U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2018–1136

Prepared in cooperation with the U.S. Army Corps of Engineers and the Bureau of Reclamation

Social Attraction Used to Establish Caspian Tern 

(Hydroprogne caspia) Nesting Colonies on Modified Islands 

at the Don Edwards San Francisco Bay National Wildlife 

Refuge, California—Final Report
Cover: Photograph showing Caspian terns (Hydroprogne caspia) on Island 11 

in Pond A16, Don Edwards San Francisco Bay National Wildlife Refuge, 

California. Photograph by Crystal Shore, U.S. Geological Survey, May 23, 2016.
Social Attraction Used to Establish Caspian Tern 

(Hydroprogne caspia) Nesting Colonies on Modified 

Islands at the Don Edwards San Francisco Bay 

National Wildlife Refuge, California—Final Report

By C. Alex Hartman, Joshua T. Ackerman, Mark P. Herzog, Cheryl Strong, David Trachtenbarg, and 

Crystal A. Shore

Prepared in cooperation with the U.S. Army Corps of Engineers and the

Bureau of Reclamation 

Open-File Report 2018-1136

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

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Suggested citation:

Hartman, C.A., Ackerman, J.T., Herzog, M.P., Strong, C., Trachtenbarg, D., and Shore, C.A., 2018, Social 

attraction used to establish Caspian tern (Hydroprogne caspia) nesting colonies on modified islands at the Don 

Edwards San Francisco Bay National Wildlife Refuge, California—Final report: U.S. Geological Survey Open-File

Report 2018-1136, 41 p., https://doi.org/10.3133/ofr20181136. 

ISSN 2331-1258 (online)
iii

Contents 

Executive Summary....................................................................................................................................................1

Introduction................................................................................................................................................................. 2

Methods......................................................................................................................................................................4

Social Attraction Measures for Caspian Terns and Snowy Plovers........................................................................ 4

Gull Dissuasion Efforts ........................................................................................................................................... 4

Evaluation of Nesting by Caspian Terns and Snowy Plovers ................................................................................. 5

Estimating Colony Size and Productivity of Caspian Terns.................................................................................... 5

Estimating Apparent Nest Density of Caspian Terns.............................................................................................. 7

Results and Discussion ..............................................................................................................................................7

Abundance of Caspian Terns and Other Prominent Nesting Species in Ponds A16 and SF2 ............................... 7

Breeding Chronology of Caspian Terns ................................................................................................................. 8

Size, Productivity, and Growth of Caspian Tern Breeding Colonies....................................................................... 8

Island Area and Nest Density of Caspian Terns................................................................................................... 10

Sightings of Color-Banded Caspian Terns ........................................................................................................... 11

Nesting of Snowy Plovers and Other Waterbirds ................................................................................................. 12

Gull Dissuasion .................................................................................................................................................... 13

Factors Influencing Overall Success of Caspian Tern Colonies ........................................................................... 13

Conclusions and Management Implications..............................................................................................................14

Acknowledgments.....................................................................................................................................................15

References Cited......................................................................................................................................................15

Figures

Figure 1. Locations of Ponds A16 and SF2 containing islands modified for Caspian terns and snowy plovers,

Don Edwards San Francisco Bay National Wildlife Refuge, California .................................................................... 17

Figure 2. Locations of islands modified for nesting Caspian terns and nesting snowy plovers in Pond A16,

Don Edwards San Francisco Bay National Wildlife Refuge, California .................................................................... 18

Figure 3. Locations of islands modified for nesting Caspian terns and nesting snowy plovers in Pond SF2,

Don Edwards San Francisco Bay National Wildlife Refuge, California .................................................................... 19

Figure 4. Arrangement of social attraction measures for Caspian terns on modified Island 11 and Islands 11

and 12 in Pond A16, Don Edwards San Francisco Bay National Wildlife Refuge, California................................... 20

Figure 5. Arrangement of social attraction measures for Caspian terns on modified Island 12 and Island 17

in Pond SF2, Don Edwards San Francisco Bay National Wildlife Refuge, California............................................... 21

Figure 6. Arrangement of social attraction measures for Caspian terns on modified Island 21 in Pond SF2,

Don Edwards San Francisco Bay National Wildlife Refuge, California .................................................................... 22

Figure 7. Arrangement of social attraction measures for snowy plovers on modified Island 10 in Pond SF2,

Don Edwards San Francisco Bay National Wildlife Refuge, California .................................................................... 23

Figure 8. Number of Caspian tern adults, incubating adults, chicks, fledglings, and active nests by date at

Pond A16, Island 11; and Pond SF2, Island 21, Don Edwards San Francisco Bay National Wildlife Refuge, 

California, March–September 2015–17 ................................................................................................................... 24

Figure 9. Number of Caspian tern nests and fledglings by date on Island 11 in Pond A16 and Island 21 in Pond 

SF2 in 2015, 2016, and 2017, Don Edwards San Francisco Bay National Wildlife Refuge, California .................... 25
iv

Figure 10. Weekly high counts of individuals of the most abundant nesting waterbird species (excluding

California gulls) observed during pond surveys at Ponds A16 and SF2, Don Edwards San Francisco

Bay National Wildlife Refuge, California, March–September 2015–17 .................................................................... 26

Figure 11. Counts of Caspian terns, American avocets, black-necked stilts, California gulls, Forster’s terns,

and western snowy plovers at Pond A16, Don Edwards San Francisco Bay National Wildlife Refuge,

California, during May–June of 2015–17 ................................................................................................................. 27

Figure 12. Counts of Caspian terns, American avocets, black-necked stilts, California gulls, Forster’s terns,

and western snowy plovers at Pond SF2, Don Edwards San Francisco Bay National Wildlife Refuge,

California, during May–June of 2015–17 ................................................................................................................. 28

Figure 13. Percentage of Caspian tern observations by location during 166 behavior surveys conducted

at Ponds A16 and SF2, Don Edwards San Francisco Bay National Wildlife Refuge, California,

March–September 2015–17 .................................................................................................................................... 29

Figure 14. Breeding chronology of Caspian terns based on observed behaviors at Ponds A16 and SF2,

Don Edwards San Francisco Bay National Wildlife Refuge, California, in 2015, 2016, and 2017............................ 30

Figure 15. Aerial photograph showing Island 11 modified for Caspian terns in Pond A16, Don Edwards San 

Francisco Bay National Wildlife Refuge, California.................................................................................................. 31

Figure 16. Aerial photograph showing Island 21 modified for Caspian terns in Pond SF2, Don Edwards San 

Francisco Bay National Wildlife Refuge, California.................................................................................................. 32

Figure 17. Total number of Caspian tern breeding pairs and fledglings, and the apparent breeding success and 

apparent nest density at Ponds A16 and SF2, and overall, Don Edwards San Francisco Bay National Wildlife 

Refuge, California, 2015–17 .................................................................................................................................... 33

Tables

Table 1. Summary of social attraction efforts at three islands in Pond A16 and four islands in Pond SF2,

Don Edwards San Francisco Bay National Wildlife Refuge, California, 2015–17 .................................................... 34

Table 2. Total number of Caspian terns observed and the number of surveys in which Caspian terns were 

observed on each island during 166 pond surveys conducted at Ponds A16 and SF2, Don Edwards San

Francisco Bay National Wildlife Refuge, California, March–September 2015–17.................................................... 35

Table 3. Breeding metrics on seven modified islands with social attraction measures for Caspian terns and

snowy plovers in Ponds A16 and SF2, Don Edwards San Francisco Bay National Wildlife Refuge,

California, 2015–17.................................................................................................................................................. 36

Table 4. Sightings of 52 color-banded Caspian terns at Ponds A16 and SF2, Don Edwards San Francisco

Bay National Wildlife Refuge, California, 2015–17. ................................................................................................. 38
v 

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

International System of Units to U.S. customary units 

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd) 

Area

square meter (m2) 0.0002471 acre 

square meter (m2) 10.76 square foot (ft2) 

Abbreviations 

BiOp Biological Opinion

DENWR Don Edwards San Francisco Bay National Wildlife Refuge

RPAs Reasonable and Prudent Alternatives

RTK Real-Time Kinematics

SBSP South Bay Salt Pond

USACE U.S. Army Corps of Engineers

USDA-APHIS U.S. Department of Agriculture Animal and Plant Health Inspection Service

USGS U.S. Geological Survey
vi 

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1 

Social Attraction Used to Establish Caspian Tern 

(Hydroprogne caspia) Nesting Colonies on Modified 

Islands at the Don Edwards San Francisco Bay National 

Wildlife Refuge, California—Final Report

By C. Alex Hartman1, Joshua T. Ackerman1, Mark P. Herzog1, Cheryl Strong2, David Trachtenbarg3, and 

Crystal A. Shore1

Executive Summary

To address the 2008/2010 and Supplemental 2014 National Oceanic and Atmospheric 

Administration Fisheries Biological Opinion for operation of the Federal Columbia River Power 

System, the U.S. Army Corps of Engineers (USACE) and the Bureau of Reclamation (Reclamation) 

developed and began implementation of Caspian tern (Hydroprogne caspia) management plans. This 

implementation includes redistribution of the Caspian terns in the Columbia River estuary and the midColumbia River region to reduce predation on salmonids listed under the Endangered Species Act. Key 

elements of the plans are (1) reduction of nesting habitat for Caspian terns in the Columbia River estuary 

and the mid-Columbia River region, and (2) creation or modification of nesting habitat at alternative 

sites within the Caspian tern breeding range. As part of this effort, USACE and Reclamation developed 

Caspian tern nesting habitat at the U.S. Fish and Wildlife Service Don Edwards San Francisco Bay 

National Wildlife Refuge (DENWR), California, prior to the 2015 nesting season. Furthermore, nesting 

habitat for western snowy plovers (Charadrius alexandrinus nivosus) also was developed to provide 

separate nesting opportunities in the same managed ponds to reduce potential conflicts with Caspian 

terns. Specifically, seven recently constructed islands within two managed ponds (Ponds A16 and SF2) 

of DENWR were modified to provide habitat attractive to nesting Caspian terns (5 islands) and snowy 

plovers (2 islands). These 7 islands were a subset of 46 islands recently constructed in Ponds A16 and 

SF2 to provide waterbird nesting habitat as part of the South Bay Salt Pond (SBSP) Restoration Project. 

 

1

U.S. Geological Survey. 

2

U.S. Fish and Wildlife Service. 

3

U.S. Army Corps of Engineers. 
2 

We used social attraction methods (decoys and electronic call systems) to attract Caspian terns 

and snowy plovers to these seven modified islands, and conducted surveys from March to September of 

2015, 2016, and 2017 to evaluate nest numbers, nest density, and productivity. Results from the 2015 

nesting season, the first year of the study, indicated that island modifications and social attraction 

measures were successful in establishing Caspian tern breeding colonies at Ponds A16 and SF2 of 

DENWR. Prior to 2015, there was no history of Caspian terns nesting in either Pond A16 or Pond SF2. 

The success of 2015 continued in 2016 and 2017. In 2017, the third and final year of the project, 

Caspian terns initiated at least 664 nests, fledged at least 239 chicks, and had a breeding success rate of 

0.36 fledged chicks per breeding pair. This represents a 171 percent increase in the number of breeding 

pairs and a 41 percent increase in the number of chicks fledged, but a 48 percent decrease in the 

fledglings produced per breeding pair in 2017 compared to 2015, the first year the colonies were

established. The two new large and growing Caspian tern nesting colonies at Ponds A16 and SF2 

demonstrate the effectiveness of social attraction measures in helping to establish tern nesting colonies 

in San Francisco Bay. Social attraction measures similar to those used in this study, but targeting other 

colonial species such as Forster’s terns (Sterna forsteri) and American avocets (Recurvirostra 

americana), may help to establish waterbird breeding colonies at wetlands enhanced as part of the SBSP

Restoration Project.

Introduction

The U.S. Army Corps of Engineers (USACE), Walla Walla and Portland Districts, and the 

Bureau of Reclamation (Reclamation) (Action Agencies) are in the process of addressing the 2008/2010 

and 2014 Supplemental National Oceanic and Atmospheric Administration Fisheries Biological Opinion 

(BiOp) for operation of the Federal Columbia River Power System that includes Reasonable and Prudent 

Alternatives (RPAs) 45, 47, 66, and 68. As part of implementing these RPAs, the Action Agencies have 

developed, and are in the process of implementing, pertinent parts of Caspian tern (Hydroprogne caspia) 

management plans (U.S. Fish and Wildlife Service, 2005; U.S. Army Corps of Engineers, 2015). This 

implementation includes redistributing nesting Caspian terns out of the Columbia River estuary and the 

mid-Columbia River region to reduce predation on salmonids listed under the Endangered Species Act. 

Previous studies have indicated that salmonids make up a small proportion of the diet of Caspian terns 

nesting in South San Francisco Bay, and that most smolts consumed by Caspian terns were hatcheryraised and non-listed (Evans and others, 2011; Collis and others, 2012), suggesting that development of 

Caspian tern nesting colonies in South San Francisco Bay may not affect endangered salmonids. 

Therefore, the Action Agencies developed Caspian tern and western snowy plover (Charadrius 

alexandrinus nivosus) habitat at the U.S. Fish and Wildlife Service Don Edwards San Francisco Bay 

National Wildlife Refuge (DENWR), California, prior to the 2015 nesting season. 

To provide waterbird nesting habitat in South San Francisco Bay, the South Bay Salt Pond 

(SBSP) Restoration Project constructed 30 islands in Pond SF2 in 2010, and added 16 islands to 4 

existing islands in Pond A16 of the DENWR in 2012 (fig. 1). However, since construction, nesting 

waterbirds have made limited use of these new islands. In 2015, seven of these islands were selected by 

the U.S. Fish and Wildlife Service, in conjunction with the USACE and Reclamation, and modified to 

provide habitat attractive to nesting Caspian terns and western snowy plovers. In Pond A16, two islands 

(Islands 11 and 12) were modified for nesting Caspian terns, and one island (Island 3) was modified for 

nesting snowy plovers (fig. 2). In Pond SF2, three islands (Islands 12, 17, and 21) were modified for 

nesting Caspian terns, and one island (Island 10) was modified for nesting snowy plovers (fig. 3). 

Islands modified for nesting snowy plovers were selected based on locations of snowy plover 

observations during previous breeding seasons, and islands modified for nesting Caspian terns were 

selected so that they were centrally located within the ponds but away from snowy plover islands. 
3 

Islands modified for Caspian terns were groomed to a slope of 4 to 1 or less, were covered in 3/8-in. 

crushed rock 18 in. deep, and 3-in. rock was placed around the island perimeter. Islands modified for 

snowy plovers were covered with 1/4-in. crushed rock (Island 10 at Pond SF2) or 3/4-in. crushed rock 

(Island 3 at Pond A16) 5 in. deep, and a 6-ft buffer free of gravel was left around the edge of each island 

for plover foraging habitat. Island modifications were completed in February 2015. 

To improve the likelihood that Caspian terns and snowy plovers would use these newly modified 

islands, social attraction measures (decoys and playback of adult vocalizations using call systems) were 

deployed beginning in 2015. Social attraction can be an effective wildlife-management technique 

whereby adult birds are attracted to potential nesting sites with decoys and bird calls to assist in the 

establishment of breeding colonies (Arnold and others, 2011; Jones and Kress, 2012). Colonial 

waterbirds, such as terns, are excellent candidate species for social attraction efforts because they are 

readily attracted to decoys of adult birds as well as sound recordings of adult vocalizations (Kress, 1983; 

Roby and others, 2002). Furthermore, the presence of conspecifics may influence selection of nest sites 

by snowy plovers (Patrick and Colwell, 2014), and the use of decoys and vocalizations may encourage 

nesting (California State Parks and Redwood National Park, 2014). 

In coordination with USACE, Reclamation, and DENWR, the U.S. Geological Survey (USGS) 

implemented social attraction measures and monitoring activities during the first three nesting seasons 

following completion of island modifications in early 2015. Social attraction measures were intended to 

attract Caspian terns and snowy plovers to these recently modified islands during the 2015, 2016, and 

2017 breeding seasons. Moreover, USGS conducted surveys twice weekly throughout the breeding 

season in all 3 years to monitor Caspian tern and snowy plover response to social attraction efforts and 

to evaluate project success.

The objectives of this project were to:

1. Deploy and maintain social attraction measures (decoys and call systems) for Caspian terns 

and snowy plovers on seven islands within Ponds A16 and SF2; 

2. Monitor and evaluate nesting by Caspian terns and snowy plovers on the seven modified 

islands as well as surrounding islands of Ponds A16 and SF2; 

3. Evaluate factors limiting breeding success of Caspian terns and snowy plovers at Ponds A16 

and SF2; 

4. Implement gull (Larus spp.) dissuasion efforts, as needed, in coordination with DENWR and 

U.S. Department of Agriculture Animal and Plant Health Inspection Service (USDAAPHIS)-Wildlife Services to limit potential negative effects of gulls on nesting Caspian terns 

and snowy plovers; and 

5. Provide information, based on field observations, to DENWR, USACE, and USDA-APHIS 

on additional predator management options for limiting potential negative effects of other 

terrestrial and avian species on nesting Caspian terns and snowy plovers. 
4 

Methods

Social Attraction Measures for Caspian Terns and Snowy Plovers

In late February to early March of 2015, 2016, and 2017, we deployed social attraction measures 

(decoys and call systems) on seven islands (three islands in Pond A16, four islands in Pond SF2). Five 

islands (Islands 11 and 12 in Pond A16; Islands 12, 17, and 21 in Pond SF2) each received 50–115

Caspian tern decoys arranged in the interior of each island, and spaced 1–1.5 m apart (table 1, figs. 4–6), 

which is the nest spacing commonly observed in Caspian tern colonies (Cuthbert and Wires, 1999). The 

number of decoys increased in later years as the manufacturer (Mad River Decoy, Waitsfield, Vermont) 

completed and delivered new decoys, and as more old decoys became available from partner agencies. 

The remaining two islands (Island 3 in Pond A16; Island 10 in Pond SF2) each received six snowy 

plover decoys in all 3 years of the study, arranged in male-female pairs at three locations on the island 

(fig. 7). We installed a call system (Murremaid Music Boxes, South Bristol, Maine) on each of the seven

modified islands and broadcast either Caspian tern colony calls or snowy plover calls continuously 

through two omni-directional outdoor speakers. Each call system was powered by two 6-volt Optima®

AGM batteries and charged by a 135-watt Kyocera© solar panel, enabling it to broadcast continuously 

without intervention. Call box and solar panels were deployed on the south side of each island, about 20 

m from the decoy arrangement. Speakers were deployed on opposite ends of the decoys and connected 

to the call box by speaker wire that was buried under the gravel. We used a 20-minute recording of a 

winter flock of snowy plovers obtained from Little River State Beach in Humboldt County, California, 

supplied by Amber Transou of California State Parks, and a 42-minute recording of a breeding Caspian 

tern colony obtained from Rice Island (lower Columbia River estuary) by Kathy Turco of Alaska’s 

Spirit Speaks. Caspian tern or snowy plover recordings were broadcast on a continuous loop at each 

island. The Caspian tern recording is the same one used in social attraction efforts at the other USACEconstructed sites in southeastern Oregon and northeastern California as part of implementing the 

Columbia River estuary Caspian tern management plans. Decoys and call systems broadcasting on a 

continuous loop remained on the islands until they were retrieved in August or September.

Gull Dissuasion Efforts

We visited Ponds A16 and SF2 at least 3–4 days a week for surveys and, if necessary, to haze 

gulls from islands (both modified islands and other islands nearby). The California gull (Larus 

californicus) population in San Francisco Bay has increased from fewer than 200 breeding gulls in 1982 

to more than 38,000 breeding gulls in 2016 (Strong and others, 2004; Tarjan and Butler 2016). Gulls

occur in large numbers around the project area, and have been identified as dominant predators of 

waterbird eggs and chicks (Herring and others, 2011; Ackerman, Herzog, Hartman, and Herring, 2014; 

Ackerman, Herzog, Hartman, and Takekawa, 2014). Thus, gull dissuasion was considered a potentially 

important component for successful nesting of Caspian terns and snowy plovers. Hazing measures 

included the use of green lasers and most often involved boating to islands and making noise to flush 

gulls from the islands. 
5 

Evaluation of Nesting by Caspian Terns and Snowy Plovers

We conducted survey bouts at Ponds A16 and SF2 beginning in early March and extending 

through mid-September in 2015, 2016, and 2017 (total of 166 survey bouts). We conducted surveys 3–4 

times per week early during the season (March–April) and then twice per week for the remainder of the 

breeding season (April–September). Both ponds were surveyed on the same day, one in the early 

morning and the other in the afternoon, and the time of day was alternated at each pond during 

consecutive survey bouts. Each survey bout was separated into three separate surveys, which were 

conducted from the surrounding levee using a 20–60× spotting scope. 

First, a pond survey was conducted where we drove around the surrounding levee of each pond 

(A16 or SF2) by truck and stopped at set vantage points to record the number and location (250×250m 

grid cell, figs. 2 and 3) of all tern, shorebird, and gull species known to nest in South San Francisco Bay, 

as well as potential nest predators (for example common raven [Corvus corax], raptors, and gulls). This 

survey was completed within 60 minutes to limit double-counting of individuals and avoid biasing 

abundance estimates at each pond.

Second, immediately following the pond survey, we conducted an additional 60-minute 

behavioral survey from a single fixed vantage point, where we scanned the pond every 10 minutes and 

documented the location and behaviors of all Caspian terns and snowy plovers. This survey was 

designed to provide breeding chronology information such as initiation of courtship, nest-building, and 

incubation, and to document factors negatively influencing nesting birds such as predation or 

disturbance.

Third, immediately following the behavioral survey, we conducted island surveys from a vantage 

point on the surrounding levee closest to each particular island, where we recorded the number of adults, 

the number of incubating adults, the number of nests with chicks, the number of chicks, and the number 

of fledglings of Caspian terns and snowy plovers observed on each island. In total, each survey bout 

consisted of about 150–180 minutes of observation. During each survey bout, we recorded color-band 

combinations of any color-banded Caspian tern observed. 

Additionally in 2016 and 2017, we conducted separate weekly 2-hr band-resight surveys of 

Caspian terns at each of the Pond A16 and SF2 nesting colonies. During these surveys, we recorded any 

color-banded Caspian terns that were observed. These band-resight surveys were conducted from a 

small portable blind installed each day of the survey on an island (Pond A16) or internal levee (Pond 

SF2) that was adjacent to the focal nesting colony. During the band-resight surveys, we recorded the 

color combination and behaviors of all banded Caspian terns observed. These data were then provided to 

Yasuko Suzuki of Oregon State University to identify the origin of banded Caspian terns and potentially 

demonstrate the movement of terns from the Columbia River basin to the modified islands at the 

DENWR. 

Estimating Colony Size and Productivity of Caspian Terns 

Because it was important to project partners to limit disturbance of these newly formed colonies 

to avoid causing nest and (or) colony failures, visits to islands with active colonies were limited. Thus, 

we could not mark and follow the fates of individual nests to estimate the total number of nests within a 

colony over the course of the breeding season. Instead, we estimated the number of Caspian tern nests 

from direct counts of the number of active nests (sum of nests being incubated and nests with chicks)

observed during our island surveys. In all years, Caspian terns were observed nesting over a 4–5-month 

period from mid-April through early September (fig. 8). Thus, the use of peak counts of active nests 

results in the underestimation of the number of total nests, as nests initiated early in the season or after 

the peak are not included. Moreover, in 2015, the number of nests initiated at both Ponds A16 and SF2 

had a bimodal distribution with two distinct peaks. Concurrent increases in the number of adult Caspian 

terns during the second peak in nesting indicated that nests during the second peak were those of new 
6 

nesting birds rather than renesting attempts (Hartman and others, 2016). As a result, using only the peak 

count in all years would underestimate production in 2015, whereas using the two peaks in 2015 but 

only the single peaks in 2016 and 2017 would underestimate the latter 2 years relative to 2015. To 

account for this, we used the following approach to estimate the total number of nests initiated in each

year. First, we identified the date when the peak number of nests was observed in each year. Next, we 

determined the number of nests observed about 40 days prior to and about 40 days after the peak. If a 

survey occurred exactly 40 days before or 40 days after the date of the peak, we used the number of 

nests observed on those days. If not, we used the survey date closest to 40 days before and after the peak 

date. We selected this 40-day interval because each nest typically can be active for about 40 days—

Caspian tern females lay 1–3 eggs with 2–3 days between laying each egg, incubate eggs over a 27-day 

period, and brood small chicks for 1 week post-hatch (Cuthbert and Wires 1999). Thus, nests active 40 

days prior to and 40 days after the date the peak number of nests was observed would represent 

additional nests that would not have been active during the peak count. Therefore, for each year, we 

calculated the total number of nests as the number of nests active during the peak count, plus the number 

of nests active 40 days prior to the peak count, plus the number of nests active 40 days after the peak 

count. Additionally, if nests were active 80 days prior to and (or) 80 days after the peak, we also added 

these nests to the total (fig. 9). By including nests that could not have been active at the time of the peak 

count, this method provided a more realistic estimate of the total number of nests initiated over the entire 

breeding season. 

We considered a chick fledged if it was similar in size to adults and had well-developed flight 

feathers, and (or) if it was observed flying. However, fledglings were observed over a 3-month period 

(mid-June through mid-September) at both Pond A16 and Pond SF2 (fig. 8), and many, if not most, of 

the fledglings observed in early September likely were not the same individuals observed in early July. 

To address this, we used the same procedure we used for estimating the total number of nests, in which 

we added the number of fledglings observed during the peak count, the number of fledglings observed 

40 days prior to the peak count, and the number of fledglings observed 40 days after the peak count to 

estimate the number of fledglings produced in each year. Although fledglings, unlike nests, can move 

around and remain on the island for an indeterminate length of time, we thought that 40 days was a 

sufficient interval to minimize double counting of fledglings over the season. It was not necessary to add 

fledgling counts that occurred 80 days prior to or after the peak because no fledglings were observed at 

these times. 

We used the total number of nests in each year to estimate the number of breeding pairs and 

estimated breeding success in each year as the number of fledglings per breeding pair by dividing the 

estimated number of fledged Caspian tern chicks produced by the estimated number of breeding pairs. 

Because Caspian terns may renest (lay a second clutch of eggs) following an unsuccessful first nesting 

attempt (Cuthbert and Wires, 1999), some late renests likely were included as new nests in estimating 

the number of breeding pairs. However, because we applied the same procedure to estimate the number 

of breeding pairs, the number of fledglings, and breeding success in all years, we could effectively 

determine the relative change in these measures among years.
7 

Estimating Apparent Nest Density of Caspian Terns

We calculated two measures of apparent nest density. First, we calculated nest density at each 

colony as a function of the total island area for that colony. Island areas were determined using RealTime Kinematics (RTK; Leica GNSS RTK Rover, Leica Geosystems, Atlanta, Georgia). During August 

24–27, 2015, we traced the gravel perimeter of each island with the RTK unit while collecting latitude, 

longitude, and elevation data at 1-second intervals. These data were imported into ArcGISTM 10.2 

(Environmental Research Systems Institute, Redlands, California) and converted to polygon shapefiles 

for determination of island area. Second, we calculated nest density at each colony as a function of only 

the area of the island used by the colony. For this second measure, we used aerial photographs of the 

colonies to map locations of tern nests on each island. We determined the area of each island used by 

nesting Caspian terns from digitized aerial colony photographs taken each year around the peak of 

nesting and provided by Real Time Research, Inc. (Bend, Oregon). From these photographs, we 

identified Caspian tern nests, created a minimum convex polygon shapefile around each colony in each 

year, and calculated colony area using ArcGISTM 10.2. We calculated the number of nests per square 

meter of island area by dividing the number of nests observed on the island survey at the time the aerial 

colony photograph was taken by the total island area. We calculated the number of nests per square 

meter of colony area by dividing the number of nests observed on the island survey at the time the aerial 

colony photograph was taken by the total colony area. Both of these estimates are of apparent nest 

densities, not accounting for influences of nest depredation or nest abandonment (Mayfield, 1961), 

because we limited access to the islands during the nesting season to limit disturbance. Therefore, these 

density estimates are biased somewhat low.

Results and Discussion

Abundance of Caspian Terns and Other Prominent Nesting Species in Ponds A16 and SF2

Weekly high counts of the most abundant surveyed nesting species, and snowy plovers, at Ponds 

A16 and SF2 during all years are shown in figure 10. May and June are the primary nesting period for 

most waterbirds, including Caspian terns, in South San Francisco Bay. Thus, the number of individuals 

observed during these months is a good estimate of the breeding population, as few to no young of the 

year fledge before July. Caspian tern numbers at both ponds consistently increased throughout the 3-

year study. At Pond A16, the average count of Caspian terns in May and June increased 96 percent from

2015 (56 birds) to 2016 (110 birds), and increased another 41 percent from 2016 (110 birds) to 2017 

(155 birds), for a 177-percent increase over the 3-year study period (fig. 11A). Similarly, at Pond SF2, 

the average count of Caspian terns in May and June increased 39 percent from 2015 (137 birds) to 2016 

(190 birds), and increased another 63 percent from 2016 (190 birds) to 2017 (310 birds), for a 126-

percent increase over the 3-year study period (fig. 12A). Other abundant, although non-target, species 

that used islands in Ponds A16 and SF2 included American avocet (Recurvirostra Americana), blacknecked stilt (Himantopus mexicanus), California gull, and Forster’s tern (Sterna forsteri). American 

avocet numbers increased more than 200 percent at both ponds in 2016 relative to 2015, before 

decreasing 59 percent (Pond A16) and 13 percent (Pond SF2) in 2017 (figs. 11B, 12B). Black-necked 

stilt numbers decreased 84 percent at Pond A16 from an average of 77 birds in 2015 to just 12 birds 

2017 (fig. 11C). Black-necked stilt numbers at Pond SF2 were consistently low, but rose from an 

average of 4 birds in 2015 to 19 birds in 2017 (fig. 12C). California gulls, a major predator of waterbird 

eggs and chicks (Herring and others, 2011; Ackerman, Herzog, Hartman, and Herring, 2014; Ackerman, 

Herzog, Hartman, and Takekawa, 2014), were abundant at Pond A16 during all years. However, at Pond 

A16, the average count of California gulls in May and June decreased 57 percent from an average of 270 

birds in 2015 to an average of 115 birds in 2017 (fig. 11D). This large decrease in California gull 
8 

numbers at Pond A16 made gull dissuasion largely unnecessary in 2016 and 2017. At Pond SF2, the 

number of California gulls in May and June was consistently low, with counts averaging 6 or fewer 

birds in all years (fig. 12D). Counts of Forster’s terns in May and June at Pond A16 consistently 

averaged 15–21 birds throughout the study (fig. 11E), but at Pond SF2, Forster’s tern numbers increased 

248 percent from an average of only 23 birds in May and June of 2015 to an average of 80 birds in May 

and June of 2017 (fig. 12E). Finally, western snowy plover observations were infrequent and numbers 

were consistently low (average ≤8 birds) at both sites. Western snowy plovers increased over the study 

period at Pond A16 (fig. 11F), but were not observed at Pond SF2 during 2016 or 2017 (fig. 12F).

Caspian terns were observed at least once on each of the 20 islands in Pond A16 (including all 3 

modified islands) and on each of 24 islands in Pond SF2, (including all 4 modified islands) (table 2). At 

Ponds A16 and SF2, Caspian terns were overwhelmingly drawn to those islands with Caspian tern social 

attraction measures. During the 166 behavioral surveys conducted from March to September of 2015, 

2016, and 2017, 92 and 93 percent of Caspian tern observations in Ponds A16 and SF2, respectively, 

occurred on islands with social attraction measures for Caspian terns (fig. 13).

Breeding Chronology of Caspian Terns

Caspian terns were observed at Ponds A16 and SF2 from mid- to late March through midSeptember, when surveys ended, during all years of the project. However, breeding chronology was 

about 2–3 weeks earlier at Pond SF2 than at Pond A16 throughout the 3-year study (fig. 14). At Pond 

SF2, courtship consistently began in late March, nest building began in mid- to late March, and adults

were first observed incubating eggs in early to late April. Brooding of tern chicks at Pond SF2 was first 

observed 4 weeks later in early to late May. In contrast, at Pond A16, courtship began in late March or 

early April, nest building began in mid- to late April, and incubation was first observed in late April. 

Brooding of chicks at Pond A16 followed 4 weeks later in late May. At both ponds, Caspian terns 

concluded nesting and brooding chicks by early September.

Caspian terns at Pond SF2 began incubating eggs about 1 week earlier in 2016, and 2 weeks 

earlier in 2017, relative to 2015 (fig. 14). At Pond A16, terns began incubating eggs about 1 week earlier 

in 2016 and 2017, relative to 2015 (fig. 14). The reason for this advancement in breeding chronology is 

unknown but may be partially due to tern familiarity with this new nesting site in the years subsequent to 

initial colonization in 2015. 

The nesting season at Ponds A16 and SF2 was longer than has been observed for Caspian terns 

in other San Francisco Bay locations. Roby and others (2009) reported that by early August 2009, three 

Caspian tern colonies in San Francisco Bay had completed nesting and were abandoned for the season. 

In contrast, pre-fledged chicks continued to be observed as late as early September at Ponds A16 and 

SF2 (fig. 8). 

Size, Productivity, and Growth of Caspian Tern Breeding Colonies 

Caspian terns consistently nested on Island 11 in Pond A16 and on Island 21 in Pond SF2 (table 

3; figs. 8, 15, and 16). Except for a single nest on Island 17 in Pond SF2 in 2015, terns did not nest on 

the other 3 islands modified for Caspian terns (Island 12 in Pond A16, Islands 12 and 17 in Pond SF2). 

Peak counts of the total number of active nests (which include both nests with eggs and nests with 

chicks) on Island 11 in Pond A16 occurred on June 5 (n=29 nests, first peak) and again on July 20 (n=44 

nests, second peak) for a combined total of 73 nests in 2015, on June 6 in 2016 (n=137 nests), and on 

June 9 in 2017 (n=167 nests). The peak number of nests at Island 11 in Pond A16 increased 88 percent 

in 2016, and increased another 22 percent in 2017, for a 129-percent increase over the 3-year study 

period. For 2015, we estimated the total number of nests on Island 11 in Pond A16 by summing the peak 

number of 44 nests observed on July 20, 22 nests observed on June 8 (42 days before the peak), and 13 

nests observed on May 1 (80 days before the peak), for a total of 79 nests (fig. 9). For 2016, we summed 
9 

the peak number of 137 nests observed on June 6, 9 nests observed on April 27 (40 days before the 

peak), 20 nests observed on July 18 (42 days after the peak), and 3 nests observed on August 26 (81 

days after the peak), for a total of 169 nests (fig. 9). Finally, for 2017, we summed the peak number of 

167 nests observed on June 9, 5 nests observed on April 28 (42 days before the peak), 52 nests observed 

on July 21 (42 days after the peak), and 3 nests observed on August 28th (80 days after the peak), for a 

total of 227 nests (fig. 9). Using these total nest values, we estimated that the Caspian tern colony at 

Island 11 in Pond A16 increased 114 percent in 2016, and increased another 34 percent in 2017, for a 

187-percent increase over the 3-year study period (fig. 17).

Peak counts of the total number of active nests on Island 21 in Pond SF2 occurred on May 29 

(n=57 nests, first peak) and again on June 19 (n=93 nests, second peak) for a combined total of 150 nests 

in 2015, on June 3 in 2016 (n=180 nests), and on June 9 in 2017 (n=334 nests). The peak number of 

nests at Island 21 in Pond SF2 increased 20 percent in 2016, and increased another 86 percent in 2017, 

for a 123-percent increase over the 3-year study period. For 2015, we estimated the total number of nests

on Island 21 in Pond SF2 by summing the peak number of 93 nests observed on June 19, 49 nests 

observed on May 8 (42 days before the peak), and 23 nests observed on July 31 (42 days after the peak), 

for a total of 165 nests (fig. 9). For 2016, we summed the peak number of 180 nests observed on June 3, 

28 nests observed on April 25 (39 days before the peak), 54 nests observed on July 15 (42 days after the 

peak), and 3 nests observed on August 22 (80 days after the peak), for a total of 265 nests (fig. 9). 

Finally, for 2017, we summed the peak number of 334 nests observed on June 9, 28 nests observed on 

April 28 (42 days before the peak), 74 nests observed on July 21 (42 days after the peak), and 1 nest

observed on August 31 (83 days after the peak), for a total of 437 nests (fig. 9). Using these total nest 

values, we estimated that the Caspian tern colony at Island 21 in Pond SF2 increased 61 percent in 2016, 

and increased another 65 percent in 2017, for a 165-percent increase over the 3-year study period 

(fig. 17). 

Summing the total number of nests on Ponds A16 and SF2 resulted in an overall estimated 245 

breeding pairs in 2015 (including the single nest observed on Island 17), 434 in 2016, and 664 in 2017 

(table 3). In recent years, only the Brooks Island Caspian tern colony produced more nests (n=681 in 

2009; Roby and others, 2009) in San Francisco Bay than was produced between Ponds A16 and SF2 in 

2017. The overall number of Caspian tern breeding pairs at Ponds A16 and SF2 increased 77 percent in 

2016, and increased another 53 percent in 2017, for a 171-percent increase over the 3-year study period 

(table 3; fig. 17).

Peak counts of the number of fledglings on Island 11 in Pond A16 occurred on July 13 (n=22, 

first peak) and again on August 21 (n=32, second peak) for a combined total of 54 in 2015, on August 1 

in 2016 (n=35), and on August 7 in 2017 (n=95). The peak number of fledglings at the Caspian tern 

colony at Island 11 in Pond A16 decreased 35 percent in 2016, and increased 171 percent in 2017, for a 

76-percent increase over the 3-year study period. For 2015, we estimated the total number of fledglings 

on Island 11 in Pond A16 by summing the peak number of 32 fledglings observed on August 21, and 22 

fledglings observed on July 13 (39 days before the peak), for a total of 54 fledglings (fig. 9). For 2016, 

we summed the peak number of 35 fledglings observed on August 1, 11 fledglings observed on June 24 

(38 days before the peak), and 5 fledglings observed on September 12 (42 days after the peak), for a 

total of 51 fledglings (fig. 9). Finally, for 2017, we summed the peak number of 95 fledglings observed 

on August 7, 3 fledglings observed on June 26 (42 days before the peak), and 8 fledglings observed on 

September 19 (42 days after the peak), for a total of 106 fledglings (fig. 9). Using these total fledgling 

values, we estimated that the number of fledglings at the Caspian tern colony at Island 11 in Pond A16 

decreased 6 percent in 2016, and increased 108 percent in 2017, for a 96 percent increase over the 3-year 

study period (fig. 17).
10 

Peak counts of the number of fledglings on Island 21 in Pond SF2 occurred on July 6 (n=75, first 

peak) and again on August 21 (n=44, second peak) for a combined total of 119 in 2015, on July 8 in 

2016 (n=55), and on July 14 in 2017 (n=120). The peak number of fledglings at the Caspian tern colony 

at Island 21 in Pond SF2 decreased 54 percent in 2016, and increased 118 percent in 2017, for a 1-

percent increase over the 3-year study period. For 2015, we estimated the total number of fledglings on 

Island 21 in Pond SF2 by summing the peak number of 75 fledglings observed on July 6, and 39 

fledglings observed on August 17 (42 days after the peak), for a total of 114 fledglings (fig. 9). For 

2016, we summed the peak number of 55 fledglings observed on July 8, and 17 fledglings observed on 

August 19 (42 days after the peak), for a total of 72 fledglings (fig. 9). Finally, for 2017, we summed the 

peak number of 120 fledglings observed on July 14, and 13 fledglings observed on August 25 (42 days 

after the peak), for a total of 133 fledglings (fig. 9). Using these total fledgling values, we estimated that 

the number of fledglings at the Caspian tern colony at Island 21 in Pond SF2 decreased 37 percent in 

2016, and increased 85 percent in 2017, for a 17-percent increase over the 3-year period (fig. 17). 

Summing the total number of fledglings on Ponds A16 and SF2 resulted in an overall estimated 169

fledglings in 2015 (including the single fledgling from the one nest observed on Island 17), 123 

fledglings in 2016, and 239 fledglings in 2017 (table 3). The overall number of Caspian tern fledglings 

produced at Ponds A16 and SF2 decreased 27 percent in 2016, and increased 94 percent in 2017, for a 

41 percent increase over the 3-year study period (table 3; fig. 17).

Dividing the estimated number of fledglings by the estimated number of breeding pairs yielded 

an apparent breeding success (number of fledglings per breeding pair) of 0.68 in 2015, 0.30 in 2016, and 

0.47 in 2017 at Island 11 in Pond A16 (table 3). Apparent breeding success at Pond A16 decreased 56 

percent in 2016, increased 55 percent in 2017, and decreased 32 percent over the 3-year study period 

(table 3; fig. 17). At Island 21 in Pond SF2, apparent breeding success was 0.69 in 2015, 0.27 in 2016, 

and 0.30 in 2017, resulting in a 61 percent decrease in 2016, a 12 percent increase in 2017, and a 56 

percent decrease over the 3-year study period (table 3; fig. 17). Overall apparent breeding success for 

Ponds A16 and SF2 combined was 0.69 in 2015, 0.28 in 2016, and 0.36 in 2017, representing a 59 

percent decrease in 2016, a 27 percent increase in 2017, and a 48 percent decrease over the 3-year study 

period (table 3; fig. 17). Decreases in apparent breeding success between years were largely the result of 

increases in the number of breeding pairs without proportional increases (and in some cases decreases) 

in the number of fledglings produced (table 3). However, limitations associated with estimating the total 

number of fledglings from peak counts likely led to underestimation of the number of fledglings 

produced and, therefore, breeding success. The average breeding success rate over the course of the 3-

year project was 0.44 fledglings per breeding pair, lower than the long-term averages on East Sand 

Island (about 0.62) and Crescent Island (0.53) in the mid-Columbia River (Roby and others, 2013), but 

greater than the average among sites in South San Francisco Bay (0.27; Collis and others, 2012). 

Island Area and Nest Density of Caspian Terns

The island area used by Caspian tern colonies increased in size each year of the study (figs. 15 

and 16)—366 m2 in 2015, 747 m2 in 2016, and 1,394 m2 in 2017 (table 3). This corresponded to a 104 

percent increase in 2016, another 87 percent increase in 2017, and a 281 percent increase over the 3-year 

study period. Owing to its much larger population, the colony on Island 21 in Pond SF2 occupied a 

much larger proportion of the island habitat than did the colony on Island 11 in Pond A16. By 2017, 29 

percent and 55 percent of the island area available was occupied by nesting terns on Island 11 in Pond 

A16 (1,603 m2) and Island 21 in Pond SF2 (1,693 m2), respectively. 
11 

Apparent nesting density as a function of total island area available (nests per square meter of 

island area) also increased over the study period (table 3). On Island 11 in Pond A16, nests per island 

area increased 258 percent in 2016, increased another 59 percent in 2017, and increased 469 percent 

over the 3-year study. Similarly, on Island 21 in Pond SF2, nests per island area increased 175 percent in 

2016, increased another 62 percent in 2017, and increased 347 percent over the 3-year study period. 

Apparent nesting density as a function of only the island area used by Caspian terns also increased over 

the study period (table 3; fig. 17). On Island 11 in Pond A16, nests per colony area increased 75 percent

in 2016, decreased 8 percent in 2017, and increased 61 percent over the 3-year study period. Similarly, 

on Island 21 in Pond SF2, nests per colony area increased 35 percent in 2016, decreased 16 percent in 

2017, and increased 13 percent over the 3-year study period. Overall nest density (nests per square meter 

of colony area) in Ponds A16 and SF2 was higher in 2017 (0.29) and 2016 (0.33) compared to 2015 

(0.23), and was lower than densities observed at San Francisco Bay sites of Brooks Island (0.9), Eden 

Landing (1.0), and Steven’s Creek (0.5; Roby and others, 2009). These results suggest that, for two 

reasons, considerable room remains for colony population increases. First, large majorities of modified 

island habitat are still not being used by nesting Caspian terns. This includes 71 percent of the area of 

Island 11 in Pond A16, 45 percent of the island area of Island 21 in Pond SF2, and 100 percent of the 

island area of Island 12 in Pond A16 and Islands 12 and 17 in Pond SF2 (total of 6,509 m2

 of island area 

modified for nesting Caspian terns not used in 2017). Second, in 2017, overall nest density between 

Ponds A16 and SF2 was 0.29 nests per colony area, lower than the 0.5–1.0 breeding pairs per m2 of 

colony area that has been observed at other colonies in San Francisco Bay (Roby and others, 2009). 

Sightings of Color-Banded Caspian Terns

We observed 52 uniquely banded Caspian terns over the course of the 3-year study, including 5 

in 2015, 26 in 2016, and 38 in 2017 (table 4). Eight banded terns were observed only on Island 11 in 

Pond A16, 37 were observed only on Islands 17 and (or) 21 in Pond SF2, 3 were observed elsewhere in 

Pond SF2, and 4 were observed on both Island 11 in Pond A16 and Island 21 in Pond SF2. A total of 27 

of these color-banded Caspian terns were observed engaging in breeding behaviors (that is, copulation, 

nest scraping, courtship feeding, incubation, or attending to, feeding, or brooding a chick), including two 

in Pond A16, 24 in Pond SF2, and one at both sites.

All information regarding site of initial banding was provided courtesy of Kirsten Bixler and 

Yasuko Suzuki (Oregon State University, written commun., November 9, 2017). Among the 27 colorbanded Caspian terns suspected or confirmed to have bred at Pond A16 or Pond SF2, 3 were banded as 

adults and 15 were banded as chicks during 2004–2009 at Brooks Island in San Francisco Bay; 2 were 

banded as chicks in 2009 at Eden Landing in San Francisco Bay; 3 were banded as chicks during 2003–

2010 at East Sand Island in the Columbia River estuary, Oregon; 1 was banded as a chick in 2012 at 

Sheepy Lake on the Lower Klamath National Wildlife Refuge, 1 was banded as a chick in 2011 on 

Goose Island in Potholes Reservoir, Washington; 1 was banded as a chick in 2010 at the Port of 

Bellingham in Bellingham Bay, Washington; and 1 for which we could not determine the banding 

location or year because the bird was missing bands (table 4). When the number of breeding Caspian 

terns previously banded at either Brooks Island or Eden Landing (n=20) were combined, 74 percent of 

the 27 color-banded terns observed breeding at Pond A16 or Pond SF2 originated from colonies within 

San Francisco Bay. This suggests that Island 11 in Pond A16 and Island 21 in Pond SF2 have been 

largely colonized by Caspian terns from other breeding sites in San Francisco Bay. Nevertheless, at least 

three Caspian terns banded as chicks at the East Sand Island colony in the Columbia River estuary

attempted to breed at either Pond A16 or Pond SF2 on the Don Edwards San Francisco Bay National 

Wildlife Refuge in 2017, and two were confirmed attending young chicks. This indicates success in one 

goal of the project—relocating Caspian terns from the Columbia River estuary to San Francisco Bay.
12 

Nesting of Snowy Plovers and Other Waterbirds

Snowy plovers were uncommon at Ponds A16 and SF2, both before and during the 3-year study 

period, with most observations occurring before or after the nesting season (fig. 10). At Pond A16, 

snowy plover numbers in May and June averaged fewer than three individuals in 2015 and 2016 (fig. 

11). However, in 2017, snowy plover numbers in May and June averaged eight individuals, and six nests 

were documented. We observed three nests on Island 11 on April 21 and another three nests on Island 

12 on May 2. Both of these islands were modified, and had active social attraction measures for nesting 

Caspian terns. No snowy plover nested on Island 3 in Pond A16, the island that was specifically 

modified for nesting snowy plovers and contained snowy plover social attraction measures. All six

snowy plover nests in 2017 were unsuccessful and no chicks were ever observed. Caspian terns did 

cause some disturbance to snowy plovers nesting on Island 11, occasionally flying overhead, landing or 

walking close to the nest and causing the attending parent to flush. However, snowy plovers would 

quickly return to the nest after such encounters. All three snowy plover nests on Island 11 in Pond A16 

were outside the perimeter of the Caspian tern colony, whereas the three nests on Island 12 were among 

the Caspian tern decoys and along the decoy perimeter (no terns nested on Island 12). At Pond SF2, 

snowy plover numbers in May and June dropped from an average of six individuals in 2015 to no birds 

in either 2016 or 2017 (fig. 12), and no nests were observed throughout the 3-year study period. 

American avocets, Forster’s terns, black-necked stilts, and black skimmers (Rynchops niger) 

were other common nesting waterbird species in Ponds A16 and (or) SF2. These species nested on 

islands or on mudflats within these ponds. At Pond A16, the number of American avocet nests increased 

from 4 in 2015 to 89 in 2016 before decreasing to 79 in 2017. The number of black-necked stilt nests 

increased from 0 in 2015 to 5 in 2016 before decreasing to 1 in 2017. No other waterbirds nested in 

Pond A16 during the study. At Pond SF2, the number of American avocet nests increased from 25 in 

2015 to 121 in 2016 before decreasing to 72 in 2017. The number of Forster’s tern nests increased from 

0 in 2015 to 8 in 2016, then to 170 in 2017. The 178 Forster’s tern nests in Pond SF2 in 2016 and 2017 

are the first documented nests of this species at Pond SF2 over the past 13 years of nest monitoring 

efforts by USGS. The number of Forster’s tern nesting colonies and the overall number of Forster’s tern 

nests have been decreasing in South San Francisco Bay over the past several years. In 2017, only 4 large 

Forster’s tern nesting colonies (one of which was in Pond SF2) were documented in South San 

Francisco Bay, down from the 10–20 colonies observed in previous years (J.T. Ackerman, M.P Herzog, 

and C.A. Hartman, U.S. Geological Survey, unpub. data, 2018). In 2015 and 2016, DENWR staff 

deployed Forster’s tern decoys and call systems on Island 22 in Pond SF2. Although no tern nests were 

observed on this or nearby islands in any year, this social attraction effort may have helped attract 

Forster’s terns to nest in other areas of Pond SF2 in 2017. There also were 15–20 black skimmer nests in 

Pond SF2 in 2017, the first documented nests of this species at Pond SF2 over the past 13 years of nest 

monitoring efforts by USGS. Black skimmers nested on several islands in Pond SF2, including all three 

islands with Caspian tern social attraction (Islands 12, 17, and 21). Black skimmers were largely 

successful, as about 30–35 chicks fledged and we observed no conflict or disturbance between black 

skimmers and Caspian terns. Caspian tern social attraction efforts, and the presence of Caspian tern 

nesting colonies in Ponds A16 and SF2 during 2015, 2016, and 2017 may have attracted other nesting 

waterbirds (particularly American avocet, Forster’s terns, and black skimmers) to nest in these ponds as 

well, and, thus, may be an indirect benefit of the Caspian tern social attraction efforts.
13 

Gull Dissuasion

California gull numbers decreased at Ponds A16 and SF2 over the 3-year study period (figs. 11 

and 12). At Pond A16, California gull numbers in May and June decreased 57 percent from an average 

of 270 in 2015 to 115 in 2017. At Pond SF2, May and June California gull numbers were consistently 

low and averaged fewer than 6 individuals in all years. Gulls typically roosted on the modified islands 

less often than on other islands. This, coupled with the much lower gull numbers relative to 2015, made 

gull hazing largely unnecessary in 2016 and 2017. We never observed gulls prospecting for nest sites, 

building nests, or engaging in other nesting behaviors at either Ponds A16 or SF2. 

The greater number of gulls at Pond A16 compared to Pond SF2 likely is owing partly to the 

greater distance of Pond SF2 to the nearest breeding gull colony and to the nearest landfill, which 

attracts large numbers of gulls (Ackerman, Eagles-Smith, and others, 2009). The distance from modified

islands in Pond A16 to the nearest gull colony is less than 3 km, whereas the distance from modified

islands in Pond SF2 to the nearest gull colony is more than 5 km (fig. 1). Additionally, the density of 

breeding gulls is much greater around Pond A16 than around Pond SF2 (fig. 1). Furthermore, the 

distance from modified islands in Pond A16 to the nearest landfill (Newby Island) is about 2 km, 

whereas the distance from modified islands in Pond SF2 to the nearest landfill (Newby Island) is more 

than 16 km. Finally, gulls likely were attracted to Pond A16 because it lies almost directly between the 

large A9/A10 gull colony and the Newby Island Landfill (Ackerman, Eagles-Smith, and others, 2009; 

fig. 1). 

Factors Influencing Overall Success of Caspian Tern Colonies

Potential egg or chick predators observed at Ponds A16 and SF2 included California gulls, ringbilled gulls (Larus delawarensis), western gulls (L. occidentalis), American crows (Corvus 

brachyrhynchos), and common ravens. Of these, California gulls were by far the most numerous, and 

are well-documented predators of waterbird eggs and chicks in South San Francisco Bay (Herring and 

others, 2011; Ackerman, Herzog, Hartman, and Herring, 2014; Ackerman, Herzog, Hartman, and 

Takekawa, 2014). California gulls were particularly abundant at Pond A16. However, California gull 

numbers at Pond A16 decreased considerably in 2016 and 2017 relative to 2015 (fig. 11), and gull 

numbers typically only exceeded 300 individuals after late July. Gulls in Pond A16 typically were 

observed roosting on islands and the surrounding pond levees, and there was little evidence that 

California gulls had a large negative influence on nesting Caspian terns. On June 2, 2016, a California 

gull was observed depredating an abandoned Caspian tern nest at the edge of the colony on Island 11 in 

Pond A16, and on July 1, 2016, a California gull unsuccessfully attempted to steal a fish from a Caspian 

tern chick. There were fewer gulls at Pond SF2 than at Pond A16 (fig. 12), and gulls were largely absent 

from the pond from mid-April to mid-July. Gulls rarely roosted on Pond SF2 islands modified for 

Caspian terns. However, on a few occasions in June 2017, two gulls were observed disturbing the 

Caspian tern colony at Pond SF2, including the successful depredation of a young tern chick. On July 3, 

USDA-APHIS removed one of these gulls, and gull disturbance of the colony ceased for the remainder 

of the breeding season. 

In late September, at the time decoys and call systems were removed, we observed several dead 

Caspian tern chicks at both Caspian tern nesting islands, which is typical among colonial waterbird 

nesting colonies. The number of dead chicks and fledglings observed was greater in 2015 (n=55) and 

2016 (n=63) than in 2017 (n=30). Causes of mortality for these chicks and fledglings is unknown, but 

there were no obvious signs of trauma, suggesting that these individuals died because of exposure, 

starvation, abandonment, and (or) other natural causes.
14 

Conclusions and Management Implications

Results from the 2015, 2016, and 2017 breeding seasons demonstrate the success of social 

attraction measures in establishing Caspian tern (Hydroprogne caspia) breeding colonies on modified 

islands at Ponds A16 and SF2 of the Don Edwards San Francisco Bay National Wildlife Refuge, 

California. Over the 3-year study, we estimate that 1,343 Caspian tern nests were initiated and 531 

chicks fledged between the two ponds. Prior to 2015, the first year of the study, there were no records of 

Caspian terns nesting at either Pond A16 or Pond SF2. However, by 2017, we estimated that 664 

Caspian tern breeding pairs nested and at least 239 chicks fledged at Ponds A16 and SF2, making these 

two ponds home to two of the largest Caspian tern breeding colonies in San Francisco Bay. The 

proportion of modified island area occupied by Caspian tern nesting colonies almost doubled in 2017 

(1,394 square meters [m2]) compared to 2016 (747 m2), encompassing 29 and 55 percent of the available 

nesting habitat on Island 11 in Pond A16 and Island 21 in Pond SF2, respectively. Even with the 

increase in island area used by Caspian terns in 2017, the complete lack of nesting on three of the five 

modified islands resulted in use of only 18 percent (1,394 m2

) of the 7,903-m2 area modified on five

islands for nesting Caspian terns in Ponds A16 and SF2. This suggests that there is considerable 

opportunity for Caspian tern breeding colonies to expand at each pond in future years. 

Of the 52 banded Caspian terns observed at Ponds A16 and SF2 in 2016, 27 were observed to be 

engaged in breeding behaviors. The other 25 were mostly observed in early spring and (or) late summer, 

suggesting that they bred at other locations and only passed through Ponds A16 and SF2 before and after 

the breeding period. Twenty of the 27 color-banded Caspian terns observed engaging in breeding 

behaviors were banded at other Caspian tern breeding sites in San Francisco Bay (Brooks Island or Eden 

Landing), whereas 3 of the 27 were banded as chicks on East Sand Island in the Columbia River estuary. 

Thus, although Caspian terns from other colonies in San Francisco Bay likely constitute a large 

proportion of the Caspian terns at the new colonies at Ponds A16 and SF2, we documented evidence of 

three individuals (two of which were observed attending chicks) moving from their natal colony of East 

Sand Island colony in the Columbia River estuary to breed at the Don Edwards San Francisco Bay 

National Wildlife Refuge.

We found little evidence that the large population of California gulls (Larus californicus) in 

South San Francisco Bay had a negative effect on nesting Caspian terns. In fact, compared to 2015, 

California gull numbers in 2016 and 2017 were markedly lower at Pond A16, and gull numbers at both 

Ponds A16 and SF2 were relatively low throughout most of the breeding season, only increasing in 

August and September. However, as the islands in Pond SF2 and especially Pond A16 are relatively 

new, continued monitoring, as well as hazing, may be necessary to ensure that this attractive nesting 

habitat is not overtaken by nesting gulls. 

Snowy plovers (Charadrius alexandrinus nivosus) did not nest on the two islands modified for 

them in any year but did nest on Islands 11 and 12 modified for Caspian terns in Pond A16 in 2017. 

Although modifying and deploying snowy plover social attraction measures on a separate island (Island 

3 in Pond A16) was not successful in keeping nesting Caspian terns and snowy plovers separate, we did 

not observe major disturbance to nesting snowy plovers by Caspian terns. Overall, there was little

observed conflict between Caspian terns and snowy plovers associated with island modifications in 

Ponds A16 and SF2. 
15 

Prior to island modification and deployment of social attraction measures in 2015, there was no 

history of Caspian terns nesting at either Pond A16 or Pond SF2. In just 3 years, the number of nesting 

Caspian terns in Ponds A16 and SF2 has increased from 0 to more than 650 breeding pairs and 230

fledglings, becoming the largest Caspian tern breeding colonies in South San Francisco Bay. Successful 

establishment of Caspian tern colonies in Ponds A16 and SF2 in 2015, and colony expansion in 2016, 

and again in 2017, demonstrate the potential of social attraction measures to help establish waterbird 

nesting colonies in San Francisco Bay. To offset the loss of managed pond habitat from tidal marsh 

restoration, the South Bay Salt Pond Restoration Project has constructed 46 new islands (16 in Pond 

A16, 30 in Pond SF2) over the past 5 years to provide nesting and roosting habitat for waterbirds (7 of 

these islands were further modified for nesting Caspian terns and snowy plovers in 2015 and are the 

focus of the current study). However, these other recently constructed islands thus far have been used

relatively little by nesting waterbirds, particularly Forster’s terns (Sterna forsteri), one of the most 

abundant colonial-breeding waterbirds in South San Francisco Bay. Continued social attraction 

measures similar to those used in this study may help to establish breeding colonies of several species at 

targeted wetlands enhanced by the South Bay Salt Pond Restoration Project. 

Acknowledgments

We thank the U.S. Army Corps of Engineers for project support and the U.S. Fish and Wildlife 

Service Don Edwards San Francisco Bay National Wildlife Refuge Complex and the South Bay Salt 

Pond Restoration Project for logistical and project support. We also thank Kimberly Sawyer, Jeanne 

Fasan, Janel Mayo, Sam Lei, Andrea Mott, Breanne Cooney, and Sami Michishit of the U.S. Geological 

Survey for assistance in the field; Kathy Turco of Alaska’s Spirit Speaks for the Caspian tern colony 

recording; Amber Transou of California State Parks for the western snowy plover recording; and the San 

Francisco Bay Bird Observatory for California gull data.

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Lyons, D.E., 2011, Recovery of coded wire tags at a Caspian tern colony in San Francisco Bay—A 
16 

technique to evaluate impacts of avian predation on juvenile salmonids: North American Journal of 

Fisheries Management, v. 31, no. 1, p. 79–87. 

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C., 2016, Evaluation of Caspian tern (Hydroprogne caspia) and snowy plover (Charadrius 

alexandrinus nivosus) nesting on modified islands at the Don Edwards San Francisco Bay National 

Wildlife Refuge—2015 annual report: U.S. Geological Survey Open-File Report 2016-1049, 36 p. 

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nest predators of American avocets (Recurvirostra americana) and black-necked stilts (Himantopus 

mexicanus) in San Francisco Bay, California: Southwestern Naturalist, v. 56, no. 1, p. 35–43. 

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C., Suzuki, Y., Wolf, C., 2009, Caspian tern nesting ecology and diet in San Francisco Bay and 

interior Oregon—Draft 2009 annual report: Prepared for the U.S. Army Corps of Engineers, 78 p.

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Management, v. 66, no. 3, p. 662–673. 

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Oregon, 12 p., 25 p., 

http://www.nwp.usace.army.mil/portals/24/docs/announcements/ea/final_cate_ea.pdf. 

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salmonids in the Columbia River Estuary—Final environmental impact statement: U.S. Fish and 

Wildlife Service, Portland, Oregon, 

https://www.fws.gov/pacific/migratorybirds/pdf/Caspian_Tern_Final_EIS.pdf. 
17 

Figure 1. Locations of Ponds A16 and SF2 (red shaded polygons) containing islands modified for Caspian terns 

and snowy plovers, Don Edwards San Francisco Bay National Wildlife Refuge, California. Green circles indicate 

California gull breeding colonies.
18 

Figure 2. Locations of islands modified for nesting Caspian terns (Islands 11 and 12; shown in blue) and nesting 

snowy plovers (Island 3; shown in red) in Pond A16, Don Edwards San Francisco Bay National Wildlife Refuge, 

California. Numbered yellow dots designate vantage points used to count birds in the pond during pond surveys.

Square grid cells are 250 by 250 meters. 
19 

Figure 3. Locations of islands modified for nesting Caspian terns (Islands 12, 17, and 21; shown in blue) and 

nesting snowy plovers (Island 10; shown in red) in Pond SF2, Don Edwards San Francisco Bay National Wildlife 

Refuge, California. Numbered yellow dots designate vantage points used to count birds in the pond during pond 

surveys. Square grid cells are 250 by 250 meters. 
20 

Figure 4. Arrangement of social attraction measures (decoys and call system) for Caspian terns on modified Island 

11 (top) and Islands 11 (bottom background) and 12 (bottom foreground) in Pond A16, Don Edwards San Francisco 

Bay National Wildlife Refuge, California. Top and bottom photographs by Crystal Shore, USGS, May 23 and April 5, 

2016, respectively. 
21 

Figure 5. Arrangement of social attraction measures (decoys and call system) for Caspian terns on modified Island 

12 (top) and Island 17 (bottom) in Pond SF2, Don Edwards San Francisco Bay National Wildlife Refuge, California. 

Top photograph by Alex Hartman, USGS, March 3, 2015. Bottom photograph by Kimberly Sawyer, USGS, March 

17, 2015.
22 

Figure 6. Arrangement of social attraction measures (decoys and call system) for Caspian terns on modified Island 

21 in Pond SF2, Don Edwards San Francisco Bay National Wildlife Refuge, California. Top and bottom 

photographs by Crystal Shore, USGS, March 17 and April 12, 2016, respectively. 
23 

Figure 7. Arrangement of social attraction measures (decoys and call system) for snowy plovers on modified

Island 10 in Pond SF2, Don Edwards San Francisco Bay National Wildlife Refuge, California. Photograph by Alex 

Hartman, USGS, March 2, 2015.
24 

Figure 8. Number of Caspian tern adults, incubating adults, chicks, fledglings, and active nests by date at (A) Pond 

A16, Island 11; and (B) Pond SF2, Island 21, Don Edwards San Francisco Bay National Wildlife Refuge, California, 

March–September 2015–17. No Caspian terns nested in Pond A16 or Pond SF2 prior to 2015.
25 

Figure 9. Number of Caspian tern nests (blue lines and circles) and fledglings (pink lines and circles) by date on 

Island 11 in Pond A16 (left graphs) and Island 21 in Pond SF2 (right graphs) in 2015 (top graphs), 2016 (middle 

graphs), and 2017 (bottom graphs), Don Edwards San Francisco Bay National Wildlife Refuge, California. Total 

number of breeding pairs was estimated in each year by summing the peak number of nests and nests observed 

about 40 days before, about 40 days after, about 80 days before, and about 80 days after the peak (orange circles). 

Total number of fledglings produced was estimated in each year by summing the peak number of fledglings, and 

fledglings observed about 40 days before and about 40 days after the peak (yellow circles). 
26 

Figure 10. Weekly high counts of individuals of the most abundant nesting waterbird species (excluding California 

gulls) observed during pond surveys at Ponds A16 (left graphs) and SF2 (right graphs), Don Edwards San 

Francisco Bay National Wildlife Refuge, California, March–September 2015–17. 
27 

Figure 11. Counts of (A) Caspian terns, (B) American avocets, (C) black-necked stilts, (D) California gulls, (E) 

Forster’s terns, and (F) western snowy plovers at Pond A16, Don Edwards San Francisco Bay National Wildlife 

Refuge, California, during May–June of 2015–17. Boxplots include the average (solid black circles), median 

(horizontal lines), 25th and 75th percentiles (bottoms and tops of boxes), 10th and 90th percentiles (error bars), and 

outlier observations (filled circles).
28 

Figure 12. Counts of (A) Caspian terns, (B) American avocets, (C) black-necked stilts, (D) California gulls, (E) 

Forster’s terns, and (F) western snowy plovers at Pond SF2, Don Edwards San Francisco Bay National Wildlife 

Refuge, California, during May–June of 2015–17. Boxplots include the average (solid black circles), median 

(horizontal lines), 25th and 75th percentiles (bottoms and tops of boxes), 10th and 90th percentiles (error bars), and 

outlier observations (filled circles).
29 

Figure 13. Percentage of Caspian tern observations by location during 166 behavior surveys conducted at Ponds 

A16 (A) and SF2 (B), Don Edwards San Francisco Bay National Wildlife Refuge, California, March–September 

2015–17. 
30 

Figure 14. Breeding chronology of Caspian terns based on observed behaviors at Ponds A16 (A) and SF2 (B), 

Don Edwards San Francisco Bay National Wildlife Refuge, California, in 2015 (white squares), 2016 (black 

squares), and 2017 (gray squares). Surveys were conducted from March to September of each year. 
31 

Figure 15. Aerial photograph showing Island 11 modified for Caspian terns in Pond A16, Don Edwards San 

Francisco Bay National Wildlife Refuge, California. Lines denote Caspian tern colony extents in 2015 (solid yellow), 

2016 (dashed green), and 2017 (dotted red). Photograph by Real Time Research, June 21, 2017. 
32 

Figure 16. Aerial photograph showing Island 21 modified for Caspian terns in Pond SF2, Don Edwards San 

Francisco Bay National Wildlife Refuge, California. Lines denote Caspian tern colony extents in 2015 (solid yellow), 

2016 (dashed green), and 2017 (dotted red). Photograph by Real Time Research, June 21, 2017. 
33 

Breeding pairs 2015Breeding pairs 2016Breeding pairs 2017

Fledglings 2015Fledglings 2016Fledglings 2017

Fledglings/breeding pair 2015Fledglings/breeding pair 2016Fledglings/breeding pair 2017

Nests/colony area 2015Nests/colony area 2016Nests/colony area 2017

Number

0

100

200

300

400

500

600

700

800

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

A16 

SF2 

Overall

Figure 17. Total number of Caspian tern breeding pairs and fledglings, and the apparent breeding success 

(fledglings per breeding pair) and apparent nest density (nests per square meter of colony area) at Ponds A16 and

SF2, and overall, Don Edwards San Francisco Bay National Wildlife Refuge, California, 2015–17. 
34 

Table 1. Summary of social attraction efforts at three islands in Pond A16 and four islands in Pond SF2, Don 

Edwards San Francisco Bay National Wildlife Refuge, California, 2015–17. 

[Note that the number of decoys varied among years because of manufacturer delivery dates and more decoys becoming 

available from partner agencies] 

Pond Island 

number Type Number of decoys

Date of decoy and call 

system deployment

Date of decoy and call 

system retrieval

2015

A16 3 Snowy plover 6 March 6 September 23

A16 11 Caspian tern 128 March 4 September 23

A16 12 Caspian tern 58 March 4 September 23

SF2 10 Snowy plover 6 March 2 September 22

SF2 12 Caspian tern 50 March 3 September 22

SF2 17 Caspian tern 51 March 5 September 22

SF2 21 Caspian tern 104 March 3 September 22

2016

A16 3 Snowy plover 6 March 2 August 28

A16 11 Caspian tern 105 March 2 September 19

A16 12 Caspian tern 105 March 1 September 19

SF2 10 Snowy plover 6 March 9 August 4

SF2 12 Caspian tern 105 March 8 September 22

SF2 17 Caspian tern 105 March 9 September 22

SF2 21 Caspian tern 105 March 8 September 22

2017

A16 3 Snowy plover 6 March 9 September 19

A16 11 Caspian tern 115 March 9 October 2

A16 12 Caspian tern 115 March 3 September 26

SF2 10 Snowy plover 6 February 24 October 11

SF2 12 Caspian tern 115 March 1 September 28

SF2 17 Caspian tern 115 February 28 October 11

SF2 21 Caspian tern 115 March 1 October 4

 
35 

Table 2. Total number of Caspian terns observed and the number of surveys in which Caspian terns were 

observed on each island during 166 pond surveys conducted at Ponds A16 and SF2, Don Edwards San Francisco 

Bay National Wildlife Refuge, California, March–September 2015–17. 

[Islands modified for Caspian terns or snowy plovers are in bold type. Total number of surveys were as follows: 62 in 2015, 

55 in 2016, and 49 in 2017. –, birds were not observed on island.]

Island

Total number of Caspian terns observed during all surveys

(number of surveys where Caspian terns observed)

2015 2016 2017

Pond A16

1 4 (5) 6 (5) –

2 5 (4) 4 (4) 6 (3) 13 1 (1) 1 (1) –

4 38 (18) 66 (25) 156 (21)

5 1 (1) 5 (4) 3 (2)

6 9 (6) 15 (11) 41 (6)

7 8 (7) 12 (9) 9 (6)

8 84 (12) 18 (13) 7 (5)

9 13 (8) 7 (4) 4 (3)

10 1 (1) 1 (1) – 2,311 1,985 (49) 3,530 (52) 5,284 (45) 212 85 (16) 8 (7) 68 (14)

13 6 (5) 3 (3) 3 (3)

14 2 (2) 1 (1) 5 (4)

15 1 (1) 4 (4) –

16 1 (1) 1 (1) –

17 1 (1) – 9 (3)

18 18 (1) 3 (2) 12 (2)

19 4 (1) 1 (1) 12 (2)

20 – 1 (1) 1 (1)

Pond SF2

2 – – 1 (1)

3 – – 1 (1)

4 – 1 (1) –

7 – 1 (1) 2 (2)

8 3 (1) – –

9 – 1 (1) – 110 3 (2) 1 (1) – 212 15 (10) 2 (2) 20 (9)

13 1 (1) 3 (1) –

14 3 (3) 6 (4) 12 (5)

16 2 (2) 2 (2) 9 (3) 217 178 (43) 211 (29) 245 (31)

18 14 (8) 65 (15) 15 (10)

19 – 18 (6) 33 (9)

20 21 (11) 16 (5) 56 (11)

2,321 4,538 (58) 6,469 (54) 9,130 (49)

22 14 (8) 6 (6) 4 (2)

24 10 (2) 2 (2) 2 (1)

25 2 (2) 9 (6) 136 (13)

26 – 2 (2) 13 (5)

27 – – 4 (3)

28 – – 1 (1)

29 1 (1) – 1 (1)

30 – 2 (1) – 1Denotes islands with social attractions for snowy plovers. 2Denotes islands with social attractions for Caspian terns. 3Denotes islands where Caspian terns nested.
36 

Table 3. Breeding metrics on seven modified islands with social attraction measures for Caspian terns and snowy plovers in Ponds A16 and SF2, Don 

Edwards San Francisco Bay National Wildlife Refuge, California, 2015–17. 

[Dash (–) denotes that the calculation is not possible. Numbers in parentheses denote the percentage of change from the previous year (%), where light gray shading 

indicates an increase and dark gray shading indicates a decrease. Peak number of nests: High count or peak number of nests over the nesting season. Estimated 

number of breeding pairs: Calculated as the high count or peak number of nests, plus the number of nests active 40 days prior to, 40 days after, 80 days prior to, and 

80 days after, the peak. See report section, “Methods” for details. Peak number of fledglings: High count or peak number of fledglings over the nesting season. 

Estimated number of chicks fledged: Calculated as the high count or peak number of fledglings, plus the number of fledglings 40 days prior to and 40 days after the 

peak. Breeding success: Calculated by dividing the number of chicks fledged by the number of breeding pairs.

 Apparent nest density (nests/island m2

): Calculated by 

dividing the number of nests on the island at the time the aerial photograph of the colony was taken by the total island area. Apparent nest density (nests/colony m2

):

Calculated by dividing the number of nests on the island at the time the aerial photograph of the colony was taken by the colony area. m2

, square meter] 

Year(s) Pond Island Peak number 

of nests

Estimated 

number of 

breeding 

pairs

Peak number 

of fledglings

Estimated 

number of 

chicks 

fledged

Breeding 

success 

(fledglings/

breeding pair)

Island 

gravel 

area (m2)

Colony area 

(m2)

Apparent nest density

(nests/island m2) (nests/colony m2)

Caspian tern

2015 A16 11 1

73 79 254 54 0.68 1,603 130 0.02 0.20

2015 A16 12 0 0 0 0 – 1,247 – – – 

2015 SF2 12 0 0 0 0 – 1,841 – – – 

2015 SF2 17 1 1 1 1 1.00 1,518 – 0.001 – 

2015 SF2 21 1

150 165 2119 114 0.69 1,693 236 0.03 0.24

2015 TOTAL 224 245 174 169 0.69 7,903 366 0.03 0.23

2016 A16 11 137(+88%) 169(+114%) 35(-35%) 51(-6%) 0.30(-56%) 1,603 266(+105%) 0.06(+258%) 0.35(+75%)

2016 A16 12 0 0 0 0 – 1,247 – – – 

2016 SF2 12 0 0 0 0 – 1,841 – – – 

2016 SF2 17 0 0 0 0 – 1,518 – – – 

2016 SF2 21 180(+20%) 265(+61%) 55(-54%) 72(-37%) 0.27(-61%) 1,693 481(+104%) 0.09(+175%) 0.22(+35%)

2016 TOTAL 317(+42%) 434(+77%) 90(-48%) 123(-27%) 0.28(-59%) 7,903 747(+104%) 0.08(+201%) 0.33(+48%)
37 

Year(s) Pond Island Peak number 

of nests

Estimated 

number of 

breeding 

pairs

Peak number 

of fledglings

Estimated 

number of 

chicks 

fledged

Breeding 

success 

(fledglings/

breeding pair)

Island 

gravel 

area (m2)

Colony area

(m2)

Apparent nest density

(nests/island m2) (nests/colony m2)

Caspian tern (Continued)

2017 A16 11 167(+22%) 227(+34%) 95(+171%) 106(+108%) 0.47(+55%) 1,603 460(+73%) 0.09(+59%) 0.32(-8%)

2017 A16 12 0 0 0 0 – 1,247 – – – 

2017 SF2 12 0 0 0 0 – 1,841 – – – 

2017 SF2 17 0 0 0 0 – 1,518 – – – 

2017 SF2 21 334(+86%) 437(+65%) 120(+118%) 133(+85%) 0.30(+12%) 1,693 934(+94%) 0.15(+62%) 0.27(-16%)

2017 TOTAL 501(+58%) 664(+53%) 215(+139%) 239(+94%) 0.36(+27%) 7,903 1,394(+87%) 0.12(+61%) 0.29(-14%)

Snowy plover

2015–17 A16 3 0 0 0 0 – 1,318 – – – 

2015–17 SF2 10 0 0 0 0 – 1,420 – – – 

2015–17 TOTAL 0 0 0 0 – 2,738 – – – 

1

In 2015, the number of Caspian tern nests under incubation had a bimodal distribution with 2 distinct peaks. Therefore, we summed the high count of nests from these 

two peaks to estimate the peak number of nests.

2

In 2015, the number of Caspian fledglings had a bimodal distribution with 2 distinct peaks. Therefore, we summed the high count from these two peaks to estimate the 

peak number of fledglings.
38 

Table 4. Sightings of 52 color-banded Caspian terns at Ponds A16 and SF2, Don Edwards San Francisco Bay National Wildlife Refuge, California, 

2015–17. 

[Banding and past breeding information provided by Kirsten Bixler and Yasuko Suzuki (Oregon State University, written commun., November 11, 2017). Tern: 2, 9, 

35, and 38 were unidentifiable because of band loss. Each tern was banded on its left leg with a color band combination, and on its right leg with either a color band 

combination or a single alphanumeric band. A dash (–) indicates that an individual did not have a particular band type. An X denotes a missing band or unread 

alphanumeric code] 

Tern Year(s) 

Left leg Right leg

Pond/

island 

Breeding 

behavior(s) 

observed? 

Banding 

location 

Banding 

year 

Age at 

banding Top Middle Bottom Top Middle Bottom Band 

color

Code 

color

Alphanumeric 

code

1 2015 Light 

blue

Light 

blue Light blue Dark blue White Metal – – – SF2/21 No Brooks Island 2005 Chick

2 2015 Orange Orange X Light 

blue White Metal – – – SF2 No Unknown Unknown Unknown

3 2015 Orange Light 

blue Metal – – – Yellow White XX95 SF2/21 No East Sand Island 2006 Chick

4 2015, 

2017 Red Orange Metal – – – Yellow Black C379 SF2 Yes Brooks Island 2008 Adult

5 2015 Light 

blue Green Metal – – – Yellow Black C073 SF2 No East Sand Island 2007 Chick

6 2016 Red Orange Dark 

green Yellow Light green Metal – – – A16/11 No East Sand Island 2004 Chick

7 2016, 

2017 Orange Dark blue Metal – – – Yellow Black J677 A16/11 Yes East Sand Island 2010 Chick

8 2016, 

2017

Dark 

green Yellow Metal – – – Yellow Black K400 A16/11 No Sheepy Lake 2012 Chick

9 2016 X Yellow Orange X Metal Dark 

green – – – SF2/21 No Unknown Unknown Unknown

10 2016 Dark 

blue Orange White Light green Metal White – – – SF2/21 No Knight Island 2003 Chick

11 2016, 

2017

Dark

blue

Dark 

blue

Dark 

blue

Dark 

green

Light 

blue Metal – – – SF2/21 Yes Brooks Island 2005 Chick

12 2016, 

2017 Red Dark blue Yellow Yellow White Metal – – – SF2/21 Yes Brooks Island 2004 Chick

13 2016, 

2017 Orange Light green

Dark 

blue Yellow White Metal – – – SF2/21 Yes Brooks Island 2004 Chick

14 2016, 

2017

Dark 

blue

Light 

green Yellow Dark green

Light 

blue Metal – – – SF2/21 No Brooks Island 2005 Chick
39 

Tern Year(s) 

Left leg Right leg

Pond/

island 

Breeding 

behavior(s) 

observed? 

Banding 

location 

Banding 

year 

Age at 

banding Top Middle Bottom Top Middle Bottom Band 

color

Code 

color

Alphanumeric 

code

15 2016, 

2017

Dark 

blue Red Dark green

Dark 

green

Light 

blue Metal – – – SF2/21 Yes Brooks Island 2005 Chick

16 2016, 

2017 Orange Light green Metal – – – Yellow Black J731 SF2/21 Yes Port of Bellingham 2010 Chick

17 2016 Orange Light 

green Metal – – – Yellow Black J520 SF2/21 No Port of Bellingham 2010 Chick

18 2016 White Light 

blue Metal – – – Yellow Black F108 SF2/21 No Goose Island 2011 Chick

19 2016, 

2017

Dark 

blue Orange Metal – – – Yellow Black E380 SF2/17&21 Yes Eden Landing 2009 Chick

20 2016, 

2017

Dark 

green Yellow Metal – – – Yellow Black K320 SF2/21 Yes Sheepy Lake 2012 Chick

21 2016 Red Orange Metal – – – Yellow Black C548 SF2/21 Yes Brooks Island 2008 Chick

22 2016,

2017

Dark 

blue Orange Metal – – – Yellow Black E240 SF2/17&21 Yes Brooks Island 2009 Adult

23 2016 Red Orange Metal – – – Yellow Black C379 SF2/21 Yes Brooks Island 2008 Adult

24 2016 White Yellow Metal – – – Yellow Black F274 SF2/17 No Sheepy Lake 2011 Chick

25 2016, 

2017 White Light blue Metal – – – Yellow Black F118 SF2/21 Yes Goose Island 2011 Chick

26 2016, 

2017 Red Orange Metal – – – Yellow Black C629 SF2/17&21 Yes Brooks Island 2008 Chick

27 2016, 

2017

Dark 

blue

Dark 

blue Metal – – – Yellow Black E858 SF2/17&21 No East Sand Island 2009 Chick

28 2016 Red Dark 

blue Metal – – – Yellow Black C881 SF2/21 No East Sand Island 2008 Chick

29 2016, 

2017 Red Orange Metal – – – Yellow Black C670 SF2/21 Yes Brooks Island 2008 Chick

30 2016, 

2017

Dark 

blue Orange Metal – – – Yellow Black E399 SF2/21 Yes Brooks Island 2009 Chick

31 2016 Orange White Metal – – – Red White A123 SF2/21 No East Sand 

Island 2010 Adult

32 2017 White White Light 

Green

Light 

green Metal Dark blue – – – SF2/21 Yes East Sand Island 2003 Chick
40 

Tern Year(s) 

Left leg Right leg

Pond/

island 

Breeding 

behavior(s) 

observed? 

Banding 

location 

Banding 

year 

Age at 

banding Top Middle Bottom Top Middle Bottom Band 

color

Code 

color

Alphanumeric 

code

33 2017 Dark 

blue

Dark 

blue Yellow Dark green

Light 

blue Metal – – – SF2/21 Yes Brooks Island 2005 Chick

34 2017 Dark 

blue Orange Light green

Light 

green Metal White – – – A16/11, SF2/21 No Brooks Island 2003 Chick

35 2017 X Yellow Red X X Light 

green – – – A16/11, SF2/21 Yes Unknown Unknown Unknown

36 2017 Orange White Metal – – – X X X A16/11 No East Sand 

Island

2006 or 

2010 Adult

37 2017 Dark 

blue Orange Metal – – – Yellow X X A16/11 No

Brooks Island 

or 

Eden Landing

2009 Adult or 

Chick

38 2017 White Orange Metal – – – Yellow Black X A16/11 No Unknown Unknown Unknown

39 2017 Red Dark 

blue Metal – – – Red White A187 A16/11, SF2/21 No East Sand Island 2013 Chick

40 2017 Orange Dark 

blue Metal – – – Yellow Black A393 SF2/21 Yes East Sand Island 2006 Chick

41 2017 Red Orange Metal – – – Yellow Black C368 SF2/21 Yes Brooks Island 2008 Adult

42 2017 Red Orange Metal – – – Yellow Black C392 SF2/21 No Brooks Island 2008 Adult

43 2017 Red Orange Metal – – – Yellow Black C548 SF2/21 Yes Brooks Island 2008 Chick

44 2017 Red Orange Metal – – – Yellow Black C556 SF2/21 Yes Brooks Island 2008 Chick

45 2017 Red Orange Metal – – – Yellow Black C615 A16/11 Yes Brooks Island 2008 Chick

46 2017 Red Orange Metal – – – Yellow Black C617 SF2/21 Yes Brooks Island 2008 Chick

47 2017 Dark 

blue Orange Metal – – – Yellow Black E276 SF2/21 No Brooks Island 2009 Adult

48 2017 Dark 

blue Orange Metal – – – Yellow Black E383 SF2/21 Yes Eden Landing 2009 Chick

49 2017 Dark 

blue Orange Metal – – – Yellow Black E386 A16/11, SF2/21 No Eden Landing 2009 Chick
41 

Tern Year(s) 

Left leg Right leg

Pond/

island 

Breeding 

behavior(s) 

observed? 

Banding 

location 

Banding 

year 

Age at 

banding Top Middle Bottom Top Middle Bottom Band 

color

Code 

color

Alphanumeric 

code

50 2017 Dark 

blue Orange Metal – – – Yellow Black E417 SF2/21 Yes Brooks Island 2009 Chick

51 2017 Dark 

green Red Metal – – – Yellow Black K198 A16/11 No Malheur Lake 2012 Chick

52 2017 Dark 

blue Orange Metal – – – X X X SF2/21 No

Brooks Island 

or 

Eden Landing

2009 Adult or 

Chick

Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

Director, Western Ecological Research Center

U.S. Geological Survey

3020 State University Drive East

Sacramento, California 95819

https://www.werc.usgs.gov/
Hartman and others—Social Attraction Measures Used to Establish Caspian Tern Nesting Colonies in San Francisco Bay, California—Final Report—Open-File Report 2018–1136

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20181136