Waterbird Nest Monitoring Program in San Francisco Bay (2005–10) 

Open-File Report 2012–1145

U.S. Department of the Interior

U.S. Geological Survey
Cover: 

Top Row (left to right): American Avocet Adult ( Ken Phenicie ); Black-necked Stilt Adult (Ken Phenicie); Forster’s Tern Adult (Ken Phenicie).

Bottom Row (left to right): Forster’s Tern Nest (Michael Kern); Forster’s Tern Chick (Michael Kern); American Avocet Chick (Ken Phenicie).

Photographs used with permission.
 

Waterbird Nest Monitoring Program in San Francisco Bay 

(2005–10) 

By Josh T. Ackerman and Mark P. Herzog

Open-File Report 2012–1145

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Ackerman, J.T., and Herzog, M.P., 2012, Waterbird nest monitoring program in San Francisco Bay (2005–10): U.S. 

Geological Survey Open-File Report 2012–1145, 16 p. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
iii

Contents

Introduction.................................................................................................................................................................... 1

Nest Abundance ............................................................................................................................................................ 2

Methods ..................................................................................................................................................................... 2

Results and Discussion.............................................................................................................................................. 2

Nest Success................................................................................................................................................................. 3

Methods ..................................................................................................................................................................... 3

Results and Discussion.............................................................................................................................................. 4

Clutch Size .................................................................................................................................................................... 4

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

Results and Discussion.............................................................................................................................................. 4

Nest Initiation Date ........................................................................................................................................................ 4

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

Results and Discussion.............................................................................................................................................. 5

Hatching Success.......................................................................................................................................................... 5

Methods ..................................................................................................................................................................... 5

Results and Discussion.............................................................................................................................................. 5

Case Studies: The Value of a Long-Term Waterbird Nest Monitoring Program for the South San Francisco Bay ........ 5

Example 1. Waterbird Response to Wetland Management........................................................................................ 6

Example 2. Effects of Gull Intrusions on Breeding Waterbird Colonies...................................................................... 6

Conclusions................................................................................................................................................................... 7

Literature Cited.............................................................................................................................................................. 8

Figures

Figure 1. Distribution and abundance of nearly 11,000 waterbird nests monitored by the USGS during 2005–10

in salt ponds and managed wetlands planned for restoration within the South Bay Salt Pond Restoration Project 

(white hatching), Don Edwards San Francisco Bay National Wildlife Refuge (red), and Eden Landing Ecological 

Reserve (orange) in the San Francsico Bay, California............................................................................................... 10

Figure 2. Estimated numbers and 95 percent confidence interval of Black-necked Stilt, American Avocet, and 

Forster’s Tern nests initiated in South San Francisco Bay, California (2005–10)........................................................ 11

Figure 3. Annual nest success and 95 percent confidence interval for Black-necked Stilt, American Avocet, and 

Forster’s Tern in South San Francisco Bay, California (2005–10)............................................................................... 12

Figure 4. Nesting period for Black-necked Stilt, American Avocet, and Forster’s Tern in South San Francisco Bay, 

California (2005–10).................................................................................................................................................... 13

Figure 5. Waterbird nest abundance and nesting success with 95 percent confidence intervals in pond A12 

during the creation of new nesting islands (all species combined) .............................................................................. 14

Table

Table 1. Summary of nesting parameter estimates for American Avocet, Black-necked Stilt, and Forster’s Tern in 

South San Francisco Bay, California (2005–10).......................................................................................................... 15
iv

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1 

Waterbird Nest Monitoring Program in San Francisco Bay 

(2005–10) 

By Josh T. Ackerman and Mark P. Herzog

Introduction

Historically, Forster’s Terns (Sterna forsteri), American Avocets (Recurvirostra americana), 

and Black-necked Stilts (Himantopus mexicanus) were uncommon residents of San Francisco Bay, 

California (Grinnell and others, 1918; Grinnell and Wythe, 1927; Sibley, 1952). Presently, however, 

avocets and stilts are the two most abundant breeding shorebirds in San Francisco Bay (Stenzel and 

others, 2002; Rintoul and others, 2003). More than 4,000 avocets and 1,000 stilts, roughly 20 percent of 

their San Francisco Bay wintering populations, breed within the estuary, making San Francisco Bay the 

largest breeding area for these species on the Pacific Coast (Stenzel and others, 2002; Rintoul and 

others, 2003). Forster’s Terns were first observed breeding in the San Francisco Bay in 1948 (110 

nests); they had increased to over 4000 individuals by the 1980s (Sibley, 1952; Gill, 1977; Harvey and 

others, 1992; Carter and others, 1990) and were estimated at 2000–3000 for 1998–2002; (Strong and 

others, 2004). 

It is hypothesized that the relatively large size of the current waterbird breeding populations is a 

result of the creation of artificial salt evaporation ponds from the 1930s through the 1950s (Gill, 1977; 

Goals Project, 1999). Until recently, these salt ponds and associated islands used by waterbirds for 

nesting have been managed relatively similarly and have supported large breeding waterbird 

populations. Recently, the South Bay Salt Pond Restoration Project has implemented plans to convert 

50–90 percent of the 15,000 acres of salt ponds in the South San Francisco Bay back to tidal marsh 

habitat. Therefore, there is concern that the Restoration Project, while benefiting other native species, 

could negatively influence local breeding populations of waterbirds that are reliant on salt pond habitats 

for both breeding and foraging. A primary goal of the South Bay Salt Pond Restoration Project is to 

maintain current breeding waterbird populations (South Bay Salt Pond Long-Term Restoration Project, 

2004); thus, specific efforts are planned to ensure that the Restoration Project enhances the habitats of 

the remaining salt ponds for breeding waterbirds. 

Here, we provide a summary of nesting ecology data for Forster’s Terns, American Avocets, and 

Black-necked Stilts, collected from 2005 to 2010 in the areas of the South Bay Salt Pond Restoration 

Project, including lands managed by the Don Edwards San Francisco Bay National Wildlife Refuge and 

Eden Landing Ecological Reserve (fig. 1). These results provide baseline conditions for breeding 

waterbirds prior to implementation of most restoration actions and can be used to both guide future 

restoration actions as well as to determine the effect of the South Bay Salt Pond Restoration Project on 

breeding waterbirds. It is imperative to continue to collect nesting waterbird data annually to assess the 

response of birds to the South Bay Salt Pond Restoration Project. 
2 

Nest Abundance 

Methods

Nest abundance is defined as the cumulative total number of nests initiated over the entire 

breeding season. Nest abundance was estimated separately for the three focal species at each site and for 

each year. To estimate true nest abundance it is important to take into account the limitations of nest 

monitoring methodology; specifically, that it is not possible to observe nests that initiate and fail within 

the interval between nest checks (typically 7 days in this study). If we only presented the number of 

nests observed, we would underestimate the true number of nests that were actually initiated. To correct 

for this, we adjusted the number of new nests we observed during each visit to include nests that we did 

not find. We did this by dividing the number of new nests by the estimated survival rate of a nest during 

that period. We then summed all the adjusted nests initiated over each visit to estimate the total number 

of nests initiated during the breeding season. This method of estimating nest abundance is more accurate 

than surveying pair numbers or counting peak numbers of nests (Seavy and Reynolds, 2009; Herzog and 

Ackerman, unpublished data). We were unable to correct nest abundance estimates for any site that 

contained less than 0.01 percent overall nest success and, therefore, instead present the total nests 

observed in these cases (9 occasions). 

Results and Discussion

Because nest monitoring effort varied across years and among sites, it is currently not possible to 

estimate the total abundance of nesting waterbirds across the entire South San Francisco Bay. While our 

estimates might be able to provide an index of breeding waterbird populations, there are no data that 

exist to estimate true baseline numbers of breeding waterbirds on lands managed by the South Bay Salt 

Pond Restoration Project for either the Don Edwards San Francisco Bay National Wildlife Refuge or 

Eden Landing Ecological Reserve. Thus, in this report, we provide population estimates and trends for 

each species within ponds where U.S. Geological Survey (USGS) nest monitoring has occurred for 

multiple years. 

Forster’s Tern nest abundance within specific ponds tends to be variable (fig. 2). Some of the 

most important and consistent breeding locations for Forster’s Terns in the San Francisco Bay are in salt 

ponds on nesting islands. In particular, salt ponds A7, A8, and A16 in the Alviso Salt Pond Complex, 

and A1, A2W, AB1, and AB2 in the Moffett Salt Pond Complex, contain nearly 60 percent of all the 

monitored breeding Forster’s Terns in the South San Francisco Bay. The important nesting islands in 

Ponds A7 and A8 will likely be submerged in 2011 when the A5/A7/A8 complex is flooded as part of 

the opening of A8 to Alviso Slough for flood control purposes. If these nesting sites do indeed become 

unusable, then more than 300 nesting pairs will be displaced. This represents 25–35 percent of the 

monitored population of Forster’s Terns in the South San Francisco Bay. Continued monitoring of 

Forster’s Tern breeding populations is necessary to determine if these birds relocate within lands 

managed under the South Bay Salt Pond Restoration Project or if they are lost from the breeding effort 

in the entire South San Francisco Bay. Similarly, salt ponds A1 and A2W are very important breeding 

habitat and contain 21 percent of all the monitored breeding Forster’s Tern nests in the South San 

Francisco Bay. Nesting islands in ponds A1 and A2W would benefit from being bolstered to avoid 

continued erosion. Careful consideration of the value of these islands as nesting sites can be a useful 

part of planning if these ponds are to be converted to tidal marsh habitat. 
3 

American Avocets are more ubiquitous in the South San Francisco Bay, nesting in more 

wetlands than Forster’s Terns. However, avocets generally nest on islands in salt ponds sympatrically 

with Forster’s Terns. The primary nesting locations for avocets are salt ponds A7, A8, and A16 in the 

Alviso Salt Pond Complex, New Chicago Marsh, and A1 and A2W in the Moffett Salt Pond Complex. 

Similar to their importance to terns, the islands within A7 and A8 currently provide nesting habitat for 

approximately 300–400 nesting pairs annually, and A8 supports by far the most American Avocets 

compared to any other monitored site (fig. 2). Continued monitoring and management activities to 

enhance other wetland areas for breeding are necessary to ensure the South Bay Salt Pond Restoration 

Project does not lose this breeding effort when A7 and A8 salt ponds are converted into tidal marsh. 

Black-necked Stilts are not as ubiquitous as avocets or Forster’s Terns. When they do nest with 

Forster’s Terns and avocets, they tend to nest at much lower densities. Instead, the vast majority of the 

entire South San Francisco Bay population of stilts nests within New Chicago Marsh in the Alviso area. 

New Chicago Marsh is a managed marsh that is dominated by pickleweed (Salicornia virgnica) 

vegetation. The average number of stilt nests initiated annually in New Chicago Marsh was 180, 

whereas in all others sites where we sampled more than 1 year, the average number of stilt nests was

less than 50 (fig. 2). The next most abundant stilt nesting area in the South San Francisco Bay is in Eden 

Landing Ecological Reserve. These data demonstrate the over-riding importance of New Chicago Marsh 

for stilt breeding in the South San Francisco Bay and could act as a model for the creation of similar 

semi-tidally inundated habitat for breeding waterbirds in the South Bay Salt Pond Restoration Project. 

Nest Success

Methods

Nest survival was estimated separately for each species, site, and year by performing an 

intercept-only logistic exposure model (Shaffer, 2004). For data prior to 2010, weekly nest visit data are 

currently not available electronically; thus, the number of exposure days was calculated as the days 

between when the nest was first detected and the date when the final fate (success/failure) was 

determined. A successful nest was defined as a nest where at least one egg hatched. For some nests, it 

was possible to determine the exact date of the nest’s fate. For example, when wet chicks were found in 

the nest bowl, we assumed that hatching occurred within the past 24 hours. However, in most cases, the 

final nest fate date was estimated in the same manner as is done for Mayfield nest success—that is, the 

date that represents the midpoint of the final visit interval that determined the nest fate (Mayfield, 1961; 

Mayfield, 1975; Johnson, 1979). Only nests where at least 1 day of exposure occurred were included in 

analyses. The USGS has proposed to digitize the complete nest histories for all 11,000 waterbird nests 

monitored in the South San Francisco Bay. Once these data are in a relational database, more powerful 

and complex analyses on the factors that affect nest survival and other reproductive parameters will be 

possible. For 2009 and 2010 data, we were able to enter nest data at the nest visit level. Thus, for these 2 

years, exposure days were estimated uniquely for each visit. 

Once daily nest survival was estimated, nest success was calculated by exponentiating the daily 

nest survival estimate by the expected nest age for a successful nest. For these waterbirds, we chose a 

nest age of 27 days, representing a day for the laying of each egg (3–4 egg clutch) and a 23–24 day 

incubation period (Robinson and others, 1997; Robinson and others, 1999; McNicholl and others, 

2001). 
4 

Results and Discussion

For all three waterbird species, nest success was highly variable among sites and years. For 

example, we observed almost complete colony failure in some years in wetlands that historically have 

been very productive. These results indicate that waterbird reproductive success is highly dependent on 

local conditions, such as predation and wetland management activities. Overall, nest success was 64

percent for Forster’s Terns, 39 percent for avocets, and 36 percent for stilts in the South San Francisco 

Bay (fig. 3). 

Clutch Size

Methods

Average clutch size was estimated separately for each species, site, and year. Only nests where 

researchers were confident that full clutch size was observed were included. Thus, nests that failed, were 

partially depredated during laying, or were found more than 8 days after incubation (as determined by 

floating eggs to determine development stage; Ackerman and Eagles-Smith, 2010) were excluded from 

analyses.

Results and Discussion

Clutch size was generally consistent across wetland sites and years for each species. Average 

clutch size for Forster’s Terns was 2.3 eggs, for avocets was 3.5 eggs, and for stilts was 3.5 eggs in the 

South San Francisco Bay. See table 1 for details.

Nest Initiation Date

Methods

Nest initiation date was defined as the date at which an individual female laid the first egg in the 

nest. Median nest initiation date was estimated separately for each species, site, and year. Only nests 

where researchers were confident that nest initiation date could be estimated were included. Nest 

initiation date was estimated by subtracting the initial clutch size, plus the average incubation stage of 

the eggs on the day the nest was first discovered, from the date the nest was found. 
5 

Results and Discussion

Stilts and avocets initiated nests earlier (10 percent of nests occurred on or before April 15) than 

Forster’s Terns (10 percent of nests occurred on or before May 15; table 1). Comparisons of the median 

nest initiation date showed that stilts nested 12 days earlier (May 3) than avocets (May 15) and almost a 

month earlier than Forster’s Terns (May 30). The nesting period (defined as the central span of days 

where 80 percent of the individuals initiated nests, centered on the mean) was shorter for Forster’s Terns 

(38 days) than stilts (48 days) or avocets (59 days), indicating that Forster’s Terns were more 

synchronous in their nesting activities, which is common for breeding seabirds. Accordingly, Forster’s 

Terns could be considered to be at a higher risk to perturbations during the shorter period for breeding. 

Management activities for Forster’s Terns can usefully focus on providing ideal breeding conditions 

during this window of time and restrict potential disturbances. 

Hatching Success

Methods

Hatching success is defined as the proportion of eggs that hatched within a nest that was 

successful (for example, where at least one egg hatched). Thus, only successful nests, where full clutch 

size and final fate for each individual egg were known, were included in our analyses. 

Results and Discussion

Overall, hatching success was 95 percent for Forster’s Terns, 94 percent for avocets, and 98 

percent for stilts in the South San Francisco Bay. Although not as variable as nest success, hatching 

success also varied among wetland sites and years. See table 1 for details.

Case Studies: The Value of a Long-Term Waterbird Nest Monitoring Program for 

the South San Francisco Bay

These data represent the foundation of a long-term adaptive management monitoring program 

for breeding waterbirds in the South San Francisco Bay and are summarized in this report and provided 

to the Integrated South Bay Avian Database (ISBA-db) for easy access by managers. These breeding 

waterbird data have been collected by the USGS since before the initial phases of the South Bay Salt 

Pond Restoration Project and have already informed managers about significant components of the 

Restoration Project. Tracking trends in nest abundance and nest survival at specific sites can inform 

managers about the direct effects of management actions (such as changes in water levels or the creation 

of new nesting islands). Coupled with ancillary data (such as methyl mercury contamination in 

waterbird eggs and fish, water quality data, and vegetation and landscape metrics), these data are an 

even stronger tool to guide the implementation of management actions under the South Bay Salt Pond 

Restoration Project as well as for the regular management of wetlands on Federal and State Wildlife 

Refuges. In the following sections, we present two scenarios for the application of science produced by 

the annual USGS Waterbird Nest Monitoring Program in South San Francisco Bay. 
6 

Example 1. Waterbird Response to Wetland Management

The South Bay Salt Pond Restoration Project in San Francisco Bay will restore 50–90 percent of 

15,000 acres of salt ponds to tidal marsh in order to reverse the loss (greater than 80 percent) of tidal 

marsh habitats within the San Francisco Bay Estuary. While the restored tidal marsh habitats will 

benefit many animals, another goal of the Restoration Project is to maintain current breeding 

populations of birds that currently use these pond habitats heavily (South Bay Salt Pond Long-Term 

Restoration Project, 2004). A way to mitigate this loss of nesting habitat is to create additional nesting 

habitat within the few ponds that remain after habitat restoration. Thus, the South Bay Salt Pond 

Restoration Project is implementing plans to reconfigure and enhance existing ponds by increasing 

foraging opportunities and the number of nesting islands. 

In cooperation with the Don Edwards San Francisco Bay National Wildlife Refuge, we 

experimentally manipulated water levels in pond A12 to examine the response of waterbird breeding 

effort and nest success to different water levels and to assess the bioaccumulation of methylmercury in 

bird eggs and fish, which was an unintended consequence of this management action (Ackerman and 

others, 2010a). We compared results from pond A12 to adjacent reference ponds and marshes. In 2008, 

a number of nesting islands were created by lowering water levels in pond A12. Our nest monitoring 

data were able to detect a strong waterbird nesting response to the Refuge’s water management action, 

with over 500 nests initiated during the following nesting season. However, at the same time, we found 

that mercury concentrations in waterbird eggs were higher at pond A12 than in reference ponds and 

marshes. Therefore, whereas water management actions had a strong positive effect of enhancing 

waterbird nesting opportunities in A12, it also could have had the unintended consequence of increasing 

the production and bioaccumulation of mercury in waterbird eggs, which is known to affect 

reproductive success in these species (Ackerman and others, 2007; Ackerman and Eagles-Smith, 2008). 

Continued adaptive management processes coupled with annual monitoring are necessary to limit the 

potential for negative outcomes (fig. 5).

Example 2. Effects of Gull Intrusions on Breeding Waterbird Colonies

Because of the USGS’s on-going nest monitoring program, we had the opportunity to couple 

nest survival data with movement patterns of predatory California Gulls. We leveraged our nest 

monitoring data in 2009 to assess the effect of California Gull (Larus californicus) predation on nest 

survival of waterbirds (Ackerman and others, 2010b). 

During 2009, we monitored nests of Forster’s Terns, avocets, and stilts at pond A16 by using 

standard USGS nest monitoring protocol. Simultaneously, we monitored California Gull intrusions in 

pond A16 by using a novel telemetry surveillance technique. We placed radio-telemetry data loggers on 

two waterbird nesting islands in pond A16 to detect gull intrusion events near the nesting colony. Fifty 

California Gulls were radio-marked in the previous spring of 2008 with transmitters that lasted for an 

estimated 18-month period. Thus, the recorded intrusions depended on previously radio-marked 

California Gulls returning to San Francisco Bay to breed, the transmitters surviving more than 1 year, 

and the radio-marked gulls actually using pond A16. Besides general information on nest success for 

waterbird species on pond A16, our results also detected seasonal and diurnal patterns in the use of A16 

by California Gulls. Of significant importance was our finding that while many gulls visited pond A16, 

there were specific gulls that spent substantially more time with the waterbird colonies than others.

Thus, management actions might not need to focus on the entire California Gull colony but rather on 

those “specialist” gulls that specifically affect waterbirds by depredating eggs and chicks (Ackerman 

and others, 2006; Ackerman and others, 2009). 
7 

Conclusions

With 6 years of USGS waterbird nest monitoring data, a significant amount of knowledge about 

the conservation needs of breeding waterbirds has been gained within San Francisco Bay. At the same 

time, we also have been able to understand what data are still needed. First, and foremost, is the need to 

recognize the value of a long-term waterbird nest monitoring program and continue it into the future. 

Given the finite amount of available breeding habitat and the significant linkages between 

environmental conditions and reproductive success, breeding waterbirds could be the taxonomic group 

most sensitive to management actions by the South Bay Salt Pond Restoration Project. As seen in this 

report, nesting waterbirds show immediate responses to wetland management and are useful indicators 

to assess these management actions. Many of the restoration actions (water management, creating and 

enhancing islands for nesting, and others) affect breeding waterbirds directly, and therefore, monitoring 

the nesting effort and success of waterbirds can give immediate feedback to the adaptive management 

process of the South Bay Salt Pond Restoration Project. 

It is a high priority within the South Bay Salt Pond Restoration Project to ensure that restoration 

efforts do not negatively affect breeding waterbird populations. To successfully achieve this goal, the 

Restoration Project is enhancing or creating new breeding waterbird habitats in order to compensate for 

losses elsewhere as a result of salt pond restoration. The Restoration Project has recognized that there 

are a number of uncertainties in many of these actions and has embraced an adaptive management 

approach as an essential method to inform management by using continued research and monitoring. 

We believe three overarching conclusions resulting from this study are: 

1. The implementation of an annual waterbird nest monitoring program, comparable to the 

monthly salt-pond surveys for birds, will develop a long-term dataset to inform wetland 

management, assess the effectiveness of restoration and management actions, and guide 

future implementation of the South Bay Salt Pond Restoration Project. The USGS has been 

able to fund this current 6-year nest monitoring dataset through studies on the effects of 

mercury contamination on avian reproductive success (Ackerman and others, 2007; 

Ackerman and Eagles-Smith, 2008). However, without these projects, the nest monitoring 

activities will not be possible.

2. A comprehensive survey is needed to estimate the total populations of breeding waterbirds in 

the South San Francisco Bay that use wetlands within the South Bay Restoration Project, the 

Don Edwards San Francisco Bay National Wildlife Refuge, and Eden Landing Ecological 

Reserve. Currently, there are no appropriate data to estimate population sizes of breeding 

waterbirds for the entire South Bay; therefore, we currently have no way to assess the effect 

of the South Bay Salt Pond Restoration Project on breeding waterbird population sizes. 

3. In order for the adaptive management process of the South Bay Salt Pond Restoration 

Project to be most effective, communication between researchers and wetland managers is 

essential. Given the high priority that has been placed on maintaining avian populations and 

the rapid rate of the Restoration Project, it is important to develop an annual meeting in 

which an informal discussion can take place between researchers and managers, where

researchers can inform managers of recent findings and, importantly, managers can help 

guide the necessary research and nest monitoring questions that need to be addressed.
8 

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Ackerman, J.T., Eagles-Smith, C.A., Takekawa, J.Y., Bluso-Demers, J., Tsao, D., and Le Fer, D., 2009, 

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9263.2009.00233.x. 

Shaffer, T.L, 2004, A unified approach to analyzing nest success: The Auk, v. 121, no. 2, p. 526–540. 

Sibley, C.G., 1952, The Birds of the South San Francisco Bay region: San Jose State University, San 

Jose, California, 42 p. 

South Bay Salt Pond Long-Term Restoration Project: Mission, Goals, Guiding Principles and 

Objectives, 2004: South Bay Salt Pond Restoration Project. 

Stenzel, L.E., Hickey, C.M., Kjelmyr, J.E., and Page, G.W., 2002, Abundance and distribution of 

shorebirds in the San Francisco Bay area: Western Birds, v. 33, no. 2, p. 69–98. 

Strong, C.M., Spear, L.B., Ryan, T.P., and Dakin, R.E., 2004, Forster’s Tern, Caspian Tern, and 

California Gull colonies in San Francisco Bay: Habitat use, numbers and trends, 1982–2003: 

Waterbirds, v. 27, no. 4, p. 411–423. 
10

Figure 1. Distribution and abundance of nearly 11,000 waterbird nests monitored by the USGS during 2005–10 in 

salt ponds and managed wetlands planned for restoration within the South Bay Salt Pond Restoration Project 

(white hatching), Don Edwards San Francisco Bay National Wildlife Refuge (red), and Eden Landing Ecological 

Reserve (orange) in the San Francsico Bay, California. 
11

Figure 2. Estimated numbers and 95 percent confidence interval of Black-necked Stilt, American Avocet, and 

Forster’s Tern nests initiated in South San Francisco Bay, California (2005–10). Numbers of nests have been 

corrected to account for nests that initiated and failed (and thus not observed) between nest checks; this method 

should be used rather than counting only the number of observed nests. 
12

Figure 3. Annual nest success and 95 percent confidence interval for Black-necked Stilt, American Avocet, and 

Forster’s Tern in South San Francisco Bay, California (2005–10). Overall, nest success was 64 percent for terns, 

39 percent for avocets, and 36 percent for stilts.
13

Figure 4. Nesting period for Black-necked Stilt, American Avocet, and Forster’s Tern in South San Francisco Bay, 

California (2005–10). Median nest initiation date and the nesting period (defined as the central span where 80

percent of all initiation dates occurred) are displayed in gray shading. The average annual nesting period was 

shorter for terns (38 days) than stilts (48 days) or Avocets (59 days).
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Figure 5. Waterbird nest abundance and nesting success with 95 percent confidence intervals in pond A12 during 

the creation of new nesting islands (all species combined). Prior to 2008, the water level was high and no nesting 

islands were present. In 2008, water levels were reduced, exposing island habitat. Our data show the immediate 

nesting response of waterbirds, with over 500 nests initiated in pond A12 the breeding season after the 

management actions. This creation of nesting habitat might have only provided improved breeding opportunities for 

the first two years because by 2010, both nest abundance and nest success were quite low.
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Table 1. Summary of nesting parameter estimates for American Avocet, Black-necked Stilt, and Forster’s Tern in 

South San Francisco Bay, California (2005–10).

Species

Total Number of 

Nests1 Clutch Size2 Nest Success2

Hatching 

Success2

Central Period of 

Nest Initiations

American Avocet 4351 3.47 (2.36 – 4.58) 0.37 (0.33–0.40) 0.93 (0.82 – 0.98) April 27–May 26

Black-necked Stilt 1047 3.48 (2.08 – 4.88) 0.24 (0.20–0.28) 0.99 (0.81 – 1.00) May 7–May 26

Forster's Tern 4684 2.27 (1.60 – 2.94) 0.61 (0.58–0.64) 0.95 (0.66 – 0.99) May 23–June 19

1 Total number of nests used for parameter estimates provided in table 1. An additional 850 additional nests were excluded 

from parameter estimates, including other species that were monitored during 2005–10. 

2 Mean (and 95% confidence intervals)
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For more information concerning the research in this report, contact the 

 Director, Western Ecological Research Center

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 3020 State University Drive East

 Sacramento, California 95819

http://www.werc.usgs.gov/
Ackerman and Herzog— Waterbird Nest Monitoring Program in San Francisco Bay (2005–10) —Open-File Report 2012–1145