Note: This program first aired on October 22, 2016.
Earlier this fall, a listener contacted me suggesting a
topic for the show. He had just replaced his glass hummingbird feeder with a
polycarbonate plastic one, in a successful attempt to thwart the yellow jackets
that were frequenting the feeder. He wondered though, about the possible
contaminants, especially BPA, that the birds might be exposed to. Anticipating
the arrival of the ruby throated humming birds in the spring and watching them
feed in our yards all summer long rank high among summer pleasures for many
Mainers. But with growing awareness that problematic plastic additives show up
anywhere, and everywhere, it has been only a matter of time until some one put
two and two together and asked this question.
Plastics are polymers, chains of individual units (monomers)
strung together chemically. Most plastics are a mix of different kinds of
hydrocarbons, with various additives to give them specific physical attributes.
These additives have various levels of fidelity to the plastic they are part
of, and some readily leach out of the plastic into the environment. Awareness
has grown in the past 10 years of this potential problem with the consumer
goods and food packaging we contact on a daily basis.
Some chemical additives may be inert, other are quite
biologically active and that is the crux of the problem. Bisphenol A, or BPA, the
additive my listener asked about, is used in polycarbonate plastic, the kind
that lexan water bottles, protective eye wear and DVDs are made of. It has
proven itself to be, as many plastic additives are, a potent estrogen mimic,
meaning, it binds to the same receptive sites in cells that naturally occurring
endogenous estrogen does.
Estrogen is the female sex hormone in all vertebrates, from
fish to mammals. It plays the same role throughout the vertebrate group, in
carefully timed pulses it guides the development of the reproductive system.
The biological or anatomical sex of an individual is the result of the relative
balance of estrogen and male sex hormones like testosterone, and the timing of
the exposure of cells to these hormones. Through the study of developmental
biology, we’ve learned that the critical period for this exposure is very early
in embryonic development.
Having an environmental estrogen out there can mess up this
system, throwing off the balance of hormones, and the timing of exposure. And
that is where most of the permanent impact of chemicals like BPA lies, by mimicking
estrogen and flooding estrogen receptors in the cells of vertebrate embryos BPA
can interfere with the normal development of the reproductive system of exposed
organisms, be they fat head minnows, Japanese quail, or human beings.
Most of the research on the impacts BPA on wildlife has been
on freshwater aquatic vertebrates, as it is easy for BPA to get into surface
water through municipal water treatment facilities and industrial run off. The
research on birds is much more limited, but that which is out there points to
embryonic exposure leading to persistent malformations of oviducts and the
shell gland (leading to thin and weak shells) in female birds, and changes to
brain development in male birds leading to reduced copulatory behavior. These
are problems, that, while initiated when the birds were embryos, don’t show up
until they reach sexual maturity.
All of this bird research has been on model organisms like
Japanese quail or domestic chickens, using exposure vectors like injecting BPA
directly into eggs, or dipping eggs in an aqueous solution containing BPA. No
one has looked at BPA’s effect on wild birds like humming birds, exposed
through the parent’s ingestion of BPA laden sugar water from your new plastic
hummingbird feeder. All we can say is that there is a demonstrated estrogenic
effect in some birds in experimental conditions, but that the impacts on wild
populations with more natural exposure are unknown. If there were negative effects to
hummingbirds, I would expect them to be reproductive.
And before you all start writing me telling me that you can
get BPA free polycarbonate and other plastics, yes, you can. It turns out that
many of the chemicals used to replace BPA are simply other bisphenol chemicals,
or are less well studied, and when they are investigated, turn out to have
similar biological actions. So just because it says BPA free, doesn’t mean it
is necessarily great.
If you are worried about the reproductive health of the
hummingbirds who visit your yard, you may want to continue your search for the
perfect glass feeder, or better yet, cultivate the original hummingbird feeder,
a yard full of flowers.
References:
Excellent review article in Dose Response, focusing on
aquatic vertebrates https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4674185/
Full text of a Swedish PHd dissertation from the University
of Uppsala on environmental endocrine disruption in birds: https://uu.diva-portal.org/smash/get/diva2:165990/FULLTEXT01.pdf
Environmental Defense Fund on BPA free: https://www.edf.org/health/three-reasons-bpa-free-wont-protect-you?utm_source=ggad&utm_medium=cpc&utm_campaign=gr-bpafree&gclid=COXaxd316c8CFQkkhgodBkkBtg
Other sources on BPA alternatives: http://www.the-scientist.com/?articles.view/articleNo/45789/title/Effects-of-BPA-Substitutes/
