Saturday, October 22, 2016

BPA (Bisphenol A) and Hummingbirds

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.


Excellent review article in Dose Response, focusing on aquatic vertebrates

Full text of a Swedish PHd dissertation from the University of Uppsala on environmental endocrine disruption in birds: