

58 m, with error polygons of 1.1 ha (Warnock and Takekawa 1995). Locations with large error polygons (>3 ha) were excluded from analyses.

For each radio-marked avocet, daily visual observations were used to categorize individuals into one of four breeding stages. If necessary, egg floatation (Alberico 1995) was used to back-calculate nest-initiation dates. The pre-incubation stage was from the time of capture in late winter through the day prior to incubation. Since avocets begin incubation with the laying of the third egg (Robinson et al. 1997), the incubation period

7 commenced with the laying of the penultimate egg and ended when either the eggs hatched or failed. The brood-rearing stage began when eggs hatched and ended when either chicks fledged or disappeared. The post-breeding stage started after the last day of brood-rearing, or after the nest failed or was depredated.

Behaviors and flock size estimates were collected opportunistically on radio- marked avocets throughout the study period, when individuals could be located visually.

No attempt was made to collect behavioral data at night. Behavioral categories, as described by Hamilton (1975), were: feeding (actively searching for food), resting (not moving), incubating (incubating eggs on nest), brooding (crouching over chicks), alert

(vigilant, not at rest), agonistic (inter- and intraspecific aggressive interactions), preening

(arranging feathers with the bill), nest-building (scraping or adding nest material to nest cup), and copulating. Flocks were defined as a group of two or more avocets.

Data Summary and Analysis

Hawth’s Tools for ArcGIS (Beyer 2004) in a geographic information system

(ArcGIS 9.1, ESRI Inc., Redlands, CA) was used to calculate the linear distance between

consecutive locations (hereafter: linear movements) for radio-marked avocets. Because

observations often were obtained more than once a day, the potential bias of multiple

daily observations on movements was examined by regressing distance data on the time

between consecutive observations (Appendix A). Slopes were not significantly different

than 0 during any breeding stage or for the entire study period. To further test for

temporal autocorrelation, locations less than 1, 2, 4, 8, and 16 hours apart were

consecutively removed from analyses (Appendix B). These removals did not

8 significantly alter average distance calculations; therefore all locations > 1 hour apart were used for analyses.

Home ranges and core areas were estimated using Home Range Tools for ArcGIS in ArcGIS 9.1 (Rodgers et al. 2005; ESRI Inc., Redlands, California). Home ranges and core areas were defined as the area encompassing the 95% and 50% utilization distribution, respectively. The fixed-kernel method, with likelihood cross-validation

(CVh) for smoothing parameter was used to estimate home range and core area sizes

(Garton and Horne 2007). Likelihood cross-validation tends to outperform least squares cross-validation (LSCV), producing estimates with better fit and less variability, and is especially useful with sample sizes < 50 (Horne and Garton 2006). Likelihood cross- validation parameters were calculated with Animal Space Use 1.1 (Horne and Garton

2007).

Because variation in observed locations may affect home range estimates

(Seaman et al. 1999), the minimum number of locations required to create fixed kernel home ranges was determined for each breeding stage. A sample of four avocets that had complete data in each breeding stage was used for this analysis. Random locations were consecutively removed and home ranges were calculated for each sub-sample. The minimum number of locations required for home range estimation was determined by visually inspecting the data for the sample size where home range size leveled off

(Appendix C).

Hawth’s Tools for ArcGIS (Beyer 2004) was used to calculate linear distances of avocets from nest locations (hereafter: average distances from nest). To examine the

9 extent to which avocets prospected for nest sites, nest locations were overlaid with pre- incubation core areas and average distances from future nests were calculated and regressed against the number of days prior to incubation. To calculate average distances, each bird contributed one distance for each day to average distance calculations. If multiple observations were obtained on a bird for a day, an average distance from nest was calculated.

Single-factor Generalized Linear Models (GLM) Analysis of Variance was used to compare linear movements, home range, core area and average distances from nests across breeding stages for radio-marked avocets. Tukey-Kramer multiple-comparison tests were used to test for differences between breeding stages. Linear movement, home range, and core area data were log10 transformed to meet the assumptions of normality.

To control for effects of individual variation in avocet space use, additional home range

and linear movement analyses were conducted on standardized scores (Afifi et al. 2004)

for the four avocets with data during each breeding stage. Linear regression was used to

test for relationships between distances from nests and number of days prior to

incubation. Individual avocets were the unit of analysis and sample sizes varied by stage

due to a high rate of radio failure in 2006. Data are presented as means ± SE.

RESULTS

Six resident breeding female avocets were radio-marked in 2005 and 9 were marked in 2006. A total of 1667 locations were collected by observers. The mean number of locations obtained for an individual’s breeding stage was 34.0 ± 2.4. The minimum number of locations required for home range estimation in each breeding stage is 10 (Appendix C). Approximately 48% of observations were obtained during the day,

19% in the morning, 19% in the evening, and 14% at night.

Linear Movements and Home Range

Avocet linear movements varied (F3,40 = 8.84, P < 0.001) among breeding stages

(Fig. 2). Movements were greater during post-breeding than incubation and brood- rearing stages (P < 0.001, P = 0.03). Pre-incubation distances were greater than incubation and brood-rearing distances (both P < 0.001). Post-breeding movements were

greater than pre-incubation (P = 0.04). Incubation and brood-rearing distances did not

differ (P = 0.53). These patterns were supported by analysis of standardized linear

movements (F3,12 = 34.02, P < 0.001) for four focal avocets (Fig. 3). Post-breeding

scores were significantly different from pre-incubation, incubation, and brood-rearing

stages (all P < 0.001); pre-incubation, incubation, and brood-rearing scores were not

significantly different (all P > 0.05).

Avocet home ranges (F3,40 = 20.24, P < 0.001) and core areas (F3,40 = 21.41, P <

0.001) differed among breeding stages (Fig. 4). Post-breeding home ranges and core

10 11

2000 11 1800 1600 1400 1200 15 1000 800 14

Distance (m) Distance 600 4 400 200 0 Pre-incubation Incubation Brood-rearing Post-breeding

Figure 2. Mean (± SE) linear distance between consecutive locations of American avocets in South San Francisco Bay, California, USA, 2005-06. Sample sizes (number of birds) are shown above histograms.

12

Home Range 2 Linear Movement

1.5

1

0.5

0

Standardized Score -0.5

-1 Pre-incubation Incubation Brood-rearing Post-breeding

Figure 3. Mean (± SE) home range size and linear movements of four American avocets sampled in each breeding stage in South San Francisco Bay, California, USA, 2005-06. Standardized scores were calculated based on individual mean home range and linear movement values across the four stages.

13

4000 Home Range (95%) 3500 ) 3000 Core Area (50%) 11 2500 2000 1500 1000 15 14 4 Home Range Size (ha Size Home Range 500 0 Pre-incubation Incubation Brood-rearing Post-breeding

Figure 4. Mean (± SE) home range and core area size of American avocets in South San Francisco Bay, California, USA, 2005-06. Sample sizes (number of birds) are shown above histograms.

14 areas were significantly greater than pre-incubation, incubation, and brood-rearing stages

(all P < 0.001). Pre-incubation home ranges and core areas were greater than incubation and brood-rearing stages (all P < 0.001). Incubation home ranges and core areas did not differ from brood-rearing home ranges (P = 0.61) and core areas (P = 0.96).

Standardized home ranges differed significantly among stages (F3,12 = 16.97, P < 0.001)

for the four focal avocets (Fig. 3). The post-breeding scores were significantly different

from pre-incubation (P = 0.01), incubation (P < 0.001), and brood-rearing stages (P <

0.001); pre-incubation, incubation, and brood-rearing scores were not significantly different (all P > 0.05).

The average distances from nests (F3, 40 = 38.78, P < 0.001) varied among stages

(Fig. 5). Post-breeding distances were significantly greater than during pre-incubation (P

= 0.001), as well as incubation and brood-rearing (both P < 0.001). Pre-incubation

distances were greater than incubation (P < 0.001) and brood-rearing stages (P = 0.003).

Distances from nest did not differ during the incubation and brood-rearing stages (P =

0.94).

Fourteen of the 15 avocet nest locations (93%) were within the pre-incubation core area boundaries, whereas 4 of 11 (36%) nests were within post-breeding core areas

(P = 0.003, Fisher’s exact test). The distance between future nest sites and daily location

2 decreased (F1, 39 = 85.12, r = 0.69, P < 0.001) with the number of days prior to incubation

2 (Fig. 6). Post-breeding distances from nest locations increased (F1, 29 = 5.15, r = 0.16, P

< 0.03) with number of days after breeding.

15

6000 11 5000

4000

3000 15 2000 Distance (m) Distance 14 1000 4

0 Pre-incubation Incubation Brood-rearing Post-breeding

Figure 5. Mean (± SE) distances from nest location, in each breeding stage, of American avocets in South San Francisco Bay, California, USA, 2005-06. Sample sizes (number of birds) are shown above histograms.

16

6000

5000

4000

3000 Y = 60.09 + 74.42X

2000 R2 = 0.69 1000 Distance From Nest (m) Nest From Distance 0 40 30 20 10 Days Prior To Incubation

Figure 6. Relationship between average (± SE) distances from nest and days prior to incubations during the pre-incubation stage for American avocets in South San Francisco Bay, California, USA, 2005-06.

17 Behavior and Flock Size

Behaviors and flock size estimates were collected from 24% of telemetry locations (Table 1). Pre-incubation observations included resting, feeding, nest-building, copulating and agonistic behaviors. Incubating, feeding, resting, agonistic, alert, preening, and nest building were observed during the incubation stage. During the brood-rearing stage, brooding, feeding, resting, and agonistic behaviors were observed.

Post-breeding avocets were only observed feeding and roosting.

Flock size was 86 ± 25 birds (maximum 1500) during the pre-incubation stage and 6 ± 1 birds (maximum 100) in the incubation stage. Flock size was 10 ± 4 birds

(maximum 100) during the brood-rearing stage and 59 ± 10 birds (maximum 1000) in the post-breeding stage.

Table 1. Recorded behaviors of radio-marked American avocets in South San Francisco Bay, California, USA. Behaviors were recorded opportunistically when individuals could be visually located.

Behaviors (%) of Radio-marked Avocets Life-history Stage n Feeding Resting Alert Incubation Brooding Agonistic Preening Nest-building Copulating Pre-breeding 139 42 47 0 0 0 1 0 7 3 Incubation 143 35 16 2 41 0 3 2 1 0 Brood-rearing 47 18 4 0 0 74 4 0 0 0 Post-breeding 164 67 33 0 0 0 0 0 0 0

18

DISCUSSION

Space use of female American avocets in South San Francisco Bay varied significantly among breeding stages. Linear movements, home ranges, core areas, and distances from nest were greater in the pre-incubation and post-breeding stages than during the incubation and brood-rearing stages. Post-breeding space use measurements were significantly larger than in the pre-incubation stage. Several other studies of avian species have shown seasonal reduction in space use during nesting stages (Grahn 1990,

Marzluff et al. 1997, Plissner et al. 2000a, Roth et al. 2004), but none has demonstrated significant differences between pre-incubation and post-breeding stages.

Few studies have analyzed the influence breeding stage has on the space use of wetland-dependent shorebird species (Plissner et al. 2000a, Haig et al. 2002), and no other study has separated brood-rearing from the post-breeding stage. Since avocets exhibit bi-parental care of precocial chicks after hatching (Hannam et al. 2003), the brood-rearing period should be treated as a distinct stage from post-breeding. This was demonstrated by significant differences in linear movements (Fig. 2) and home range sizes (Fig. 4) between the two stages. However, assessing space use of brood-rearing avocets was difficult since avocet chicks often did not survive long and obtaining visual confirmation of breeding stages was challenging. In this sample of 15 radio-marked avocets, only two females raised chicks to fledging and two more raised chicks long enough to assess space use. More information from the brood-rearing stage is needed to understand avocet space use requirements during this important breeding stage.

19 20 The differences in home ranges, core areas, and movements between the pre- incubation and post-incubation stages suggest space use of female avocets may be influenced by breeding activities. The pre-incubation home range and core area sizes were substantially smaller than in post-breeding (Fig. 4). There was more variation in distances from nest earlier in the pre-incubation stage and more variation in home range, linear movements, and distances from nest in the post-breeding stage than during pre- incubation. This suggests that individual avocets vary more in their space use when not constrained by breeding activities.

Pre-incubation core areas contained the future nest sites for all but one of the radio-marked avocets, demonstrating female avocets spend a substantial period of time at their future colony prior to nesting. The average distance from the nest during the pre- incubation stage (Fig. 6) suggests that avocets may be prospecting future nest sites, or engaging in pair-bonding behaviors, up to three weeks prior to nesting. It is likely that avocets repeatedly visit colony sites in the late winter and spend more time at those sites as water levels drop and temperatures increase. Similarly, research on other avocet populations indicates that nest prospecting occurs sometime prior to nesting, not in the post-breeding period (Robinson and Oring 1997, Plisnner et al. 1999).

Since only female avocets were radio-marked for this study, sex differences in space use of American avocets have yet to be determined. There is evidence that behavioral differences exist between avocet sexes, which may influence space use patterns during the annual cycle. Male avocets have been observed spending more time foraging while swimming than females (Dinsmore 1977); and they tend to use plunging

21 foraging methods (head and upper breast enter the water to capture food) more often than females (Hamilton 1975). These observations suggest that males will forage in slightly deeper water than females, which could contribute to sex differences in space use.

Sex differences may play a role in nest prospecting and site selection. Pairing occurs when a female persistently associates with a male until she is accepted as a mate

(Hamilton 1975). It is possible that females are following males during the pre- incubation period and males are playing a larger role in nest prospecting. Nest site selection in Great Basin breeding avocets is likely determined by social factors associated with breeding colonies and the individual experience of males (Robinson and Oring

1997). If this pattern persists for avocets breeding in San Francisco Bay, then space use data on male avocets would be beneficial in understanding breeding site selection and nest prospecting.

Conservation Implications

The San Francisco Bay has a legacy of mercury contamination from historic gold

and mercury mining (Wiener et al. 2003), which is thought to contribute to low

reproductive success of birds (Schwarzbach et al. 2005, Schwarzbach et al. 2006). In the

South San Francisco Bay, mercury concentrations were 61% higher in breeding female

avocets than in pre-breeding birds (Ackerman et al. 2007a), indicating female avocets

were accumulating contaminants during the nesting period. Mercury concentrations were

highest among avocets using pond A8 in the Alviso salt pond complex (Ackerman et al.

2007b), where eight (53%) of radio-marked avocets nested. Small incubation stage home

22 ranges, with nest-centered core areas, may result in the rapid accumulation of mercury when colony sites are highly contaminated.

The rapidly increasing California gull population in the South San Francisco Bay may present another threat to San Francisco Bay avocet populations. The California gull population has increased from under 1000 birds in 1982 to over 30,000 in 2006 (Strong et al. 2004, Ackerman et al. 2006). Ackerman et al. (2006) reported 61% of radio-marked avocet chicks and 15% of camera-monitored avocet nests were depredated by California gulls. Also, local landfill use by gulls decreased by 73% between April and August 2006

(Ackerman 2006), suggesting gulls reduced landfill foraging and switched to predation of nests and chicks of avocets and other breeding birds. The largest California gull colony was located in the Alviso salt pond complex (Ackerman et al. 2006), where 80% of radio- marked female avocets bred. To mitigate for California gull population increases, managers may be able to manipulate water levels early in the nest prospecting period to attract avocets to more suitable colony sites and discourage nesting near the Alviso gull colony.

As part of tidal marsh restoration efforts, federal and state agencies are planning to convert many salt ponds into tidal marsh habitat (Steere and Schaefer 2001, Siegel and

Bachand 2002, Life Science 2003). Salt ponds are known to be valuable habitat for foraging, roosting, and nesting waterbirds (Rufino et al. 1984, Masero and Perez-Hurtado

2001, Luis et al. 2002, Warnock et al. 2002). A decrease in salt pond habitat may adversely affect breeding American avocet populations in the San Francisco Bay.

Rintoul et al. (2003) reported that 93% of avocets nested in salt pond habitat. Likewise,

23 the majority (73%) of radio-marked avocets in this study nested in salt ponds, whereas no avocets nested in tidal marsh or other tidally influenced habitats. Conversion of salt ponds to tidal marsh will decrease availability of breeding habitat for American avocets in the South San Francisco Bay, with possible population consequences.

In a human-altered estuary like the San Francisco Bay, natural resources managers can utilize spatial information to implement conservation strategies that maximize the potential for continued survival of American avocets and other breeding waterbirds. Radio-telemetry data can be used to determine where and when avocets are exposed to mercury and other contaminants (Ackerman et al. 2007b). Furthermore, space-use data within distinct breeding stages can be used to make predictions about levels of contaminant exposure throughout the annual cycle. Most importantly, pre- incubation stage core areas are beneficial for approximating future avocet nest colony locations. These data can be used by wildlife managers to implement management strategies that will discourage birds from nesting in highly contaminated areas or wetlands near California gull colonies.

LITERATURE CITED

Ackerman, J. T., J. Y. Takekawa, C. Strong, N. Athearn, and A. Rex. 2006. California gull distribution, abundance, and predation on waterbird eggs and chicks in South San Francisco Bay. Final Report, U. S. Geological Survey, Western Ecological Research Center, Davis and Vallejo, California 61 pp.

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Appendix A. The distance and time between consecutive locations, in each breeding stage, of American avocets in San Francisco Bay, California, USA. The fitted lines indicate there is no correlation in the time between locations and the distances moved (Pre-incubation, r2 = 0.01, P = 0.02; Incubation, r2 = 0.00, P = 0.892; Brood-rearing, r2 = 0.01, P = 0.420, Post-breeding, r2 = 0.0150, P = 0.031). All locations >1 hour apart were used for analyses.

16000 1800 Brood-rearing 14000 Pre-incubation 1600 12000 1400 1200 10000 1000 8000 800 6000 600 2 Distance (m) 2 Distance (m) R = 0.0087 4000 R = 0.0128 400 2000 200 0 0 0 1000 2000 3000 4000 5000 0 500 1000 1500 2000 2500 3000 Time (minutes) Time (minutes)

6000 25000 Incubation Post-breeding 5000 20000 4000 15000 3000 10000 2000 Distance (m) Distance (m) R2 = 0.0026 1000 5000 R2 = 0.0203

0 0 0 500 1000 1500 2000 2500 3000 0 1000 2000 3000 4000 5000 Time (minutes) Time (minutes)

29

30 Appendix B. Average distances between consecutive locations of American avocets in each breeding stage, San Francisco Bay, California, USA. To test for temporal autocorrelation in linear movement data, locations less than 1, 2, 4, 8, and 16 hours apart were consecutively removed from analysis. These removals did not significantly alter average distance calculations; therefore all locations > 1 hour apart were used for analyses.

2500

< 1 Hour 2000 < 2 Hours < 4 Hours 1500 < 8 Hours < 16 Hours 1000 Distance (m)

500

0 Pre-incubation Incubation Brood-rearing Post-breeding

31 Appendix C. Average home range sizes of four American avocets in South San Francisco Bay, California, USA. Locations were randomly selected and removed to test for the minimum number of locations necessary to estimate home range size in each stage. The number of locations was visually determined to be 10.

7000

6000

5000

4000

3000 Hectares 2000

1000

0 5 6 7 8 9 101112131415161718192021222324252627 Pre-incubation Locations

2500

2000

1500

Hectares 1000

500

0 56789101112131415161718192021222324 Incubation Locations

500 450 400 350 300 250

Hectares 200 150 100 50 0 5 6 7 8 9 10111213141516 Brood-rearing Locations

70000

60000

50000

40000

30000 Hectares 20000

10000

0 5 6 7 8 9 10111213141516171819202122 Post-breeding Locations