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A LOOK AT ENDOCRINE DISRUPTORS: Assessing the threat of hormone-disrupting environmental contaminants to birds
Introduction
Many environmental contaminants of human origin have
the potential to act like hormones or to interfere with hormone action.
These contaminants are often referred to as "endocrine disruptors." Chemicals
of very different types, including industrial chemicals like PCBs, insecticides
such as DDT, common pesticides, and many fungicides, can act in this way.
These chemicals have the potential to interfere with the development of sexual and reproductive structures in embryos and young birds. During early life stages of all vertebrates, including birds, hormones permanently influence the development of the reproductive tract and the brain, notably those areas that control reproductive behavior. An alteration or disruption in the development of these reproductive structures may become evident only after the bird grows into an adult and cannot breed properly. Disruption of hormonal action by these contaminants in adult birds can cause a change in reproductive function, especially if the bird is exposed to the chemical during the breeding season. However, in contrast to the exposure of an embryo, a single exposure to an adult bird is unlikely to cause permanent harm.
What is known about this problem?
What is "endocrine disruption"?
Endocrine disruption refers to the interference or alteration,
by environmental contaminants, of the hormonal signals that control normal
physiological function in animals. This mode of action is not a new discovery.
There is abundant published evidence for changes in hormone levels in adult
organisms upon exposure to such compounds, which may be environmental contaminants
or naturally occurring chemicals. What is new for wildlife toxicology is
the emphasis on the permanent effects of endocrine-disrupting contaminants
on embryonic development and the recognition of the long delay after exposure
until the appearance of an effect. In addition, proponents of the
importance of this mechanism point to the variety of chemicals of human
origin that are known to have endocrine effects. Not all endocrine-active
compounds necessarily alter normal function of the animal, but we have
good information on this for only a handful of these chemicals.
Does endocrine disruption occur in wild birds?
We don't know, but we suspect it does. There is good
experimental evidence from hatchling gulls that were exposed to DDE (a
metabolite of the pesticide DDT) that was injected into their eggs. These
birds developed abnormal gonads that were more like ovaries than testes
in some respects. It is not known how these physical changes seen in embryos
might affect the later reproductive capability of these birds.
The information from studies on bird populations in the
wild is more equivocal, in part because effects cannot be attributed with
certainty to contaminants acting through a hormonal mechanism. Numerous
studies correlate hormonal changes in affected birds with changes in breeding
behavior or reproductive success, but the data so far do not establish
endocrine disruption as the ultimate cause. In fact, very few studies have
looked for endocrine disruption specifically in wild bird populations.
Studies that have found a correlation between increased contaminants and
changes in breeding behavior or reproductive success include:
- female-female pairings in Western gulls off the
California coast during the late 1960s and early 1970s; this phenomenon
declined when DDT contamination of the local environment declined
- the drop in population levels of gulls, terns,
cormorants and eagles (all fish-eaters), breeding on the Great Lakes and
a subsequent increase in population levels after the use of organochlorines
ended
- thinner eggshells produced in many birds exposed
to DDT
What birds are at risk for endocrine disruption?
The most serious problems can occur in developing birds.
Even in the shell, an embryo is exposed to chemicals deposited into the
egg by a female. Endocrine disruptors accumulate in fat cells. Birds generally
must use fat stores to produce eggs so contaminants stored in fat will
be transferred to eggs. Birds that accumulate fat-soluble contaminants
will be at risk if those contaminants are also endocrine disruptors.
Fish-eating birds, and other birds that feed at the top of food webs, are
those that accumulate fat-soluble contaminants most rapidly.
Hatchlings are also at risk as they are still developing under the influence of hormonal signals. Their exposure to contaminants from food may permanently alter development. Birds feeding at the top of food webs often feed their young contaminated prey, as do songbirds that feed their offspring insects or other food items from habitats where pesticides have been applied. Unfortunately, pesticides are often applied at the very time of year when young birds are present. As we learn more about which contaminants are endocrine disruptors and more about their distribution in the environment both temporally and by habitat, we will be able to say more specifically which species are at risk for endocrine disruption.
What chemicals are endocrine disruptors in birds?
Actually, very few have been tested in birds. DDT,
its metabolite DDE and other organochlorine pesticides and some forms of
PCBs show endocrine effects and effects on hormonally controlled traits
in birds. Many studies have shown altered hormone concentrations that accompany
impaired reproduction (when exposed to petroleum crude oil or parathion
for example) but often these studies cannot say if altered hormone levels
result from the exposure directly or because reproduction is impaired in
other ways. More recent studies in fish and mammals have found hormone
level effects resulting from exposure to vinclozolin and other fungicides,
or pthalate esters, an industrial chemical.
How is the exposure to, or effects of, endocrine
disruptors measured?
This is an area of great interest for the research community
right now. For the action of sex steroids on development of reproductive
function, the best way to demonstrate a functional effect is to expose
breeding birds and then test the breeding capability of their offspring
when they reach breeding age. This strategy is very time consuming and
not feasible for testing birds in the wild. Measurement of traits in young
birds that are hormonally controlled and that clearly indicate later reproductive
capability would be very useful for measuring endocrine disruption in wild
birds. Such traits include the presence of egg yolk proteins in the blood
of male birds or the condition of sexually dimorphic plumage used for courtship
and breeding. These measures, and others, need to be evaluated through
careful experimentation for their ability to predict the reproductive capability
of a bird as an adult. Traits with similar properties that could
be measured in the egg or early embryo would also be useful.
Policy issues
The 1996 Food Quality Protection Act, which amended the
Federal Insecticide, Fungicide and Rodenticide Act and the Federal Food,
Drug, and Cosmetic Act, and amendments to the Safe Drinking Water Act require
the Environmental Protection Agency (EPA) to test chemicals for estrogenic
and other hormonal effects. EPA convened a panel of experts, known as the
Endocrine Disruptor Screening and Testing Committee (EDSTAC), which was
charged with recommending to EPA a screening and testing program to determine
if chemicals disrupt estrogens, androgens (male steroid hormones), and
thyroid hormones. The final EDSTAC report was submitted to EPA in August
1998 and EPA is now implementing a testing program based on that report.
EPA activity in 2000 comprises four primary efforts: organize validation
of screening assays through an Endocrine Disruptor Standardization and
Validation Task Force; continue the development of the Endocrine Disruptor
Priority Setting process; harmonize endocrine screening tests of
international interest with the Organization for Economic Cooperation and
Development Endocrine Testing and Assessment Task Force to; and conduct
demonstration studies of several screening assays. Initial testing will
focus on a class of pesticides known as organochlorines. Some of these
chemicals, including DDT and dieldrin, have already been banned from use
(but are still present in the environment); others are still in use.
Other federal agencies are engaged in a wide range of research activities relating to endocrine disruptors, including research to measure potential endocrine disruption in the field and developing indicators of exposure and effect in wildlife species at individual and population levels. However, the National Science and Technology Council - a cabinet-level council that serves as the principal means for coordinating science and technology issues across the federal government - found that the majority of federally-funded research on endocrine disruptors focuses on human health effects rather than effects on the rest of the ecosystem.
Federal policy on endocrine disruptor research and protection against harm from these substances is noteworthy in that it recognizes harmful endpoints other than cancer, which, in the past, was the primary focus of regulations of harmful effects of chemicals in our environment. Second, although the Food Quality Protection Act and Safe Drinking Water Act are intended to protect human health, the research efforts are following EDSTAC's recommendation that testing include assessment of effects in wide range of animal taxa.
What further information is needed?
There is a great need to develop appropriate and sensitive
short-term procedures that recognize chemicals with endocrine activity
using birds - especially embryonic birds - as subjects. More important
for protecting ecological health is an assessment of which chemicals are
harmful to wild bird populations through an endocrine mechanism and a determination
of harmful exposure levels. In this context, there are broad subject
areas that would benefit from basic scientific study, such as:
- endocrinology of higher trophic level species,
especially fish eating birds, so that there is baseline information on
the birds that are likely to be exposed,
- determination of the endocrine activity of several
heavily used pesticides and herbicides that are present in the habitats
of songbirds,
- for all wild birds, more information on the natural
history of thyroid and growth hormone particularly will be important to
help judge the effects of contaminants that affect the actions of those
hormones,
- an understanding of how exposure to multiple
contaminants that individually exist in
the environment below levels known to be harmful
might cumulatively cause harmful
effects
Why should we worry about the effects of endocrine
disruptors on birds?
We all live in the same environment. The chemical compounds
that threaten birds and other wildlife also threaten humans. But birds
are important for many other reasons:
• Birds protect our forests and crops - an average,
a pair of adult warblers removes caterpillars from more than a million
leaves in the two to three weeks from the time the young hatch until they
leave the nest. In the Pacific northwest, for instance, 24 species of Neotropical
migrants feed on western spruce budworm and Douglas-fir tussock moth, the
two most destructive defoliating insects found in the region. Birds
also play an important part in Integrated Pest Management, an agricultural
strategy to reduce harmful insects while reducing the use of chemical pesticides.
•Fruit-eating birds help distribute seeds to promote
forest growth and the birds that drink nectar, such as hummingbirds, help
to pollinate plants.
•Expenditures related to bird-watching, bird feeding,
and other wildlife watching exceeded $29.2 billion in 1996. More than 57
million Americans enjoy feeding and watching birds. Birding is now
the fastest-growing outdoor recreation, with a 150% increase in the past
decade.
Further reading:
Dickerson, R.L. and R.J. Kendall. 1998.
Annual review issue: Endocrine disruptors. Environmental Toxicology
and Chemistry 17; 1-127.
Feyk, L.A. & J.P. Giesy. 1998. Xenobiotic
modulation of endocrine functions in birds. In, R.J. Kendall, R.L.
Dickerson, J.P. Giesy & W.P. Suk Eds. Principles and processes for
evaluating endocrine disruption in wildlife SETAC Press, 121-140.
Fry, D.M., C.K. Toone, S.M. Speich, and R.J. Peard.
1987. Sex ratio skew and breeding patterns of gulls: demographic
and toxicological considerations. Studies in Avian Biology 10; 26-43.
Guillette, L.J., D.A. Crain, A.A. Rooney, and D.B.
Pickford. 1995. Organization versus activation: the role
of endocrine-disrupting compounds (EDCs) during embryonic development in
wildlife. Environmental Health Perspectives 103; 157-164.
Heinz, G.H. 1998. Contaminant effects on
Great Lakes fish-eating birds: a population perspective. In, R.J.
Kendall, R.L. Dickerson, J.P. Giesy & W.P. Suk Eds. Principles and
processes for evaluating endocrine disruption in birds SEATAC Press, 141-154.
MacLellan, K.N.M., D.M. Bird, D.M. Fry, and J.L.
Cowles. 1996. Reproductive and morphological effects
of o,p'-Dicofol on two generations of captive American kestrels. Archives
of Environmental Contamination and Toxicology 30; 364-372.
This publication was reviewed by professional ornithologists and other scientific experts under the auspices of the Ornithological Council. You may contact the Council for further information.
Citation: Ornithological Council (2000).. Bird Issue Brief Vol
2, No. 1, 1st Ed.