Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

1 

A Dual Life-Stage Approach to 

Monitoring the Effects of Mercury 

Concentrations on the Reproductive 

Success of Forster’s Terns in San 

Francisco Bay 

Annual Report 

Josh T. Ackerman and Collin A. Eagles-Smith 

U. S. Geological Survey, WERC, 

Davis Field Station
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

2 

A Dual Life-Stage Approach to Monitoring the 

Effects of Mercury Concentrations on the 

Reproductive Success of Forster’s Terns in San 

Francisco Bay 

Annual Report 

Josh T. Ackerman1

 and Collin A. Eagles-Smith1

U. S. GEOLOGICAL SURVEY 

1 U. S. Geological Survey, Western Ecological Research Station, Davis Field Station, 

One Shields Avenue, University of California, Davis, CA 95616; 

jackerman@usgs.gov and ceagles-smith@usgs.gov 

Prepared for: 

San Francisco Estuary Institute 

Regional Monitoring Program, Exposure and Effects Workgroup 

Don Edwards San Francisco Bay National Wildlife Refuge 

South Bay Salt Pond Restoration Project 

California Department of Fish and Game 

U. S. Fish and Wildlife Service 

U. S. Geological Survey 

Davis, California 

[2008] 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

3 

U.S. DEPARTMENT OF THE INTERIOR 

Dirk Kempthorne, Secretary 

U.S. GEOLOGICAL SURVEY 

Mark Meyer, Director 

Suggested citation: 

Ackerman, J. T., and C. A. Eagles-Smith. 2008. A Dual Life-Stage Approach to Monitoring the 

Effects of Mercury Concentrations on the Reproductive Success of Forster’s Terns in San 

Francisco Bay. Annual Report, U. S. Geological Survey, Western Ecological Research Center, 

Davis, CA 47 pp. 

The use of firm, trade, or brand names in this report is for identification purposes only and does 

not constitute endorsement by the U.S. Geological Survey. 

For additional information, contact: 

Center Director 

Western Ecological Research Center 

U. S. Geological Survey 

3020 State University Dr. East 

Modoc Hall, 3rd Floor, Room 3006 

Sacramento, CA 95819 

916-278-9490; steven_schwarzbach@usgs.gov 

Acknowledgments: 

This research was funded from generous contributions by the Regional Monitoring Program’s 

Exposure and Effects Workgroup administered by the San Francisco Estuary Institute, CALFED 

Bay-Delta Program’s Ecosystem Restoration Program, and the USGS Western Ecological 

Research Center, and the efforts of Meg Sedlak and Jay Davis. We thank Sarah Stoner-Duncan, 

Joel Shinn, Kathy Baumberger, Leslie Yen, and Bronwyn McCulloh for field assistance. We 

also thank Meg Sedlak, Jay Davis, and the San Francisco Estuary Institute, Clyde Morris, Eric 

Mruz, Cheryl Strong, Joy Albertson, Mendel Stewart, Joelle Buffa, and the staff at the Don 

Edwards San Francisco Bay National Wildlife Refuge, Carl Wilcox, Larry Wyckoff, John 

Krause, and the staff of the Eden Landing Ecological Reserve (California Department of Fish 

and Game), Danielle Le Fer, San Francisco Bay Bird Observatory, Mark Herzog, PRBO 

Conservation Science, John Takekawa, and Nicole Athearn for logistical support. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

4 

EXECUTIVE SUMMARY 

Problem Statement 

• In order to facilitate long-term monitoring of mercury contamination in San Francisco 

Bay wildlife, the appropriate sampling tools and biosentinel species need to be developed 

that are indicative of local exposure conditions and are at risk to potentially impaired 

reproduction. Additionally, appropriate mercury toxicity thresholds which incorporate 

risks to multiple lifestages need to developed. 

• Forster’s Terns are ideal and practical biosentinel species for monitoring mercury 

contamination in San Francisco Bay wildlife because they occupy a high trophic position, 

mainly forage along the Bay’s margins where mercury methylation rates are elevated, 

and nest at high densities at several sites throughout the Bay making sampling eggs 

logistically feasible. 

• Forster’s Tern eggs have the highest mercury concentrations of any of the 17 aquatic 

birds sampled in San Francisco Bay to date; hence, continued monitoring of Forster’s 

Terns is essential for examining risk to wildlife. 

Study Results

Objective 1. Link mercury concentrations in eggs to those of down feathers in just-hatched 

chicks. 

• Total mercury concentrations in down feathers were highly correlated with total mercury 

concentrations in the reconstructed fresh whole-egg homogenate (n = 94, r

2

 = 0.96, p < 

0.0001). 

• Albumen mercury concentrations were correlated with mercury concentrations of down 

feathers from chicks found in the same nest (n = 28, r

2

 = 0.79, p < 0.0001). 

• Down feather mercury concentrations in recaptured chicks were correlated with mercury 

concentrations of down feathers sampled from the same chick during the first capture 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

5 

event that occurred at hatching (n = 88, r

2

 = 0.74, p < 0.0001). 

• These results demonstrate the utility of using chick down feather mercury concentrations 

([THg]df; μg g-1 fw) to predict concentrations in fresh whole eggs ([THg]we; μg g-1 fww), 

and vice versa, using the equation: [THg]we = (e-2.517±0.043) × [THg]df

0.962±0.020

• Our results demonstrate the utility of using down feathers of chicks up to 10 days of age 

to non-lethally predict mercury in eggs, and thus provide the ability to develop toxicity 

thresholds for eggs that incorporate in ovo mercury’s effects on both egg hatchability and 

subsequent chick mortality. 

Objective 2. Determine toxic thresholds in Forster’s Tern eggs by comparing mercury 

concentrations in randomly collected eggs to concentrations in failed-to-hatch and abandoned 

eggs. 

• In 2007, we monitored 683 Forster’s Tern nests at 5 colony sites in San Francisco Bay. 

• Forster’s Tern nest success varied considerably among colonies from 13% to 72%, and 

hatching success ranged from 72% to 82%. 

• Nest success tended to decline with the geometric mean egg mercury concentration 

estimated for each colony, but this trend was not statistically significant (n = 5 colonies, r

= -0.74, p = 0.15). 

• In 2007, fresh egg mercury concentrations (μg g-1 fww) were highest in failed-to-hatch 

eggs (1.73 ± 0.13), followed by abandoned eggs (1.38 ± 0.09), and randomly sampled 

eggs from successful nests (1.27 ± 0.05; F2,129 = 3.09, p < 0.05). 

• To improve our statistical power and account for the inherent temporal and spatial 

variability in mercury levels, we used our archived egg samples and incorporated nest 

data from our CALFED funded research in 2005 and 2006 to improve our sample size. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

6 

• For all 3 years of data, fresh egg mercury concentrations were higher in failed-to-hatch 

eggs (1.74 ± 0.13 μg g-1 fww) than in randomly sampled eggs from successful nests (1.20 

± 0.04 μg g-1 fww; F2,341 = 13.58, p < 0.001), but abandoned eggs (1.43 ± 0.07 μg g-1

fww) did not differ significantly from either failed-to-hatch or random eggs (all p > 0.05). 

 

Objective 3. Examine effects of mercury on chick mortality by comparing mercury 

concentrations in down feathers of alive and dead Forster’s Tern chicks. 

• In 2007, we captured and banded 358 individual Forster’s Tern chicks and found 

(salvaged) 73 dead chicks at ≤10 days of age. 

• Colony-wide mortality of chicks tended to increase with mercury concentrations in live 

chick down feathers, but this trend was not statistically significant (n = 5 colonies, r = 

0.65, p = 0.27). 

• Geometric mean mercury concentrations in known-dead chicks (20.64 ± 1.07 μg g-1 dw) 

tended to be higher than those that were assumed to have died (20.03 ± 0.71 μg g-1 dw) or 

survived (19.82 ± 0.91 μg g-1 dw), but these results were not statistically significant 

(F2,246 = 0.26, p = 0.77). 

Conclusions and Management Implications 

• Forster’s Terns were recently added to the San Francisco Bay Regional Monitoring 

Program’s long-term plan for avian contaminant monitoring. In order for these egg 

monitoring results to be related to potential toxicity risk, egg toxicity levels for Forster’s 

Terns should be established. 

• Our results indicate that current mercury exposure is likely causing reduced hatching 

success in Forster’s Terns nesting in San Francisco Bay. Approximately 27% of all 

Forster’s Tern eggs sampled exceeded the geometric mean mercury concentration for 

failed-to-hatch eggs (1.74 ± 0.13 μg g-1 fww), and 97% of tern eggs were above the San 

Francisco Bay’s current TMDL monitoring target for eggs (0.50 μg g-1 ww). 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

7 

• However, establishing the egg toxicity threshold is difficult due to high variability among 

years and colony sites, indicating the importance of defining the egg toxicity threshold 

over multiple years and sites. 

• We also have shown a strong predictive link between mercury concentrations in chicks 

and eggs, suggesting that incorporating effects of mercury to both lifestages into a single 

tissue matrix – eggs – is possible once these thresholds are established. 

• Continued assessment and monitoring of Forster’s Tern mercury exposure and 

reproduction are warranted. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

8 

A DUAL LIFE-STAGE APPROACH TO MONITORING THE EFFECTS OF 

MERCURY CONCENTRATIONS ON THE REPRODUCTIVE SUCCESS OF 

FORSTER’S TERNS IN SAN FRANCISCO BAY 

Annual Report 

By Josh T. Ackerman and Collin A. Eagles-Smith 

INTRODUCTION 

The San Francisco Bay Estuary has a legacy of mercury contamination from historical mining 

activities (reviews by Davis et al. 2003, Wiener et al. 2003a) that has resulted in elevated 

mercury concentrations in several waterbird species. Reproduction is the most sensitive 

endpoint of mercury toxicity in birds, and it is thought that current levels may impair the 

reproduction of waterbirds breeding within the estuary (Schwarzbach et al. 2006, Ackerman et 

al. 2007a, 2008a,b). 

Recent CALFED-funded research by the U. S. Geological Survey (USGS) has provided evidence 

that several waterbird populations are at high risk to mercury contamination (Ackerman et al. 

2007a). For example, as many as 58% of the Forster’s Terns breeding in the South San 

Francisco Bay are considered to be at high-risk or extra high-risk to mercury contamination due 

to present-day mercury concentrations in their blood (Figure 1). 

Figure 1. Percentage of waterbird 

breeding populations that are at 

risk to reduced reproductive 

success and declining populations 

due to mercury contamination in 

the South San Francisco Bay. Data 

are from birds captured on nests 

while incubating; blood was used 

as the sample matrix (Ackerman et 

al. 2007a).

Percent of Population at Risk

Breeding Birds Only

Avocet

Stilt

Caspian tern

Forster’s tern

Low Risk Moderate Risk High Risk Extra-High Risk

6%

5%

10%

58%

Risk Factor:

High + Extra High

Percent of Breeding Population at Risk

0 20 40 60 80 100

Percent of Population at Risk

Breeding Birds Only

Avocet

Stilt

Caspian tern

Forster’s tern

Low Risk Moderate Risk High Risk Extra-High Risk

6%

5%

10%

58%

Risk Factor:

High + Extra High

Percent of Breeding Population at Risk

0 20 40 60 80 100
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

9 

Forster’s Terns are ideal biosentinel species to indicate mercury contamination and risk in San 

Francisco Bay wildlife because, as fish eaters, they occupy a relatively high trophic position 

within the Bay’s food web, and, as such, bioaccumulate mercury to potentially toxic levels. 

Additionally, Forster’s Terns forage mainly within the Bay’s margins in shallow water habitats, 

such as salt ponds and marshes (Figure 2, Ackerman et al. 2008b), where mercury methylation 

rates are elevated. Forster’s Terns also nest at relatively high densities at several sites within the 

Bay, making sampling logistically feasible. Therefore, Forster’s Terns are effective and practical 

biosentinel indicators of mercury contamination and risk to wildlife within the San Francisco 

Bay. 

Furthermore, Forster’s Tern egg mercury concentrations are the highest of any aquatic bird 

sampled in the San Francisco Bay (Figure 3). As this report will demonstrate, current mercury 

concentrations are likely causing reduced egg hatchability and may be reducing chick survival as 

well. Therefore, any monitoring program within the San Francisco Bay Estuary should 

incorporate those species that biomagnify mercury to the highest concentrations and are currently 

Figure 2. Foraging locations of Forster's Terns in the South San Francisco Bay in 2006. Terns 

mainly foraged within salt pond and marsh habitats along the Bay's margins. Modified from 

Ackerman et al. 2008b.
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

10 

experiencing toxicological effects. Forster’s Terns also are related to the endangered California 

Least Tern, and can act as a biosentinel for exposure to these endangered species which are 

identified in the San Francisco Bay methylmercury TMDL as species of critical concern that are 

thought to be at especially high risk (California Regional Water Quality Control Board San 

Francisco Bay Region 2006). Thus, continued monitoring of Forster’s Tern mercury 

concentrations is essential for examining risk to wildlife within the San Francisco Bay Estuary. 

Unfortunately, the effects of mercury on avian reproductive success are difficult to quantify in 

the wild and have been little studied, particularly within the San Francisco Bay estuary. 

Moreover, the development of an appropriate monitoring matrix that encompasses sensitive 

*Clapper

Rail

*Forster’s

Tern

*Caspian

Tern

§Black Rail

*Snowy

Plover

*Mallard

*Killdeer

*California

Gull

*Red-winged

Blackbird

*Least Tern

†Double-crested Cormorant

†Great

Egret

†Brandt’s

Cormorant

†Snowy

Egret

†Black-crowned

Night Heron

0.0

0.5

1.0

1.5

*Stilt

*Avocet

Egg [THg] (μg g-1 fww)

*Clapper

Rail

*Forster’s

Tern

*Caspian

Tern

§Black Rail

*Snowy

Plover

*Mallard

*Killdeer

*California

Gull

*Red-winged

Blackbird

*Least Tern

†Double-crested Cormorant

†Great

Egret

†Brandt’s

Cormorant

†Snowy

Egret

†Black-crowned

Night Heron

0.0

0.5

1.0

1.5

*Stilt

*Avocet

Egg [THg] (μg g-1 fww)

Figure 3. Geometric ± SE mean mercury concentrations in 17 species of aquatic bird eggs (μg g-1 fresh wet 

weight [fww]) in the San Francisco Bay Estuary, California. Of the birds studied, Forster’s Terns have the 

highest egg mercury concentrations, other fish and invertebrate eating waterbirds have moderate mercury 

concentrations, and aquatic dependent songbirds have the lowest mercury concentrations. *unpublished data 

from Ackerman and Eagles-Smith. †data from Schwarzbach and Adelsbach 2003. §data from Tsao et al. 

2008.
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

11 

reproductive endpoints, such as egg hatchability and chick survival, has yet to be accomplished. 

OBJECTIVES

In order to facilitate long-term monitoring of mercury contamination in San Francisco Bay 

wildlife and link exposure to effects on avian reproduction, we need to develop sampling tools 

for biosentinel species that can be effectively applied in the field as well as that relate mercury 

concentrations to toxic threshold levels in sensitive life-stages, such as eggs and chicks. 

Together, the three objectives detailed below will allow scientists and managers to monitor 

mercury concentrations in one tissue-matrix – eggs – and assess potential effects on both egg 

hatchability and chick survival. 

Objective 1. Link mercury concentrations in eggs to those of down feathers in newly-hatched 

chicks. 

Egg toxicity thresholds typically have been based on egg hatchability (Fimreite 1971, Heinz and 

Hoffman 2003a, Albers et al. 2007, Heinz et al. 2008). Yet, in ovo mercury exposure not only 

can affect egg survival, but it can also effect subsequent chick growth, behavior, and survival 

after hatching (Heinz 1974, Heinz 1979). For example, Kenow et al. (2003) conducted a doseresponse study to quantify the effects of mercury exposure on chick development, although they 

found no effects of dietary mercury exposure on chick growth. Instead, they concluded that the 

observed reductions in asymptotic chick mass and health indices likely were due to in ovo

mercury exposure (Kenow et al. 2003, 2007a). Incorporating the effects of in ovo mercury 

exposure on egg hatchability and subsequent chick survival into a single tissue matrix is difficult 

because it involves translating mercury concentrations from one avian life-stage to another. If 

this were possible, then egg toxicity thresholds potentially could be refined to incorporate 

mercury effects on both eggs and chicks. 

 

Chicks are especially vulnerable to the effects of residual in ovo mercury exposure shortly after 

hatching when maternally deposited mercury levels are still relatively high. Thereafter, chick 

mercury concentrations rapidly decline as chicks age and dilute their body burden of mercury 

through growth in size and depuration into growing feathers (Monteiro and Furness 2001, 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

12 

Fournier et al. 2002, Ackerman et al. 2007a, Kenow et al. 2007b). Chick mortality associated 

with mercury contamination often occurs within the first week after hatching (Heinz 1974, 

Finley and Stendell 1978, Ackerman et al. 2008a), indicating that in ovo mercury exposure can 

influence post-hatch survival. Incorporating this early chick mortality into egg toxicity 

thresholds is hampered by our inability to translate mercury concentrations in chicks to 

equivalent concentrations in eggs. 

Down feathers of newly hatched chicks are potentially useful tools for estimating mercury 

concentrations in the eggs from which they hatched. Down feathers are grown in ovo during the 

embryonic phase and contain about 38% of the total body burden of mercury in newly hatched 

chicks (Becker et al. 1993, 1994). Mercury concentrations in down feathers can be correlated 

with whole-body mercury burdens in chicks (Becker et al. 1993), and therefore may also be 

useful for estimating whole-egg mercury concentrations. If down feather mercury concentrations 

are, in fact, correlated with concentrations in the whole-egg, then down feathers can be sampled 

non-lethally as a proxy for egg mercury concentrations in studies assessing the effect of in ovo

mercury exposure on subsequent chick mortality. 

 

In Objective 1, we developed equations to predict mercury concentrations in eggs using mercury 

concentrations in chick down feathers, and vice versa. 

Figure 4. Forster’s Tern chick blood mercury concentrations in relation to chick age. Note: 

Mercury concentrations are log transformed. Ackerman et al. 2007a. 

Hatch Fledge

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Blood mercury concentration 

loge [THg] (μg/g ww)

Chick Age

R2=0.68

0 4 8 12 16 20 24 28 32 36

Hatch Fledge

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Blood mercury concentration 

loge [THg] (μg/g ww)

Chick Age

R2=0.68

0 4 8 12 16 20 24 28 32 36
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

13 

Objective 2. Determine toxic thresholds in Forster’s Tern eggs by comparing mercury 

concentrations in randomly collected eggs to concentrations in failed-to-hatch and abandoned 

eggs. 

Egg hatchability is the most commonly measured endpoint of the effect of mercury on impaired 

reproduction (Heinz and Hoffman 2003a, Heinz et al. 2008). However, mercury also has been 

shown to affect parental nest attendance and tenacity, and may cause increased nest 

abandonment rates (Evers et al. 2004). Despite considerable laboratory work to develop toxicity 

thresholds for bird eggs (Fimreite 1971, Heinz and Hoffman 2003a, Albers et al. 2007, Heinz et 

al. 2008), there still are no defined thresholds where mercury concentrations negatively affect 

egg hatchability for birds breeding in San Francisco Bay. Furthermore, toxicity thresholds 

developed using laboratory egg-injection techniques for other species are considered 

inappropriate for application to wild birds because the injected mercury is thought to be much 

more toxic than maternally derived mercury (Heinz et al. 2008). Instead these laboratory 

techniques are most useful in assessing the relative sensitivities of avian embryos to mercury 

toxicity among species (Heinz et al. 2008). 

Heinz et al. (2008) assessed the relative sensitivities of 26 species of birds to methylmercury 

chloride injected into the egg. Although they did not test embryo sensitivity to mercury 

specifically for Forster’s Terns, they did find that three closely related tern species (Common 

Tern, Royal Tern, and Caspian Tern) were at moderate sensitivity to mercury exposure. The 

LC50 for Common Terns was 0.87 μg g-1 ww and 0.40 μg g-1 ww for Royal Terns (there was not 

enough data for an accurate LC50 estimate for Caspian Terns). These toxic concentrations are far 

below the geometric mean ± SE mercury concentrations observed in all Forster’s Terns eggs in 

San Francisco Bay between 2005 and 2007 (1.29 ± 0.02 μg g-1 fww; Ackerman and EaglesSmith, unpublished data). Hence, there is cause for concern that mercury may be impairing the 

hatchability of tern eggs in the Bay. 

In Objective 2, we compared mercury concentrations in randomly collected eggs from successful 

nests to concentrations in failed-to-hatch and abandoned eggs. 

 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

14 

Objective 3. Examine effects of mercury on chick mortality by comparing mercury 

concentrations in down feathers of alive and dead Forster’s Tern chicks. 

Chicks also are a sensitive life-stage for mercury toxicity, and in ovo mercury concentrations that 

do not impact hatchability may still impair subsequent chick growth, behavior, and survival 

(Heinz 1974, 1979; Kenow et al. 2003). Data from our CALFED-funded study demonstrated 

that mercury concentrations in Forster’s Tern chick’s blood are extremely elevated during the 

first few days after hatching and then rapidly decline as chicks age and dilute their body burden 

of mercury through growth in size and depuration into growing feathers (Figure 4; Ackerman et 

al. 2007a). Mercury concentrations in chick’s blood begin to rapidly increase again when feather 

and body growth slows near the age of fledgling (about 28 days old). Our research also has 

shown that chick mortality mainly occurs at ages where mercury is elevated in blood – either just 

after hatching or during fledging. Chick down feathers are a valuable sample matrix because 

they are formed in ovo during embryo development and, hence, may be related to egg mercury 

concentrations (Objective 1). Therefore, down feathers likely provide a valuable link between 

egg mercury concentrations and chick survival. 

In Objective 3, we compared mercury concentrations in down feathers of alive, apparently 

healthy chicks to concentrations in dead chicks. 

STUDY AREA 

We studied mercury concentrations in eggs and chicks of Forster’s Terns during the 2005-2007 

nesting seasons (April to August) in South and North San Francisco Bay, California (Figure 5). 

Our main study site was in the South Bay at the Don Edwards San Francisco Bay National 

Wildlife Refuge (37.4o N, 122.0o W). Most tern colonies occurred on islands within former salt 

evaporation ponds within the Alviso (Ponds A5, A7, A8, A16), Newark (Ponds N6, N7) and 

Moffett (Pond A1) salt pond complexes of the Don Edwards San Francisco Bay National 

Wildlife Refuge, Eden Landing Ecological Reserve (Ponds B4, B7), or Napa-Sonoma Marsh 

Wildlife Area (Pond 2). 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

15 

METHODS

Objective 1. Link mercury concentrations in eggs to those of down feathers in newly-hatched 

chicks. 

We significantly expanded Objective 1 beyond the original proposal’s scope in three ways. First, 

we completed this objective for three species (including American Avocets and Black-necked 

Stilts), rather than just for Forster’s Terns. Including the two additional species allowed us to 

observe the egg to chick correlations over a much wider range of mercury concentrations than 

had we only sampled terns. Second, we conducted a similar egg to chick study by using 

Figure 5. Map of Forster’s Tern colonies monitored for eggs and chicks in San Francisco Bay. Additional 

tern colonies are located throughout the bay in several other locations, but were not monitored. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

16 

subsampled egg albumen and newly hatched chicks. This further strengthened our primary (egg 

to chick) result by alleviating any concerns about our sampling eggs just before they hatched at 

the pipping stage. Third, we assessed whether mercury concentrations in down feathers changed 

as chicks age, possibly due to the continued production of down feathers after hatching, in order 

to identify the appropriate age(s) for sampling feathers from chicks. Together, these three 

studies within Objective 3 provide a robust equation to estimate egg mercury concentrations 

using chick down feather samples. 

Whole Egg and Chick Down Feather Mercury Concentrations 

To link mercury concentrations in down feathers of chicks to whole-egg concentrations, we 

collected eggs immediately before they hatched. We attempted to collect eggs of each species at 

several nesting sites to yield a range of mercury concentrations common in San Francisco Bay 

(Ackerman et al. 2007a, Ackerman et al. 2008a, Ackerman et al. 2008b). We entered colonies 

weekly and marked each new nest we found with a uniquely numbered anodized aluminum tag 

(Ben Meadows Company, Janesville, WI, USA) placed at the nest and a colored pin flag placed 

2 m from the nest. We recorded Universal Transverse Mercator coordinates of each nest site 

(Garmin GPSMAP 76, Garmin International Inc., Olathe, KS, USA) to facilitate re-location of 

the nest. Each nest was re-visited once every seven days, the stage of embryo development was 

determined by floating (Hays and LeCroy 1971, Alberico 1995), and clutch size and nest fate 

(hatched, depredated, or abandoned) were determined. For those nests that were nearing the full 

incubation term (about 24 days), we randomly collected one egg from nests that were at the 1- to 

4-star pipping stage (also called pip to ring stage; about 21-24 days in incubation; Robinson et al. 

1997, Robinson et al. 1999, McNicholl et al. 2001). Collecting pipping eggs within three days of 

hatching ensured that we could assess mercury concentrations in both the whole-egg homogenate 

and down feathers from the fully developed embryo. We stored collected eggs in the refrigerator 

for less than 10 days prior to processing. We opened each egg with acid-rinsed, stainless steel 

scissors and removed all egg contents into a 2-ounce polypropylene jar. Using tweezers, we 

removed about 15 down feathers (0.03 ± 0.01 [SD] g) from the mantle of each chick and placed 

them in polypropylene cryovials. We then immediately froze both the egg and feathers until 

processing and analysis of mercury concentrations. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

17 

Micro-Sampled Egg and Chick Down Feather Mercury Concentrations 

Because mercury concentrations in down feathers collected from pipping chicks, still in the egg, 

could be different than mercury concentrations in down feathers collected from chicks once they 

had hatched, we also used a non-lethal egg sampling technique, called micro-sampling, to verify 

that egg mercury concentrations were correlated with down feathers in recently hatched chicks. 

We have described the methodological details of this technique elsewhere (Stebbins et al., in 

press), so we will only briefly describe it here. During our routine nest monitoring procedures 

(described above), we selected nests where all eggs were ≤3 days in incubation as determined 

from floating (Hays and LeCroy 1971). We randomly selected one egg from the clutch for 

micro-sampling, dipped the egg in a dilute betadine (1%) solution, and wiped it clean with 

isopropanol. Using a handheld cordless rotary tool (Dremel Rotary Tool, 7.2V Cordless 

MultiPro, Racine, WI, USA) with a diamond-tipped grinding bit, we breached the egg shell in 

two locations: one hole was placed at the top of the egg above the air cell to act as a vent during 

albumen extraction and the other hole was place one-third of the way up from the bottom of the 

egg as a site for albumen extraction. We then used a sterile 20-gauge needle attached to a 1-ml 

sterile syringe to carefully extract 200 μl (Forster’s terns and stilts; about 1.1% of egg content 

fresh mass) or 300 μl (avocets; about 1.1% of egg content fresh mass) of albumen from the egg. 

We quickly sealed the extraction hole and the vent hole with a cordless hot glue gun 

(ColdHeatTM Cordless Glue Gun, Bellevue, WA, USA) and then applied a layer of cyanoacrylate 

glue over the hot glue to ensure an adequate seal when parents rotated the egg during incubation. 

We marked the egg we sub-sampled with a blue permanent marker and then returned the egg to 

its nest once the glue had dried. The albumen micro-sample was transferred to a clean cryovial 

and stored on ice until it was returned to the laboratory within 5 hrs. Thereafter, the albumen 

was stored in a freezer at -20 ̊C until mercury analysis. 

To match the albumen sample with a down feather sample from the same chick, we returned to 

the nest weekly to monitor the embryo’s development. When the clutch hatched, we attempted 

to collect down feathers (mantle) from the same chick that hatched from the micro-sampled egg. 

However, this was not always possible, especially when multiple eggs hatched in the clutch 

before we had revisited the nest. Therefore, we categorized the down feather sample into one of 

three groups: feathers known to be sampled from the chick hatching from the micro-sampled 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

18 

egg, feathers known to be sampled from a sibling (not micro-sampled) egg, and unknown 

feathers sampled either from the micro-sampled or sibling egg. We considered the sibling and 

unknown feather samples still to be useful, since sibling egg mercury concentrations are often 

highly correlated, however we acknowledge that intra-clutch variation in mercury concentrations 

in down feathers can be 10-39% (Becker 1992, Becker et al. 1994). 

Mercury Concentrations in Down Feathers as Chick Age 

In order to use down feather mercury concentrations to predict egg mercury concentrations, it is 

necessary to understand if and how mercury concentrations change in down feathers as chicks 

age. Therefore, we used Forster’s tern chicks and mark-recapture methodology to assess 

whether mercury concentrations in down feathers changed as chicks aged. Monitoring methods 

for chicks are described below in Objective 3. Our weekly colony monitoring yielded two down 

feather samples taken from the same individual separated by 7 days. For this analysis, we only 

used those chicks that were first captured at 0-3 days of age when they were first found, thus the 

maximum age of chicks during the subsequent recaptures were 7-10 days of age. 

Statistical Analyses for Objective 1 

We reconstructed mercury concentrations in the whole egg by combining mercury 

concentrations determined for the down feathers sampled from pipping chicks and the remaining 

whole egg-homogenate. To do so, we weighed (dry weight; dw) the entire sample of down 

feathers removed from the embryo (Mdf) and the remaining whole-egg homogenate (Meh) 

separately before determining their respective mercury concentrations (dw; accuracy to 0.0001 

g). We then multiplied the weight of the down feathers removed from the embryo by its specific 

mercury concentration ([THg]df) and added the product of the weight of the remaining whole-egg 

homogenate (dw) and the average mercury concentration of three sub-samples of the whole-egg 

homogenate ([THg]eh). This resulted in the total mercury burden in the whole egg, and we 

divided this quantity by the combined mass (dw) of the removed down feathers and the 

remaining whole-egg homogenate to yield the mercury concentration of the reconstructed wholeegg homogenate at pipping ([THg]we dry weight at pipping; Equation 1). 

Equation 1: [THg]we dry weight at pipping = ((Mdf)([THg]df) + (Meh)([THg]eh))/(Mdf + Meh) 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

19 

Next, we converted the mercury concentration of the reconstructed whole-egg homogenate at 

pipping from a dry weight to a wet weight (ww) basis using Equation 2: 

Equation 2: [THg]we wet weight at pipping = ([THg]we dry weight at pipping) × (1-[% 

moisture/100]) 

Since pipping eggs may have lost a substantial amount of mass from the time of laying (due to 

respiration and moisture loss), we then adjusted the wet weight mercury concentration of the 

reconstructed whole-egg homogenate at pipping ([THg]we wet weight at pipping) to a fresh egg 

wet weight mercury concentration (fww) by dividing the total mass (ww) of the egg content at 

processing (Mec) by the predicted fresh egg mass (ww) at laying (Mfe) and multiplying that value 

by the wet weight mercury concentration at pipping (following Stickel et al. 1973; Equation 3): 

Equation 3: [THg]we fresh wet weight = [THg]we wet weight at pipping × (Mec/ Mfe) 

The fresh egg mass (ww) was estimated using egg morphometrics following Hoyt (1979) using 

Equation 4: 

Equation 4: Mfe = 0.548 × egg length × egg width2

We used Analysis of Covariance (ANCOVA, JMP® version 4.0.4; Sall et al. 2001) to examine 

whether there was an interaction between the effects of species and the reconstructed whole-egg 

homogenate mercury concentrations on down feather total mercury concentrations. We then 

used linear regression to test whether: 1) mercury concentrations in down feathers were 

correlated with concentrations in the reconstructed fresh whole-egg homogenate, 2) mercury 

concentrations in albumen were correlated with concentrations in down feathers sampled from 

chicks found in the same nest, and 3) mercury concentrations in down feathers sampled from 

recaptured chicks were correlated with concentrations in down feathers sampled from chicks 

captured for the first time. We used t-tests to determine whether regression slope coefficients 

differed from a value of one. Lastly, we used ANCOVA to examine whether fresh wet weight 

mercury concentrations differed between pipping eggs and randomly sampled eggs, and we 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

20 

controlled for colony site and calendar date statistically by including them as main effects in the 

model. All data were loge-transformed for analysis, and we report all egg concentrations in fresh 

wet weight (fww); the mean (±SE) moisture content in pipping eggs was 74.85 ± 0.20% (avocet), 

74.03 ± 0.44% (stilt), and 78.71 ± 0.18% (Forster’s terns). Albumen is reported in wet weight 

(ww), and down feathers are reported in fresh weight (fw). 

Objective 2. Determine toxic thresholds in Forster’s Tern eggs by comparing mercury 

concentrations in randomly collected eggs to concentrations in failed-to-hatch and abandoned 

eggs. 

We monitored Forster’s Tern nests at several sites from late April to August in 2007. We 

entered colonies weekly and each new nest we found was marked with a uniquely numbered 

anodized aluminum tag (Ben Meadows Company, Janesville, WI) placed at the nest and a 

colored pin flag placed 2 m from the nest. We recorded Universal Transverse Mercator 

coordinates of each nest site (Garmin GPSMAP 76, Garmin International Inc., Olathe, KS) to 

facilitate re-location of the nest. Each nest was re-visited once every seven days, the stage of 

embryo development was determined by floating (Hays and LeCroy 1971, Alberico 1995), and 

clutch size, overall nest fate (hatched, failed, abandoned, or depredated), and the fate of each 

individual egg (hatched, failed-to-hatch, abandoned, or depredated) was determined. 

To assess whether mercury concentrations in failed-to-hatch and abandoned eggs were higher 

than expected, we randomly collected one egg from several nests at the 9 to 12 day incubation 

stage. We then followed the fate of the remaining eggs in the nest (average clutch size is 2.9 

eggs) and classified the nest as successful if ≥1 of the remaining eggs hatched. For our random 

egg sample, we used only those random eggs that were collected from successful nests. In 

addition, we collected failed-to-hatch and abandoned eggs during routine nest monitoring. We 

defined failed-to-hatch eggs as those eggs that did not successfully hatch despite the fact that 

other siblings within the clutch successfully hatched. Importantly, eggs from nests that were 

depredated, abandoned, or where all eggs were either infertile or dead were excluded from our 

definition of failed-to-hatch eggs. We defined abandoned eggs as those clutches that were 

naturally abandoned by their parents without any obvious sign of depredation, disturbance, or 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

21 

flooding. Additionally, we excluded from our analyses any eggs that contained signs of physical 

damage, such as cracks or dents in the shell, and any heavily decomposed eggs or highly 

desiccated eggs with moisture content as later determined to be below 70%, to ensure that 

comparable samples were used across all egg categories. 

After egg collection, we stored eggs in the refrigerator until processing within three months. 

Before processing we measured the length and breadth of each egg (±0.01 mm) with digital 

calipers, and the whole mass of each egg (±0.01 g) with a digital balance. We opened each egg 

with scissors, removed all egg contents into a tared two ounce polypropylene jar, and recorded 

egg content mass. We then immediately froze the egg until mercury analysis which was 

completed within seven months of egg collection. 

 

Statistical Analyses for Objective 2 

We used ANCOVA to test whether egg mercury concentrations differed among the three 

experimental groups: random eggs from successful nests, failed-to-hatch eggs, and abandoned 

eggs. We included colony site and year as covariates to control for their potential effects on egg 

hatchability and mercury levels. 

Objective 3. Examine effects of mercury on chick mortality by comparing mercury 

concentrations in down feathers of alive and dead Forster’s Tern chicks. 

During the chick rearing period, we accessed Forster’s Tern colonies weekly. At each visit, we 

hand-captured every chick at the colony, banded newly hatched chicks with stainless steel U. S. 

Geological Survey leg bands or recorded band numbers from recaptured chicks, and collected 

10-15 down feathers from the chick’s rump for mercury analysis. We also measured the 

structural size of chicks (mm) at each visit in order to estimate their age using an equation we 

developed based on Forster’s Tern chicks with known hatching dates (chick age [days] = [0.11 x 

wing chord] + [1.11 x culmen] - [0.018 x culmen2] + [1.34 x tarsus] - [0.035 x tarsus2] - 22.15; n 

= 472, r

2

 = 0.98; J. T. Ackerman, unpublished data). We measured exposed culmen and short 

tarsus (tarsometatarus bone) lengths with digital calipers (±0.01 mm with Fowler® electronic 

digital calipers, Newton, Massachusetts, USA) and flattened wing length with a wing board 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

22 

(±1.0 mm). We also salvaged all the dead tern chicks we found on or near the nesting colonies, 

and sampled their down feathers and measured their structural size to estimate their age as 

described above. We evaluated the fate of each banded chick and categorized fates as: survived, 

assumed dead, known dead, and unknown. We considered a chick to have survived if its age at 

its last capture was ≥18 days, since on the following visit 7-days later it would have been 25 days 

of age or older and they fledge at 25-28 days of age. We assumed a chick to have died if it was 

≤10 days old at their initial capture and it was never recaptured again. We considered a chick to 

have definitively died if its body was recovered at any point after banding. For chicks that were 

recaptured at least once, but whose age at last capture was ≤18 days, we classified their fate as 

unknown. 

Statistical Analyses for Objective 3 

We used ANCOVA to test whether mercury concentrations in down feathers differed among 

three experimental groups: survived, assumed dead, and known dead. We included colony site 

and date as covariates to control for there potential effects on chick survival. We controlled for 

the potential effect of age by including only those chicks that were ≤10 days of age. 

Mercury Determination 

We analyzed down feathers and remaining whole-egg homogenate samples for total mercury (U. 

S. Geological Survey, Davis Field Station Mercury Lab), since more than 95% of mercury in 

avian eggs and feathers is methyl mercury (Thompson and Furness 1989, Heinz and Hoffman 

2004, Evers et al. 2005). Prior to analysis, we thawed albumen samples to room temperature and 

ensured sample homogeneity by inverting the cryovials several times and thoroughly mixed the 

albumen by stirring with a clean pipette tip. We pipetted 50-100 μl of albumen from each 

cryovial and weighed (to the nearest 0.0001 g, Ohaus Adventurer Balance, model AR0640, 

Ohaus Corporation, Pine Brook, New Jersey, USA) each aliquot into a nickel or quartz sample 

vessel. For eggs, we measured the length and breadth of each egg to the nearest 0.01 mm using 

digital calipers and measured a total egg weight to the nearest 0.01 g on a digital balance. We 

then carefully cut an approximately 25 mm diameter hole in the top of the egg using clean, 

stainless steel scissors and removed the embryo and any remaining contents into a clean, glass 

petri dish with stainless steel forceps. Total content mass was measured with a digital balance to 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

23 

the nearest 0.01 g. After sub-sampling down feathers, the embryo and remaining egg contents 

were stored frozen as described above. Prior to analysis, we dried the entire egg contents at 50-

60°C for 48 hrs or until completely dried and re-weighed the egg contents to determine moisture 

content. We then ground the dried egg contents to a powder in a Wiley mill, followed by further 

grinding in a mortar and pestle. Just prior to analysis we weighed 0.02-0.05 g of homogenized 

sample into a nickel sample vessel to the nearest 0.0001 g. For feathers, we washed and 

mechanically scrubbed each feather in a 1% Alconox solution (Alconox, White Plains, NY, 

USA) to remove surface debris. We then dried the feathers at 60°C for 24 hrs, weighed them to 

the nearest 0.0001 g (Mettler Toledo, Model AT201, Greifensee, Switzerland) and transferred 

each feather sample into a quartz sample vessel. Following EPA Method 7473 (U. S. EPA 

2000), we analyzed each albumen, egg, and feather sample for total mercury on a Milestone 

DMA-80 Direct Mercury Analyzer (Milestone Inc., Monroe, Connecticut, USA) as described in 

Ackerman et al. (2007b, 2008c). Quality assurance measures included analysis of two certified 

reference materials, two system and method blanks, two duplicates, one matrix spike, and one 

matrix spike duplicate per sample batch. Recoveries of certified reference materials, calibration 

checks, and matrix spikes, respectively, averaged (± SE) 98.47 ± 0.78% (n = 33), 102.36 ± 

1.09% (n = 49), and 103.02 ± 1.51% (n = 17). Absolute relative percent difference for all 

duplicates and matrix spike duplicates, respectively, averaged (± SE) 5.23 ± 1.54% (n = 7) and 

4.86 ± 2.10% (n = 4) for down, and 3.65 ± 0.66% (n = 25) and 2.44 ± 0.45% (n = 13) for eggs. 

 

RESULTS AND DISCUSSION 

Objective 1. Link mercury concentrations in eggs to those of down feathers in newly-hatched 

chicks. 

Whole Egg and Chick Down Feather Mercury Concentrations 

We collected 94 pipping eggs at 22 ± 1.5 (SD) days in incubation (Figure 6). Total mercury 

concentrations (mean ± SD) in down feathers were 2.88 ± 2.06 μg g-1 fw for avocets (range: 

0.47-8.75 μg g-1 fw), 5.96 ± 2.97 μg g-1 fw for stilts (range: 2.61-13.69 μg g-1 fw), and 18.32 ± 

5.60 μg g-1 fw for terns (range: 9.20-32.39 μg g-1 fw). We sub-sampled the remaining whole-egg 

homogenate and determined mercury concentrations in each of the three sub-samples to reduce 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

24 

any variation that might occur due to the advanced stage of embryo development and mercury 

partitioning among different tissues. However, we found little variation among our three subsamples of the remaining whole-egg homogenate; 

the coefficient of variation among the three egg 

sub-samples averaged 2.5% (range: 0.1-9.6%). 

Total mercury concentrations (mean ± SD) in the 

reconstructed fresh whole-egg homogenate was 

0.23 ± 0.15 μg g-1 fww for avocets (range: 0.04-

0.62 μg g-1 fww), 0.43 ± 0.21 μg g-1 fww for 

stilts (range: 0.15-0.95 μg g-1 fww), and 1.35 ± 

0.47 μg g-1 fww for terns (range: 0.69-2.79 μg g-1

fww). 

To assess the relationship between total mercury concentrations in down feathers and those in the 

reconstructed fresh whole-egg homogenate (fww), we first tested whether there were species 

differences in the relationships. There was no interaction between species and reconstructed 

whole-egg homogenate mercury concentrations on down feather total mercury concentrations 

(ANCOVA: species x egg: F2,88 = 1.44, p = 0.24; species: F2,88 = 5.31, p = 0.01; egg: F1,88 = 

306.06, p < 0.0001). We therefore pooled all data among species to test whether total mercury 

concentrations in down feathers were correlated with concentrations in the reconstructed wholeegg homogenate. Total mercury concentrations in down feathers were highly correlated with 

total mercury concentrations in the reconstructed fresh whole-egg homogenate (linear regression: 

n = 94, r

2

 = 0.96, p < 0.0001; Figure 7). After back-transforming the linear regression equations, 

the equations for predicting chick down total mercury concentrations ([THg]df; μg g-1 fw) from 

fresh whole-egg total mercury concentrations ([THg]we; μg g-1 fww) were: 

Equation 5: [THg]df = (e2.590 ±0.025) x [THg]we1.000±0.021

 and, conversely, 

Equation 6: [THg]we = (e-2.517±0.043) x [THg]df

0.962±0.020

The slope coefficient (from Equation 5) of 1.000 ± 0.021 (SE) did not differ from 1.0 (t92=0.01, p

Figure 6. Forster’s Tern nest with a pipping egg. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

25 

= 0.99). 

To examine the potential for differential mercury partitioning into down feathers, we also 

performed the linear regression analysis using the dry weight total mercury concentration of the 

reconstructed whole-egg homogenate at pipping, rather than the fresh wet weight egg mercury 

concentration. The linear regression equation (linear regression: n = 94, r

2

 = 0.97, p < 0.0001) to 

predict chick down total mercury concentrations ([THg]df; μg g-1 fw) from whole-egg total 

mercury concentrations at pipping ([THg]we; μg g-1 dw at pipping) was: 

Equation 7: [THg]df = (e0.971 ±0.025) x [THg]we0.940±0.017

The slope coefficient of 0.940 ± 0.017 (SE) differed from 1.0 (t92=3.53, p ≤ 0.001). 

Lastly, we compared fresh wet weight mercury concentrations in pipping eggs to randomly 

Figure 7. Mercury concentrations in down feathers (μg g-1 fresh weight [fw]) of pipping chicks 

were highly correlated (r

2

 = 0.96) with mercury concentrations in the reconstructed fresh whole-egg 

homogenate (μg g-1 fresh wet weight [fww]) for Forster’s terns (circles), American avocets 

(squares), and black-necked stilts (triangles) in South San Francisco Bay, California, USA. The 

linear regression equation describing the data was: loge Y=2.590+1.000(loge X). Chick down feather [THg] (μg g-1 fw)

Fresh whole egg [THg] (μg g-1 fww)

0.01 0.1 1 10

0.1

1

10

100

Chick down feather [THg] (μg g-1 fw)

Fresh whole egg [THg] (μg g-1 fww)

0.01 0.1 1 10

0.1

1

10

100
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

26 

sampled eggs to verify that pipping eggs were representative of the population. Mercury 

concentrations in pipping eggs did not differ from randomly sampled eggs for avocets (F1,264 = 

0.46, p = 0.50), stilts (F1,50 = 0.09, p = 0.77), or Forster’s terns (F1,173 = 0.01, p = 0.93). 

Micro-Sampled Egg and Chick Down Feather Mercury Concentrations 

We pooled data among all three species for these statistical analyses because we did not have a 

large enough sample of nests to test for interactions among species (stilts: n = 1, avocets: n = 5, 

terns: n = 22). We also randomly selected a down feather sample to correlate with the albumen 

sample when multiple chicks had hatched from the same nest containing the micro-sampled egg. 

Albumen mercury concentrations were correlated with mercury concentrations of down feathers 

from chicks found in the same nest (linear regression: n = 28, r

2

 = 0.79, p < 0.0001; Figure 8). 

We found similar results when we used only those samples where we were able to positively 

match the micro-sampled egg and chick as the same individual (linear regression: n = 6, r

2

 = 

0.77, p = 0.02). After back-transforming the linear regression equation from the full dataset, 

which was based on loge-transformed data (±SE), the equation for predicting chick down feather 

total mercury concentrations (μg g-1 fw) from albumen total mercury concentrations ([THg]alb; 

μg g

-1 ww) was: 

Equation 8: [THg]df = (e2.291±0.085) x [THg]alb

0.888±0.090 

 and, conversely, 

Equation 9: [THg]alb = (e-1.919±0.261) x [THg]df

0.889±0.090 

The slope coefficient (from Equation 8) of 0.888 ± 0.090 (SE) did not differ from 1.0 (t26 = 1.24, 

p = 0.22). 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

27 

Mercury Concentrations in Down Feathers as Chick Age 

To examine whether down feather mercury concentrations changed with age after hatching, we 

compared mercury concentrations in down feathers collected from chicks that were first captured 

when they were ≤3 days of age to their down feather mercury concentrations upon their next 

capture 7-days later (≤10 days of age). Down feather mercury concentrations in recaptured 

chicks were correlated with mercury concentrations of down feathers sampled during the first 

capture event from recently hatched chicks (linear regression: n = 88, r

2

 = 0.74, p < 0.0001; 

Figure 9). After back-transforming the linear regression equation, which was based on logetransformed data (±SE), the equation for predicting recaptured chick down feather total mercury 

concentrations ([THg]df-recapture; μg g-1 fw) from first-captured chick down feather total mercury 

concentrations ([THg]df-first capture; μg g-1 fw) was: 

0.1

1.0

10.0

100.0

0.1 1.0 10.0 100.0

Chick down feather [THg] (μg g-1 fw)

Albumen [THg] (μg g-1 ww)

0.1

1.0

10.0

100.0

0.1 1.0 10.0 100.0

Chick down feather [THg] (μg g-1 fw)

Albumen [THg] (μg g-1 ww)

Figure 8. Mercury concentrations in down feathers (μg g-1 fresh weight [fw]) of newly hatched chicks 

found in the nest were correlated (r

2

 = 0.79) with albumen mercury concentrations (μg g-1 wet weight [ww]) 

micro-sampled from an egg in the same nest when the eggs were ≤3 days incubated in South San Francisco 

Bay, California, USA. Symbol patterns (Forster’s terns [circles], American avocets [squares], and blacknecked stilts [triangles]) indicate whether the feathers were sampled from: the same chick that hatched from 

the albumen micro-sampled egg (filled), a sibling chick from the same nest that was not micro-sampled 

during incubation (partially filled), or an unknown chick from the same nest sampled either from the microsampled or sibling egg (open). The linear regression equation describing the data was: loge

Y=2.291+0.888(loge X). 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

28 

Equation 10: [THg]df-recapture = (e0.384±0.166) x [THg]df-first capture

0.798±0.052

 and, conversely, 

Equation 11: [THg]df-first capture = (e0.485±0.176) x [THg]df-recapture

0.922±0.060

The slope coefficient (from Equation 10) of 0.798 ± 0.052 (SE) differed from 1.0 (t86=3.88, p ≤

0.0001). 

 

Conclusions: Objective 1 

Together, our results in Objective 1 demonstrate the utility of using chick down feather mercury 

concentrations to predict concentrations in eggs, and vice versa. These results have several 

applications, including for mercury monitoring programs as well as in research for assessing 

toxicity thresholds. Often mercury monitoring programs are based on sampling eggs (Evers et 

Figure 9. Mercury concentrations in down feathers (μg g-1 fresh weight [fw]) of recaptured Forster’s 

tern chicks (≤10 days of age) were correlated (r

2

 = 0.74) with mercury concentrations in down feathers 

of the same chicks sampled just after they hatched (≤3 days of age) in South San Francisco Bay, 

California, USA. The stippled line indicates a one-to-one line. The linear regression equation 

describing the data was: loge Y=0.384+0.798(loge X). 

10 100

10

100

Recaptured chick down feather 

[THg] (μg g-1 fw)

First capture chick down feather 

[THg] (μg g-1 fw)

4

4 10 100

10

100

Recaptured chick down feather 

[THg] (μg g-1 fw)

First capture chick down feather 

[THg] (μg g-1 fw)

4

4
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

29 

al. 2003, Hill et al. 2008), however sampling wild bird eggs is sometimes not possible due to 

permitting restrictions, especially with endangered species (Schwarzbach et al. 2006), or not 

desired since it necessarily results in the destruction of eggs. Currently, there are only a few 

ways to sample mercury concentrations in eggs non-lethally. These techniques include using 

chorioallantoic membranes left behind in the eggshell post-hatch (Heinz and Hoffman 2003b) 

and micro-sampling a viable egg by extracting a small amount of albumen (Ackerman and 

Eagles-Smith, unpublished data). However, chorioallantoic membranes can be difficult to find 

and should be collected shortly after hatching (Cobb et al. 1994, G. H. Heinz, pers. comm.) and 

albumen micro-sampling requires considerable training and must be done within a short time 

window when eggs have been incubated for ≤3 days (Ackerman and Eagles-Smith, unpublished 

data). In contrast, sampling down feathers of chicks can occur over a longer time period (up to 

10 days post-hatch) and is relatively easy.

In addition to the value of this technique for monitoring mercury non-lethally, the ability to 

predict mercury concentrations in eggs from chick down feathers, or vice versa, can improve our 

assessment of egg toxicity thresholds. Because avian embryos are especially sensitive to 

methylmercury (reviews by Scheuhammer et al. 2007, Thompson 1996, Wolfe et al. 1998, 

Wiener et al. 2003b), egg toxicity thresholds traditionally have been developed by examining 

effects of in ovo mercury concentrations on egg survival (Fimreite 1971, Heinz and Hoffman 

2003a, Albers et al. 2007, Heinz et al. 2008). However, chick growth (Spalding et al. 2000b, 

Longcore et al. 2007), behavior (Bouton et al. 1999), and survival (Heinz 1976, Finley and 

Stendell 1978, Albers et al. 2007, Burgess and Meyer 2008, Evers et al. 2008) also can be 

affected by methylmercury. The ability to combine both of these sensitive reproductive 

endpoints into a single tissue matrix, could improve our understanding of mercury toxicity levels 

that cause reproductive impairment. For instance, in ovo mercury concentrations that impair egg 

hatchability are likely to be higher than those concentrations that could impair subsequent chick 

growth and survival, since the chick must first hatch before it has the opportunity for its growth 

and survival to be affected by residual mercury exposure. 

To illustrate the utility of this tissue conversion equation, we used the mercury concentration in 

eggs commonly associated with impaired hatchability to estimate the corresponding mercury 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

30 

concentration that would occur in down feathers of a newly hatched chick. Egg mercury 

concentrations >1.0 μg g-1 ww often cause impaired hatchability and embryonic mortality in 

birds (review by Scheuhammer et al. 2007). Using our equation developed in this report 

(Equation 5), a concentration of 1.0 μg g-1 ww in the whole fresh egg is equivalent to 13.3 (13.0-

13.7) μg g-1 fw in down feathers of recently hatched chicks. There is limited data on chick 

toxicity to compare this value to, but Ackerman et al. (2008a) found that mercury concentrations 

(geometric mean ± SE) in down feathers of dead stilt chicks at hatching (16.43 ± 2.19 μg g-1 fw) 

were higher than levels in randomly-sampled live chicks of similar age (9.98 ± 0.77 μg g-1 fw). 

Down feather mercury concentrations in live stilt chicks correspond to a predicted fresh egg 

concentration of 0.74 μg g-1 fww, whereas down feathers from dead stilt chicks correspond to a 

predicted fresh egg concentration of 1.19 μg g-1 fww (Equation 6). These data suggest that 

although eggs with mercury concentrations above >1.0 μg g-1 ww can still hatch, the residual 

effects of maternally derived mercury on early chick mortality may continue to impair overall 

reproduction. This is particularly important because it means that commonly used endpoints, 

such as egg hatchability, likely underestimate the risk of mercury contamination to avian 

reproduction. However, by using the equation we derived between mercury concentrations in 

down feathers and eggs, inclusion of early chick mortality into the egg toxicity criterion allows 

us to integrate toxicity risk for two life-stages into a single tissue matrix. 

Furthermore, toxicity thresholds for egg hatchability are often estimated using lab experiments 

where a measured amount of methylmercury chloride is dissolved in corn oil and injected into 

the egg’s aircell and allowed to diffuse into the embryo (Heinz et al. 2006). The egg is then 

artificially incubated and hatchability is compared to concentrations of methylmercury that were 

injected. Whereas this technique has proven useful in estimating the relative sensitivities of 

avian embryos to mercury toxicity among species, the injected mercury is thought to be more 

toxic than maternally derived mercury (Heinz et al. 2008). Moreover, the amount of injected 

mercury that is actually assimilated by the growing embryo is not fully known (Heinz et al. 

2006; G. H. Heinz, pers. comm.). Perhaps sampling down feathers from successfully hatched 

and failed-to-hatch chicks in these lab studies could refine egg toxicity thresholds by comparing 

assimilated- to injected-egg mercury concentrations. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

31 

Objective 2. Determine toxic thresholds in Forster’s Tern eggs by comparing mercury 

concentrations in randomly collected eggs to concentrations in failed-to-hatch and abandoned 

eggs. 

 

We monitored 683 Forster’s Tern nests at several sites in San Francisco Bay in 2007, including 

Ponds A1, A7, A16, Eden Landing Ecological Reserve, and Moffett. Mercury concentrations in 

randomly collected eggs differed among colonies (ANOVA: F5,129 = 2.71, p = 0.02; Table 1). 

Mayfield nest success (Mayfield 1975, Klett et al. 1986) varied considerably among colonies 

from just 13% in Moffett to 72% in Pond A16 (Table 1). Nest success tended to decline with the 

geometric mean egg mercury concentration estimated for each colony using randomly sampled 

eggs, but this trend was not significant (n = 5, r = -0.74, p = 0.15; Figure 10). Instead, the 

greatest factor influencing nest success was depredation. The proportion of eggs hatching in a 

successful nest (i.e., ≥1 egg hatched) ranged from 72% to 82%, and was reduced by both partial 

clutch depredation and failed-to-hatch eggs. 

Colony Egg Geometric Mean [THg] μg g-1 fww

Colony Nest Success 

(Mayfield)

Figure 10. Colony nest success tended to decline with the 

geometric mean egg mercury concentration for each colony.

0.5 1.0 1.5 2.0 2.5 3.0 3.5

0.0

0.2

0.4

0.6

0.8

1.0

Colony Egg Geometric Mean [THg] μg g-1 fww

Colony Nest Success 

(Mayfield)

Figure 10. Colony nest success tended to decline with the 

geometric mean egg mercury concentration for each colony.

0.5 1.0 1.5 2.0 2.5 3.0 3.5

0.0

0.2

0.4

0.6

0.8

1.0
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

32 

Table 1. Mercury concentrations in random eggs from successful nests, abandoned, and failed-to-hatch 

Forster’s Tern eggs, colony nest success, and egg hatching success by colony in South San Francisco Bay, CA in 2007.

Colony # Nests 

Monitored

Random Eggs 

[THg] μg g-1 fww

Abandoned Eggs 

[THg] μg g-1 fww

Failed-to-Hatch Eggs 

[THg] μg g-1 fww

Nest Success 

(Mayfield)

Hatching Success 

(in successful nests)

A1 185 1.37 ± 0.08 1.32 ± 0.10 1.56 ± 0.19 67% 72%

A7 103 1.27 ± 0.13 1.62 ± 0.45 1.42 39% 74%

A8 0 na na 1.62 na na

A16 258 1.21 ± 0.06 1.56± 0.17 1.95 ± 0.03 72% 74%

Eden Landing 119 1.11 ± 0.19 1.02 ± 0.19 2.62 40% 82%

Moffett 18 3.08 2.43 ± 0.38 na 13% 75%

Total 683 1.27 ± 0.05 1.38 ± 0.09 1.73 ± 0.13 --- ---

In 2007, mercury concentrations in Forster’s Tern eggs differed among random eggs from 

successful nests, abandoned, and failed-to-hatch eggs, after controlling for potential effects of 

colony site (Figure 11: top panel; ANCOVA: egg type: F2,129 = 3.09, p < 0.05, colonies: F5,129 = 

2.72, p = 0.02). For all sites combined in 2007, mercury concentrations (geometric mean ± SE) 

were highest in failed-to-hatch eggs (1.73 ± 0.13 μg g-1 fww), followed by abandoned eggs (1.38 

± 0.09 μg g-1 fww), then randomly collected eggs from successful nests (1.27 ± 0.05 μg g-1 fww). 

However, abandoned eggs did not differ from failed-to-hatch eggs or random eggs from 

successful nests, due in part to limited sample sizes and relatively low statistical power (0.59) in 

2007. To improve our statistical power and account for the inherent temporal and spatial 

variability in mercury levels, we used our archived egg samples and evaluated nest data from our 

CALFED funded research in 2005 and 2006 (Ackerman et al. 2007a) to increase our sample size. 

We went through our nest records and classified eggs from 2005 and 2006 similarly to our 

approach in 2007. 

After pooling all 3 years of data, mercury concentrations differed significantly among egg 

collection status (F2,341 = 13.58, p < 0.001), colony (F8,341 = 12.18, p < 0.0001), and year (F2,341 = 

12.48, p < 0.001). Mercury concentrations (geometric mean ± SE) were higher in failed-to-hatch 

eggs (1.74 ± 0.13 μg g-1 fww) than in randomly sampled eggs from successful nests (1.20 ± 0.04 

μg g

-1 fww), but abandoned eggs (1.43 ± 0.07 μg g-1 fww) did not differ significantly from either 

failed-to-hatch or random eggs from successful nests (Figure 11: bottom panel; Tukey’s multiple 

comparisons tests: both p > 0.05). These results indicate the importance of collecting data across 

multiple years and colony sites in the derivation of effects thresholds. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

33 

1.0

1.2

1.4

1.6

1.8

2.0

1.0

1.2

1.4

1.6

1.8

2.0

2007

2005-07

Random

Abandoned

Failed-tohatch

Random

Abandoned

Failed-tohatch

Egg [THg] (μg g-1 fww) Egg [THg] (μg g-1 fww)

Figure 11. Mercury concentrations in failed-to-hatch Forster’s Tern eggs were higher than randomly 

sampled eggs from successful nests during 2007 and for all 3 years combined.

1.0

1.2

1.4

1.6

1.8

2.0

1.0

1.2

1.4

1.6

1.8

2.0

1.0

1.2

1.4

1.6

1.8

2.0

1.0

1.2

1.4

1.6

1.8

2.0

2007

2005-07

Random

Abandoned

Failed-tohatch

Random

Abandoned

Failed-tohatch

Egg [THg] (μg g-1 fww) Egg [THg] (μg g-1 fww)

Figure 11. Mercury concentrations in failed-to-hatch Forster’s Tern eggs were higher than randomly 

sampled eggs from successful nests during 2007 and for all 3 years combined.

Conclusions: Objective 2 

Together, our results in Objective 2 indicate that current mercury concentrations may be reducing 

hatching success of Forster’s Tern eggs and suggest that the large variability among years and 

colonies make determining toxic threshold concentrations in eggs difficult. Defining a threshold 

mercury concentration for reduced egg hatchability in the wild is complicated by external factors 

such as disturbance, predation, and food availability. Thus, studies should span multiple colony 

sites and years to account for the inherent variability in reproductive success. 

We found that mercury concentrations in failed-to-hatch eggs were higher than in random eggs 

during 2007 as well as during 2005-2007 using the expanded dataset. Additionally, we found 

that colony-specific nest success tended to decline with increasing colony-wide egg-mercury 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

34 

concentrations. Importantly, we found that mercury concentrations in failed-to-hatch eggs were 

about 45% higher than in randomly collected eggs from successful nests (2005-2007). 

Approximately 27% of all Forster’s Tern eggs sampled between 2005 and 2007 exceeded the 

geometric mean mercury concentration for failed-to-hatch eggs (1.74 ± 0.13 μg g-1 fww), 

suggesting that hatchability of Forster’s Tern eggs in San Francisco Bay may currently be 

affected by mercury exposure and bioaccumulation. However, the degree to which mercury 

itself is affecting tern reproduction is still unclear. Depredation was generally the greatest factor 

influencing nest success, and variability in mercury levels, predation pressure, and disturbance 

among years and colony sites can confound interpretation of potential effects, as well as the 

development of toxicity thresholds. To account for this inter-annual and inter-colony variability, 

egg toxicity thresholds should be developed over multiple years. 

Objective 3. Examine effects of mercury on chick mortality by comparing mercury 

concentrations in down feathers of alive and dead Forster’s Tern chicks. 

We captured and banded 358 individual Forster’s Tern chicks (Figure 12, Table 2), 341 of which 

were ≤10 days old at their first capture. Through a mark-recapture methodology, we followed 

their fate to fledging (survived or died). We considered a chick to have died if it was ≤10 days 

old at their initial capture and it was never recaptured again. We considered a chick to have 

survived if its age at its last capture was ≥18 days, since on the following visit 7-days later it 

would have been 25 days of age or older and they fledge at 25-28 days of age. For chicks that 

Figure 12. Forster’s Tern chicks were banded soon after hatching (left panel), their down feathers 

sampled for mercury, and compared to chicks found dead (right panel) at the colony during weekly visits. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

35 

were recaptured at least once, but whose age at last capture was ≤18 days, we classified their fate 

as unknown and excluded them from analyses. Of the 341 chicks that we banded at ≤10 days of 

age, 141 individuals were assumed to have died, 41 were known to have died, 76 survived, we 

could not assign a fate to 40 individuals, and we excluded 43 chicks that were manipulated 

(blood sampled). In total, we found 73 dead Forster’s Tern chicks at ≤10 days of age on colonies 

(Figure 12: right panel), 41 were previously banded and 32 were not banded (they had hatched 

between colony visits and died before our next colony visit). 

We estimated colony-wide chick mortality in three ways, including the number of chicks banded 

that were never recaptured again, the number of chicks handled (banded or found dead 

unbanded) that were found dead, and the number of chicks banded that were later found dead 

(Table 2). In general, colony-wide mortality of chicks tended to increase with mercury 

concentrations in live chick down feather, but this trend was not significant (Figure 13; n = 5, r = 

0.65, p = 0.27) and most apparent for the second chick mortality estimate (i.e., number handled 

that were found dead). Future research should use more sophisticated mark-recapture models 

(Program MARK) to estimate survival rates, but that is beyond the scope of this study. 

Table 2. Mercury concentrations in alive and dead Forster’s Tern chicks and three estimates of chick mortality by 

colony in South San Francisco Bay, CA in 2007. 

Colony # Chicks 

Monitored

Alive Chick 

Down [THg] μg g-1 fw

Dead Chick 

Down [THg] μg g-1 fw

Chick Mortality 

(# never recaptured / 

total # banded)

Chick Mortality 

(total # found dead / 

total # handled)

Chick Mortality 

(# banded found dead 

/ total # banded)

A1 136 20.76 ± 0.69 18.24 ± 1.45 31% 23% 17%

A7 32 21.60 ± 1.67 29.08 ± 1.83 47% 14% 3%

A8 0 26.45 ± 3.84 na --- --- ---

A16 148 19.51 ± 0.73 22.18 ± 2.01 41% 16% 10%

Eden Landing 42 18.07 ± 0.75 23.21 ± 2.77 50% 5% 5%

Moffett 0 43.45 28.12 --- --- ---

Total 358 20.06 ± 0.44 20.76 ± 1.15 39% 19% 12%
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

36 

As expected based on our results from Objective 1, colony-wide mercury concentrations in live 

chick down feathers was positively correlated with colony-wide geometric mean mercury 

concentrations in eggs (n = 5, r = 0.99, p = 0.0002; Figure 14). 

Random Egg [THg] μg g-1 fww

Alive Chick Down Feather 

[THg] μg g-1 fw

Figure 14. Colony-wide mercury concentrations in live chick down feathers were 

positively correlated with colony-wide (randomly collected) egg mercury concentrations.

0.5 1.0 1.5 2.0 2.5 3.0 3.5

10

20

30

40

50

Random Egg [THg] μg g-1 fww

Alive Chick Down Feather 

[THg] μg g-1 fw

Figure 14. Colony-wide mercury concentrations in live chick down feathers were 

positively correlated with colony-wide (randomly collected) egg mercury concentrations.

0.5 1.0 1.5 2.0 2.5 3.0 3.5

10

20

30

40

50

We analyzed total mercury concentrations in down feathers of each assumed-dead, known-dead, 

and assumed-survived chicks. While controlling for the potential effects of date (F1,246 = 0.04, p

= 0.83) and colony site (F5,246 = 1.81, p = 0.11), we found no differences in down feather 

mercury concentrations among chick fates (F2,246 = 0.26, p = 0.77). However, geometric mean Chick Mortality ( found dead / total handled)

Alive Chick Down Feather [THg] μg g-1 fw

0.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28

16 17 18 19 20 21 22 23 24

Chick Mortality 

( found dead / total handled)

Alive Chick Down Feather [THg] μg g-1 fw

0.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28

16 17 18 19 20 21 22 23 24

Figure 13. Colony-wide chick mortality tended to increase with 

mercury concentrations in live chick down feathers. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

37 

mercury concentrations in known-dead chicks (20.64 ± 1.07 μg g-1 dw) were slightly higher than 

those that were assumed to have died (20.03 ± 0.71 μg g-1 dw) or survived (19.82 ± 0.91 μg g-1

dw; Figure 15). 

Conclusions: Objective 3 

Together, our results in Objective 3 are inconclusive as to whether current mercury 

concentrations in ovo have an important effect on chick survival. Although colony-wide 

mortality trended higher with chick down mercury levels, this result was not significant. 

Additionally, chicks that were found dead tended to have slightly higher mercury concentrations 

than chicks that were known to have survived to ≥18 days of age, but this difference was not 

statistically different. 

Examining the effects of mercury exposure on chick survival is complicated due to high 

variability in mercury concentrations among chicks of differing ages (Karasov et al. 2007). Any 

effects of mercury exposure on chick survival are likely to occur shortly after hatching or during 

fledging when blood concentrations are at their highest levels. Blood mercury concentrations in 

chicks are relatively high immediately after hatching, due to in ovo exposure, then rapidly 

19

20

21

22

Chick down feather [THg] (μg g-1 fw)

Chick type

Survived to 

18-day of age

Never 

Recaptured 

(assumed died)

Found Dead

19

20

21

22

Chick down feather [THg] (μg g-1 fw)

Chick type

Survived to 

18-day of age

Never 

Recaptured 

(assumed died)

Found Dead

Figure 15. Mercury concentrations in down feathers of Forster’s Tern chicks found dead at colonies 

were not statistically different than chicks that survived to ≥18 days of age (near fledging). Only chicks 

that were ≤10 days of age were included in the analyses. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

38 

decline as chicks age and dilute their body burden of mercury through growth in size and 

depuration into growing feathers (Monteiro and Furness 2001, Fournier et al. 2002). Blood 

mercury concentrations then begin to increase just before and during fledging when body growth 

and feather production slows, while chicks continue to acquire mercury through their diets. This 

U-shaped pattern of blood mercury dilution followed by accretion as chicks age has been 

observed in several species (Spalding et al. 2000b, Kenow et al. 2003), including Forster’s Terns 

(Ackerman et al. 2007a, Figure 4). Therefore, juvenile terns may experience a period of higher 

risk to mercury toxicity shortly after hatching and again at the time of fledging when feather 

production ceases. 

Elsewhere we did not detect an effect of blood mercury concentrations on survival of 

postfledging (>28 days of age) Forster’s Terns using radio-telemetry, instead size-adjusted mass 

had an overwhelming influence on survival (Ackerman et al. 2008c). In fact, cumulative 

survival probability was 61% lower for terns with the lowest, compared to the highest, observed 

masses (Ackerman et al. 2008c). Mercury exposure in ovo or soon after hatching can influence 

chick growth rates and fledging mass, therefore in ovo mercury may have had a residual effect 

on postfledging tern survival via reduced growth rates and fledging masses. If so, then future 

work should focus on examining whether in ovo mercury concentrations are having a detrimental 

effect on Forster’s Tern chick growth rates. 

MANAGEMENT IMPLICATIONS AND RECOMMENDATIONS 

Mercury contamination in San Francisco Bay has necessarily been a main focus of water quality 

agencies in the San Francisco Bay Region, and protection of wildlife is among the top issues of 

concern. In fact, the San Francisco Bay Regional Water Quality Control Board recently included 

a bird egg monitoring target into its methylmercury TMDL (California Regional Water Quality 

Control Board San Francisco Bay Region 2006). However, sensitivity to mercury exposure can 

vary widely among bird species (Heinz et al. 2008) and lifestages, and little information is 

known about the sensitivity of high-risk San Francisco Bay species, such as Forster’s Terns. As 

such, there is a need to link mercury exposure to adverse effects in species that breed in the Bay 

as the first step in evaluating population level impacts of mercury, and ultimately developing a 

compensation strategy for any potential reduced reproduction. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

39 

In particular, Forster’s Terns were recently added to the Regional Monitoring Program’s longterm plan for avian contaminant monitoring, along with Double-crested Cormorants (J. Davis, 

pers. comm.). Starting in 2009, Forster’s Tern eggs will be monitored for contaminant exposure 

every three years as part of this long-term monitoring plan. In order for these egg monitoring 

results to be related to potential toxicity risk, egg toxicity levels for mercury should be 

established. 

Effects of mercury on avian reproduction can impact several lifestages, and post-hatch effects on 

chick survival and growth are important considerations for reproductive impairment. Thus, there 

is great value in incorporating effects of multiple, sensitive lifestages into a single threshold 

value. Our dual-lifestage approach can serve as a predictive tool for simultaneously estimating 

potential impacts across lifestages. The avian egg is an extremely valuable indicator of mercury 

exposure, and potential effects across multiple lifestages, because it links maternal exposure to 

hatchability and early-chick survival (Figure 16). 

Maternal Transfer

Fail-to-Hatch

Hatch

Mortality

Survive

Abandon

Maternal Transfer

Fail-to-Hatch

Hatch

Mortality

Survive

Abandon

Figure 16. Conceptual model demonstrating the utility of using eggs as a monitoring tool for multiple lifestages, 

incorporating effects to adults, chicks, and eggs into a single tissue monitoring matrix – eggs. Once toxicity 

thresholds are developed for each lifestage shown, they can be translated into equivalent concentrations in eggs. 

Thereafter, toxicity thresholds for eggs will incorporate mercury’s effect on hatchability, chick growth and survival, 

and the probability of adult nest abandonment. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

40 

Our results indicate that current mercury exposure is likely causing reduced hatching success in 

Forster’s Terns nesting in San Francisco Bay and continued assessment and monitoring of their 

mercury exposure and reproduction are warranted. In particular, geometric mean mercury 

concentrations observed in Forster’s Terns eggs in San Francisco Bay (1.29 μg g-1 fww) are 

above mercury toxicity levels developed in the lab for two closely relate species: Common Terns 

LC50 was 0.87 μg g-1 ww and Royal Terns LC50 was 0.40 μg g-1 ww. Although injected mercury 

is likely more toxic than maternally derived mercury, current mercury concentrations in Forster’s 

Tern eggs in the Bay are considered high. For example, 99% of Forster’s Tern eggs we sampled 

were above the toxic threshold concentration calculated for Royal Terns, 97% were above the 

San Francisco Bay’s TMDL monitoring target for eggs, 80% were above the toxic threshold 

concentration calculated for Common Terns, and 27% were above the geometric mean mercury 

concentrations in failed-to-hatch eggs that we observed in San Francisco Bay (Figure 17). 

We also have shown a strong link in mercury concentrations between chick and eggs, suggesting 

that incorporating effects using a dual-lifestage approach is possible. We found only limited 

evidence that chick survival is being impaired by mercury. However, the inherent variability in 

mercury concentrations and reproductive success across years make defining a clear toxic 

threshold difficult, and data spanning multiple years are necessary to control for this variability 

and establish more definitive thresholds. Additionally, wildlife in San Francisco Bay are 

exposed to potentially deleterious levels of other contaminants of concern, such as selenium, 

PCBs, and PBDEs. These bioaccumulative pollutants may interact with mercury to enhance or 

reduce its effect on avian reproduction, thus confounding the interpretation of risk from mercury 

alone. A robust assessment of compounding effects of these multiple contaminants together will 

be necessary to partition risk among the different chemical classes. We recommend that support 

of additional research to achieve this goal be a priority in San Francisco Bay. 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

41 

% of Forster’s Tern Eggs 

Above Egg Mercury Criteria

100 80 60 40 20 0

Successful

Forster’s tern eggs

Abandoned

Forster’s Tern eggs

Failed-to-hatch

Forster’s Tern eggs

TMDL egg 

monitoring 

target

Common Tern 

egg LC50

Failed-to-hatch

Forster’s Tern eggs

Figure 17. The percentage of Forster’s Tern eggs we sampled in San Francisco Bay that were above 

various toxic threshold concentrations. Overall, 97% of Forster’s Tern eggs were above the San 

Francisco Bay’s TMDL monitoring target for eggs (0.50 μg g-1 fww; California Regional Water Quality 

Control Board San Francisco Bay Region 2006), 80% were above the toxic threshold concentration 

calculated for Common Terns (0.87 μg g-1 ww; Heinz et al. 2008), and 27% were above the geometric 

mean mercury concentrations in failed-to-hatch eggs that we observed in San Francisco Bay (1.74 μg g-1

fww; this report). 
Ackerman and Eagles-Smith, Mercury Effects on Forster’s Tern Reproduction

42 

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