Birds – and in particular, their feathers – are excellent bioindicators for the presence of chemicals in the environment. Why?

Birds are pretty much everywhere and are part of almost all food chains. So, unwillingly and unpaid, they collect environmental samples in a huge number of locations and conveniently store them in their feathers. These feathers can then (relatively) easily be collected and analyzed without destroying the research assistant (i.e., the bird collecting the samples).

Of course, I could have written this in a much more academic tone:

“Feather analysis, a largely non-invasive technique, offers a unique temporal and spatial record of environmental pollution, capturing both endogenous incorporation of contaminants from diet and metabolism, and exogenous accumulation from airborne particulate matter.”

But hey, who wants to read this when instead, you can have my bad jokes?

Feathers have another advantage – they do not substantially change their composition once they have been formed. So, the feathers contain information not only about the presence, but also about the past. This is particularly interesting when looking at feathers of museum specimens that may be decades or even hundreds of years old.

Let us look at a few more aspects of feathers as indicators for the presence of chemicals:

• How are the chemicals incorporated into the feathers?
• How can chemists detect the chemicals in the feathers?
• Which chemicals can typically be found in feathers, focusing on those that should not be there in an unpolluted environment?

How are the chemicals incorporated into the feathers?

The incorporation of chemicals (and yes, I am mostly using the term “chemicals” in a sense of “bad, toxic chemicals that should not be there if there wasn’t any pollution” – a usage that would normally annoy me as a chemist, but hey, I can sometimes pretend to be flexible) has two main pathways, an internal and an external one.

The internal one means chemicals that are in the bloodstream of the bird get incorporated into feathers as these are built up. Once the feathers are fully built, they do not substantially change their internal chemical composition anymore. That means that whatever chemicals can be found incorporated into the keratin matrix of a feather must have been present during the formation of the feather, not afterwards – a perfect time capsule.

The external pathway proceeds via chemicals accumulating on the surface of feathers. Feathers have a large surface area and can easily trap airborne pollutants such as soot (carbon black). Birds also actively apply preening oils secreted from a gland to their feathers, and these oils in turn bind some chemicals (particularly organic chemicals) on the surface of the feathers.

Whether the external or internal pathway is more important depends on the nature of the chemicals – we will get to that a bit later. It is an important distinction, for example, to understand whether a toxic chemical comes from the bird’s food (internal pathway) or the air it flies in (external pathway).

How can chemists detect the chemicals in the feathers?

Here is what a scientist does with a feather sample. First, the feather is cleaned (assuming the focus is on the internal pathway as described above) – this removes chemicals that are only on the surface of the feathers and thus did not go through the bird’s bloodstream. Then, the feathers undergo an extraction process, either with an organic solvent (for organic chemicals) or with an acid (for metals). Only then, the real analysis starts.

For organic chemicals in feathers, the main methods are GC-MS (Gas Chromatography and Mass Spectroscopy) and LC-MS (Liquid Chromatography and Mass Spectroscopy). Both methods work by first driving the sample through a long column, with either a gas or a liquid as the driver. Different chemicals go through this column at different speeds, so the different chemicals in the sample are being separated. At the end, the molecular mass of each chemical (and to some extent, its chemical structure) is determined by a mass spectrometer.

For the detection of metals, several methods with somewhat frightening names, such as Inductively Coupled Plasma Mass Spectrometry, Inductively Coupled Plasma Optical Emission Spectroscopy, Atomic Absorption Spectroscopy, and X-ray Fluorescence, can be used. Most of these methods utilize the different wavelengths at which different metals absorb light, though the first one works by turning the metals into ions and then determining the mass of these ions. This is a particularly sensitive method.

Which chemicals/pollutants can typically be found in feathers?

The feather analysis typically focuses on three groups of pollutants: heavy metals, persistent organic pollutants, and soot/particle matter. Recent research has also added microplastics to the spectrum.

Feathers are known to accumulate heavy metals and thus are good indicators of metal pollution. Common heavy metals identified in feather analysis include:

• Copper (Cu): Copper is an essential trace element but toxic at elevated levels, and its presence in feathers reflects both dietary intake and environmental exposure.
• Iron (Fe): While essential, iron can also be a pollutant. Its accumulation in feathers can come from natural and anthropogenic sources.
• Mercury (Hg): Mercury has a strong affinity for keratin, making feather analysis a good way for assessing mercury exposure.
• Lead (Pb): Studies on blue tit nestlings have demonstrated that increased dietary lead directly results in higher lead concentrations in feathers, proving the internal pathway.
• Zinc (Zn): Zinc, like copper, is an essential element but can accumulate to toxic levels.
• Manganese (Mn), Chromium (Cr)

Some studies have shown that urbanization and increased traffic are associated with higher metal concentrations in feathers.

Feathers are also a good way to monitor persistent organic pollutants, such as

• Polychlorinated Biphenyls
• Dichlorodiphenyltrichloroethane (DDT) and Metabolites
• Polybrominated Diphenyl Ethers
• Per- and Polyfluoroalkyl Substances

For these chemicals, both an internal pathway into the feathers and an external one via affinity to preening oils are relevant.

Black carbon, a significant component of atmospheric particulate matter, is another important pollutant identifiable through feather analysis. Birds, particularly those in urban or industrial areas, act as natural “air filters,” with soot clinging to their feather surfaces. This has actually been used to track historical black carbon levels in the air by comparing feathers from museum specimens collected over a period of 135 years. For example, a sharp drop in black carbon on birds was observed during the Great Depression due to reduced coal consumption, followed by a rebound during World War II, and a subsequent decline as natural gas replaced coal for heating.

Photo: Blue Tit, Visselhoevede, Germany, July 2018