Ecotoxicology (2009) 18:514–521
DOI 10.1007/s10646-009-0309-2
The corticosterone stress response and mercury contamination
in free-living tree swallows, Tachycineta bicolor
Melinda D. Franceschini Æ Oksana P. Lane Æ
David C. Evers Æ J. Michael Reed Æ Bart Hoskins Æ
L. Michael Romero
Accepted: 24 March 2009 / Published online: 10 April 2009
Ó Springer Science+Business Media, LLC 2009
Abstract We determined mercury concentrations in tree
swallows, Tachycineta bicolor, from Massachusetts and
Maine with different levels of contamination. Baseline and
stress-induced plasma corticosterone concentrations from
adults and nestlings (Massachusetts only) were compared
with mercury concentrations. In Massachusetts, adult
baseline corticosterone was negatively correlated with
blood mercury, but showed a nearly-significant positive
correlation with feather mercury. There was a negative
relationship between baseline corticosterone and blood
mercury in nestlings and between baseline corticosterone
and egg mercury. There was no relationship between
mercury and stress-induced corticosterone in any of the
groups, or with baseline corticosterone in Maine sites
where mercury levels were lower. The findings suggest
blood and egg mercury may be a better indicator of current
condition than feather mercury. Further, mercury contamination may not alter stress-induced corticosterone concentrations in tree swallows but appears to have a
significant impact on baseline circulating corticosterone.
Keywords Corticosterone Mercury Stress Tree swallows
M. D. Franceschini (&) J. M. Reed L. M. Romero
Department of Biology, Tufts University, Medford,
MA 02155, USA
e-mail: [email protected]
O. P. Lane D. C. Evers
BioDiversity Research Institute, Gorham, ME 04038, USA
B. Hoskins
US Environmental Protection Agency, New England
Regional Laboratory, North Chelmsford, MA 01863, USA
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Introduction
Mercury (Hg) is a toxic heavy metal and common environmental contaminant present in many aquatic sites.
There are natural sources of Hg contamination as well as
several anthropogenic sources resulting from historical
and present day use (Wiener et al. 2003). Anthropogenic
sources include direct mining and smelting of Hg and
mercury ores and the use of Hg for mining gold and silver,
effluent from Hg usage in industrial applications such as
chlor-alkali plants and atmospheric deposition from the
generation of electrical power mainly through coal burning.
Mercury is found in aquatic environments as inorganic
mercury (Hg) and as organic methylmercury (MeHg),
which is more toxic and easily biomagnifies through
aquatic food chains, increasing risk to wildlife and humans
(Wiener et al. 2003). The biomagnification of MeHg places
high trophic level birds living in contaminated environments at risk for toxicity through ingestion of contaminated
prey (Evers et al. 2005; Wolfe et al. 2007). While field
efforts have emphasized the adverse impacts of mercury on
piscivores (Evers et al. 2008), recent evidence now demonstrates that many avian invertivores, such as songbirds,
also fill the same high trophic level position of piscivores
and can therefore attain mercury body burdens that equal or
exceed associated piscivores (Cristol et al. 2008; Evers
et al. 2005). Field studies have documented adverse
reproductive impacts to invertivores in areas with known
(Brasso and Cristol 2008) and unknown point sources
(D. Evers, personal communication).
Determination of Hg concentrations in tissues is necessary for confirming exposure and comparison of levels
between sites can help evaluate relative exposure. Investigation of the relationship between tissue levels and their
adverse effects are important for assessing relative risk to
The corticosterone stress response and mercury contamination
wildlife living in contaminated environments (Burger and
Gochfeld 1997; Evers et al. 2007; Wiener et al. 2003;
Wolfe et al. 2007). The identification of physiological,
biochemical and histological biomarkers of contaminant
effects is a prevalent concern in the field of ecotoxicology,
which seeks to establish the impact of chemicals on individual organisms, populations, communities and ecosystems. Measurable changes at the suborganismal level can
allow for early assessment of environmental effects on
organism health, identifying potential hazards prior to
lethal results and significant impacts at the population and
ecosystem levels (e.g., Becker 2003; Evers et al. 2005;
Hansen 2003; Mayer et al. 1992).
The adrenocortical stress response is a necessary component of the overall vertebrate stress response and a critical
mechanism for coping with acute, adverse environmental
conditions (Wingfield and Romero 2001). Within minutes
after exposure to a stressful stimulus, glucocorticoids (GCs,
e.g., corticosterone in birds, cortisol in humans) are released
into the bloodstream via the hypothalamic-pituitary-adrenal
(HPA) or -interrenal (HPI) axis. GC measurement allows for
analysis of relative stress levels between individual animals
and animal populations within a species (Cockrem 2005;
Cockrem and Silverin 2002). GCs are involved in mediation
of glucose metabolism, reproduction, growth, tissue repair,
musculoskeletal health and immune, cardiovascular and
neurologic function (Sapolsky 1998, 2001, 2000). Therefore, abnormal GC synthesis or regulation (either too high or
too low) can affect numerous physiological processes,
negatively impacting health, survival and fitness. For these
reasons, measurement of GC concentrations can provide
information about the health of an animal and relative health
of animal populations.
There are a limited number of studies investigating the
effects of Hg on the adrenocortical stress response in freeliving birds with mixed results (Bowerman et al. 2002;
Franceschini 2007; Heath and Frederick 2005; Wayland
et al. 2002, 2003). Birds are useful bioindicators for several
reasons such as: ease of location, identification and study;
high public interest; many widespread species allowing
comparison over large geographical areas; they occupy
various positions in the food chain including higher trophic
levels; and non-lethal tissue sampling of blood, feathers
and eggs can be performed (Becker 2003). Mercury is
known to have several toxic effects in birds, primarily
related to accumulation in the brain, eggs, kidneys and liver
(e.g., Burgess and Meyer 2008; Evers et al. 2008; Scheuhammer et al. 2008; Wolfe et al. 1998).
Tree swallows are used widely as indicators of local
contamination (Brasso and Cristol 2008; Custer et al. 1998,
2000, 2006; Harris and Elliott 2000; Longcore et al. 2007;
McCarty and Secord 1999; Neigh et al. 2006; Smits et al.
2000; Wayland et al. 1998). Tree swallows are widely
515
distributed, feed near their nests, eat mainly emergent
aquatic insects and readily use nest boxes placed in
appropriate aquatic habitats, facilitating sampling of incubating females and nestlings. For these reasons, they are an
excellent model species for studying Hg effects on the
stress response. There is currently little known about how
local Hg contamination alters physiological endpoints,
however, Brasso and Cristol (2008) demonstrated adverse
reproductive impacts in association with elevated environmental mercury loads. Mercury levels in insects collected by the adult swallows to be fed to nestlings were
significantly higher than those on reference sites, providing
supportive evidence that the observed fitness effects were
due to local biomagnification at contaminated sites. The
major goal of the present study was to investigate the
corticosterone stress response as a bioindicator of
Hg-induced health effects.
Methods
In 2003, adults and nestlings (11–13 days old) were sampled
from nest boxes located in several different eastern Massachusetts (MA) sites with varying degrees of Hg contamination. Tree swallows were sampled from reference sites and
from polluted sites associated with an ongoing United States
Environmental Protection Agency (EPA) investigation of
the impact of Hg contamination on the Sudbury River and
surrounding areas from the Nyanza Superfund Site located in
Ashland, MA (Wiener and Shields 2000). Sampling from
contaminated sites occurred on the Sudbury River and in
Heard Pond in the Great Meadows National Wildlife Refuge
(Sudbury, MA) as well as two other contaminated areas,
Reservoir 1 and Reservoir 2 (Ashland, MA). Reference sites
were the Charles River (Medfield, MA), the Sudbury Reservoir (Marlborough, MA) and Delaney Wildlife Management Area (Stow, MA). In 2004, a small number of adults
was sampled from two salt marsh sites in southeastern Maine
(ME), one with lower (Scarborough) and the other with
higher (Wells) Hg contamination.
Sampling methods for corticosterone analysis were as
follows: adult females and males were captured in nest
boxes during incubation or the first few days following
hatching. Nestlings, if present, were not disturbed at this
time. Instead, nestlings were manually removed from nests
for Hg and corticosterone sampling 11–13 days following
hatching. With few exceptions, all nestlings within a nest
were sampled. Initial blood samples were collected from
both adults and nestlings within 3 min of capture to reflect
pre-capture (baseline) levels (Romero and Reed 2005). A
subset of birds was subsequently restrained in cloth bags
for 30 min, except while measurements were being taken.
Another blood sample was collected at the end of the
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516
30 min of restraint, prior to being returned to the nest box,
in order to measure corticosterone secretion resulting from
the stress of capture, handling, and restraint. This sample
reflects the animal’s ability to mount a physiological stress
response (Wingfield and Romero 2001).
Samples were stored on ice in the field, centrifuged in
the laboratory and the plasma separated and frozen at -4°C
until assayed. Plasma samples were assayed by radioimmunoassay (RIA) following the methods of Wingfield et al.
(1992). Briefly, tritiated corticosterone was added to all
plasma samples for determination of recoveries. Following
equilibration, samples were extracted with redistilled
dichloromethane. The supernatant extracts were dried and
re-suspended in phosphate buffer for assay. Dextran-coated
charcoal was used to separate bound from unbound steroid.
All samples were assayed in duplicate and compared to a
standard curve, with concentrations adjusted by the
recovery and reported as ng/ml of plasma. Inter-assay
variability and intra-assay variability were \8 and \12%,
respectively, as determined using a standard pool in each
assay. Accuracy of measurement of a standard sample was
96% and the sensitivity was 0.6 ng/ml.
Mercury concentrations were measured in blood and
feathers from Massachusetts adults and in the blood only
for Massachusetts nestlings and Maine adults. The outer
two tail feathers and a blood sample for Hg were collected
at the time of capture. Feather and blood Hg samples were
collected several ([5) days before or at the time of corticosterone response sampling. Blood was collected for Hg
and corticosterone at the same time in nestlings. Eggs were
also collected for Hg analysis from most clutches. We
marked, measured and weighed every egg, collecting the
heaviest egg in the clutch (assumed to be the first laid egg).
The egg was collected after at least 3 eggs were present to
avoid nest abandonment. In the analysis, the representative
egg was used as an index for the nestlings from the corresponding nest and as a measurement associated with the
laying female.
The majority of nestling blood samples and a few adult
samples were analyzed at Texas A & M University Trace
Element Research Laboratory (TERL) in College Station,
Texas. Most adult blood samples, all blood from Sudbury
Reservoir and Reservoirs 1 and 2, the majority of egg
samples, and all feathers were analyzed at Brooks Rand
Laboratory (BRL) in Seattle, Washington.
Blood samples sent to TERL were analyzed for total Hg
according to TERL SOP-0301. This method utilized a
Milestone DMA 80 to combust blood samples in nickel
boats in an oxygen-rich atmosphere. Combustion products
were passed through a heated catalyst to complete oxidation and then through a gold column which trapped Hg.
Upon completion of combustion, the gold trap was heated
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M. D. Franceschini et al.
and the Hg released for analysis by atomic absorption. All
analyses were for total Hg because it has been shown that
95% of total Hg measured in songbird blood is in the form
of MeHg (Rimmer et al. 2005).
All samples sent to BRL were analyzed in accordance
with The US Environmental Protection Agency’s Method
1631 and samples were prepared as outlined in the
Appendix to 1631 (http://www.brooksrand.com/FileLib/
1631guid.pdf). Blood samples were processed by cutting
off both ends of capillary tubes, and forcing the blood out
of the tubes by air displacement into digestion vials containing 1 ml of 2% KOH solution. Capillary tubes were
weighed before and after removing the blood to determine
the total mass of blood in the sample preparation. Blood
samples were digested using 1 ml of nitric/sulfuric acid
mixture due to the small sample size digested. After
digestion, blood samples were further oxidized with bromine monochloride (BrCl) and diluted to 20 ml prior to
analysis. The feather samples were homogenized using precleaned homogenization equipment, digested with nitric
acid/sulfuric acid and heat, and further oxidized with BrCl.
Both blood and feather samples were then analyzed with
SnCl2 reduction, single gold amalgamation and Cold Vapor
Atomic Fluorescence Spectroscopy (CVAFS) detection
using a BRL Model III CVAFS mercury analyzer. Blanks,
method duplicates and matrix spikes were also analyzed.
Blood and egg concentrations are reported in lg/g wet
weight. Feather concentrations are reported in lg/g fresh
weight.
Statistical analyses
For evaluating responses to Hg exposure in adults in both
MA and ME, a series of linear regressions between blood,
feather and egg (females only) Hg concentrations and
plasma corticosterone concentrations (baseline and 30 min)
were conducted using SAS GLM. No sex differences were
found for blood and feather Hg or for baseline and stressinduced corticosterone so sexes were combined in the
analyses. Separate regressions were run because of missing
values and site was not included in the analysis due to low
sample size at some of the sites.
For nestlings, blood Hg and egg Hg (as an index for the
nest) concentrations were compared with plasma corticosterone concentrations (baseline and 30 min). The analysis
was performed in SAS by Mixed Model ANCOVA with
site as a covariate and nest box as a random effect to
control for sampling more than one nestling per nest box.
Pseudo-r2 values were calculated for the nestling analysis
using the method described by Cox and Snell (1989). An
alpha equal to or less than 0.05 was taken to be statistically
significant.
The corticosterone stress response and mercury contamination
Results
For Massachusetts adult tree swallows sampled in 2003,
linear regressions indicate a significant negative correlation
between blood Hg and baseline corticosterone (n = 28,
r2 = 0.18, P = 0.02; Fig. 1) and a negative but not significant correlation between egg Hg and female baseline corticosterone (n = 21, r2 = 0.13, P = 0.11; Fig. 1). In contrast,
feather Hg and baseline corticosterone showed a nearlysignificant positive correlation (n = 23, r2 = 0.16, P =
0.06; Fig. 1). There was no correlation between blood or egg
Hg and baseline corticosterone in Maine adults in 2004
(blood: n = 13, r2 = 0.17, P = 0.16; egg: n = 11, r2 =
0.11, P = 0.31). However, the absolute levels of Hg were
lower in Maine adults (blood: 0.134–0.346 ww lg/g; egg:
0.048–0.156 ww lg/g) compared to Massachusetts adults
(blood: 0.119–0.996 ww lg/g; egg: 0.030–0.252 ww lg/g).
There was no relationship between blood, egg or feather Hg
and stress-induced corticosterone in tree swallow adults
sampled in Massachusetts in 2003 (blood: n = 17, r2 = 0.08,
P = 0.26; egg: n = 20, r2 = 0.095, P = 0.21; feather:
n = 14, r2 = 0.03, P = 0.56) or Maine in 2004 (blood:
n = 13, r2 \ 0.01, P = 0.45; egg: n = 11, r2 \ 0.01,
P = 0.94).
For nestling tree swallows sampled in 2003, blood Hg
was significantly negatively correlated with baseline but not
Fig. 1 Comparison of blood, egg and feather mercury concentrations
with baseline plasma corticosterone concentrations in adult tree
swallows sampled in Massachusetts in 2003. Baseline corticosterone
517
with stress-induced corticosterone (baseline: F1,125 = 4.05,
r2 = 0.5, P \ 0.05; stress-induced: F1,120 = 3.16, r2 =
0.72, P = 0.08; Table 1). Similarly, egg Hg was significantly negatively correlated with baseline but not stressinduced corticosterone (baseline: F1,125 = 4.18, r2 = 0.55,
P = 0.04; stress-induced: F1,119 = 0.49, r2 = 0.78,
P = 0.73; Table 1).
Discussion
Our findings suggest that exposure to environmental Hg
has an impact on circulating corticosterone concentrations
in both adult and nestling tree swallows. Blood Hg was
negatively correlated with baseline corticosterone concentrations in both adults and nestlings in Massachusetts
in 2003 (Fig. 1; Table 1). This pattern was not seen in
Maine adults in 2004, but the difference is most likely
due to the lower overall Hg concentrations in the Maine
birds. The lack of significant correlations between stressinduced corticosterone and Hg in both adults and nestlings suggests that these levels of Hg exposure are not
altering the ability to mount a GC stress response.
Remarkably, over 50% of the variation in nestling baseline blood corticosterone concentrations could be
explained by the amount of Hg contamination. This
concentrations were measured within 3 min of covering nest boxes.
(Blood: N = 28; Egg: N = 22; Feather: N = 23)
Table 1 Comparison of blood and egg mercury concentrations with baseline and stress-induced plasma corticosterone concentrations in nestling
tree swallows
Mean ? SE
Blood mercury (ww lg/g)
df
P
Pseudo r2
0.05 ? 0.003
(-)a Baseline corticosterone (ng/ml)
3.30 ? 0.36
4.05
1,125
0.05
0.50
Stress-induced corticosterone (ng/ml)
28.34 ? 2.22
3.16
1,120
0.08
0.72
Egg mercury (ww lg/g)
a
a
F
0.13 ? 0.01
(-) Baseline corticosterone (ng/ml)
3.30 ? 0.36
4.18
1,125
0.04
0.55
Stress-induced corticosterone (ng/ml)
28.34 ? 2.22
0.49
1,119
0.73
0.78
(-) Indicates the negative direction of the relationship
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518
suggests that nestlings may be much more sensitive to Hg
than adults.
Although the relationship between feather Hg and
baseline corticosterone was not significant, it was nearly
so, suggesting that an increased sample size would indeed
have uncovered a significant relationship (Fig. 1). However, even if the relationship were significant, it is in an
opposite direction than the relationship between baseline
corticosterone and blood Hg. This suggests that feather Hg
concentrations are unlikely to reflect the local Hg contamination at the time of sampling, but rather reflect Hg
exposure during feather growth several months earlier.
Heavy metal levels in feathers are reflective of blood
concentrations during the 3 weeks when feathers are
forming. After this period, the blood supply ceases, leaving
a record in the feather (e.g., Burger and Gochfeld 1997).
Molt in tree swallows begins in July on the breeding
grounds and continues through October during migration
(Robertson et al. 1992). In terms of body molt, the rump
region is one of the later areas to molt so at least some tail
feather growth may occur after leaving the breeding
grounds. Therefore, tail feather Hg levels likely reflect a
combination of contamination at both migratory feeding
grounds as well as where the birds bred the previous year.
Since some but not all tree swallows return to breed at the
same sites in multiple years (Butler 1988; DeSteven 1980),
it is unknown how many adults will have molted at the
same site as the sampling year. Consequently, blood and
egg levels are better indicators of local contaminant
exposure at the time of sampling.
Free-living fish and amphibians from sites contaminated
by mixtures of organic pollutants and heavy metals
(including Hg) show alterations in circulating and stressinduced GC concentrations (Gendron et al. 1997; Hontela
1998, 1997, 1995, 1992; Hopkins et al. 1999, 1997; Norris
et al. 1999). Lockhart et al. (1972) reported lower plasma
cortisol concentrations in northern pike (Esox lucius) from
a lake heavily contaminated with Hg (6–16 lg/g muscle
Hg) compared with a reference lake (\0.3 lg/g muscle
Hg). In controlled laboratory studies, changes in blood
cortisol concentrations were induced in catfish (Clarius
batrachus) and in rainbow trout (Oncorhynchus mykiss)
following chronic exposure to Hg (Bleau et al. 1996;
Kirubagaran and Joy 1991).
Comparatively few studies have been conducted in
avian species and other taxa. For example, Wayland et al.
(2002, 2003) found a positive relationship between stressinduced corticosterone and cadmium concentrations and an
inverse relationship with selenium concentrations in
northern common eiders (Somateria mollissima borealis)
but the results were inconsistent between years. Wikelski
et al. (2001) found increased baseline and stress induced
GC concentrations in marine iguanas (Amblyrhynchus
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M. D. Franceschini et al.
cristatus) following acute exposure 7 days after an oil spill.
These results contrast with our findings, but 7 days of
exposure could have a different impact than long-term
chronic exposure or, in the case of nestlings, exposure
during development.
Controlled studies of nestlings of other species show
decreased corticosterone in response to crude oil exposure
which is more consistent with our findings (e.g., Gorsline
and Holmes 1982; Holmes et al. 1979; Miller et al. 1978).
However, Bowerman et al. (2002) found a suggestive
positive relationship between increased response to ACTH
and PCB concentrations in wild nestling bald eagles
(Haliaeetus leucocephalus), also different than our findings. Yolk sac plasma corticosterone concentrations in
herring gulls (Larus argentatus) were inversely related to
yolk sac concentrations of polychlorinated dibenzop-dioxins/polychlorinated dibenzofurans (PCDDs/PCDFs),
total polychlorinated biphenyls (PCBs), non-ortho PCBs and
2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents (TEQs)
(Lorenzen et al. 1999). This negative relationship between
circulating corticosterone and contaminant concentrations in
an avian species is consistent with our findings. A couple of
studies have observed a relationship between organic contaminants and corticosterone concentrations in tree swallows. Martinovic et al. (2003) measured corticosterone in
tree swallow nestlings from several sites contaminated with
chlorinated hydrocarbons. Similar to our findings, they did
not detect any relationship between contaminants and stressinduced corticosterone but did find differences in baseline
concentrations between sites with different levels of contaminants and a negative correlation between polychlorinated dibenzofuran (PCDF) and baseline corticosterone.
Similar to the present study, Franceschini et al. (2008) found
lower baseline corticosterone concentrations and unaltered
stress-induced concentrations in tree swallow nestlings
associated with high levels of 2,3,7,8-tetrachlorodibenzo-pdioxin (TCDD) contamination. However, altered stressinduced but not baseline corticosterone concentrations were
associated with polychlorinated biphenyls (PCBs) contamination in both tree swallow nestlings and adult females.
The very few available published studies on the impact
of Hg on plasma corticosterone in free-living birds show
inconsistent findings. In contrast to our findings, no relationship was found between corticosterone and blood Hg in
bald eagle nestlings (Bowerman et al. 2002), feather Hg in
adult white ibises (Eudocimus albus) (Heath and Frederick
2005) or liver Hg in northern common eiders (Wayland
et al. 2002, Wayland et al. 2003). We recently found a
positive relationship between stress-induced corticosterone and blood Hg in breeding, adult, male common
loons (Gavia immer) sampled in the wild and inhibition
of the stress response, but no relationship with baseline corticosterone, in captive juvenile loons fed MeHg
The corticosterone stress response and mercury contamination
(Franceschini 2007). While these results indicate a
relationship between Hg and corticosterone, they are
inconsistent with the tree swallow findings presented here.
It is important to note that this is a correlative study and
that Hg may not be the specific causative agent of altered
corticosterone concentrations. Other contaminants were not
measured and may have contributed to the changes in
corticosterone. The blood Hg levels reported here are
comparatively low, suggesting sensitivity of baseline corticosterone to Hg in this species. It is also possible that
other contaminants or unknown variables are correlated
with mercury, directly impacting corticosterone concentrations through individual and/or additive effects.
It appears that contaminants impact GCs but possibly
without predictable patterns. One of the reasons for the
varied results may be that terrestrial species are receiving
sporadic doses of contaminants through consumption of
prey and in many cases are migrating and being exposed to
different environments over time. Also, individual contaminants are likely to have different influences on GC
synthesis and regulation so comparisons between studies
investigating different pollutants are difficult to interpret.
In addition, it is not surprising that some studies report
contradictory results given the mix of contaminants at
polluted sites. To the best of our knowledge, this is the first
avian field study showing a clear correlative relationship
between Hg and baseline circulating corticosterone
concentrations.
Since GCs are involved in numerous physiological
processes, a decrease in baseline circulating corticosterone
is likely to have a significant physiological impact
adversely affecting health and fitness. Multiple physiological systems are likely to be affected, including glucose
metabolism and energy balance, growth, reproduction, and
immune, cardiovascular and neurologic systems (Sapolsky
1998, 2001, 2000). The extensive body of literature from
experimental studies in rats point to the complexity of GC
functions and how these functions differ depending on
multiple factors (Dallman and Bhatnagar 2001). Although
it is unclear what the specific impact of Hg inducing a
suppression of baseline corticosterone will be, current
theory posits that baseline corticosterone is necessary for
mounting a robust fight-or-flight response (Sapolsky et al.
2000). Consequently, it is possible that decreased baseline
concentrations could negatively impact an animal’s ability
to survive stressors such as predator attacks, even if point
measurements of stress-induced levels are unaltered.
In conclusion, this study suggests that chronic Hg
exposure may be impacting glucocorticoid synthesis and/or
regulation in tree swallows, resulting in decreased baseline
circulating concentrations. Additional field studies in tree
swallows from several sites would be helpful for determining whether this species and this assay can be used as a
519
widespread indicator of Hg effects. These data support the
use of blood and/or egg Hg concentrations over feather Hg
concentrations for assessing local contaminant effects, and
that physiological impacts might best be seen in chicks.
Additional research could focus on determining the
mechanism of Hg influence on glucocorticoid synthesis,
secretion and regulation. In order to fully understand the
hazards of Hg pollution in wildlife and humans, field
studies are needed which investigate the relationship
between Hg, altered corticosterone concentrations and
direct indicators of survival and fitness.
Acknowledgments This project was made possible through a grant
from Tufts University Institute for the Environment to MDF, and
grants from the US National Science Foundation (IBN-0235044 and
IOB-0542099) to LMR.
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