What Museum Collections Of Century-Old Birds Tell Us About Our Dirty Air

Century-old museum bird specimens reveal that the air over America’s Manufacturing Midwest during the late-19th and 20th Centuries was even more polluted than previously thought

by GrrlScientist for Forbes | @GrrlScientist

Ten Horned Larks (Eremophila alpestris pratensis) at The Field Museum, showing that specimens collected in nonindustrial regions do not exhibit comparable levels of soiling to birds collected within the US Manufacturing Belt. The five specimens on the left were collected in Illinois, inside the US Manufacturing Belt. The five specimens on the right were collected along the western coast of North America, outside of the US Manufacturing Belt. All 10 specimens were collected during nonmolting months (January–April) between 1903 and 1922. (Credit: Shane G. DuBay and Carl C. Fuldner | doi:10.1073/pnas.1710239114)

A team of researchers recently announced they’ve developed a method for measuring the amount of black carbon found on the plumage of birds held in museum collections. The researchers used their clever technique to measure black carbon on the plumages of more than 1,300 common backyard birds that had been collected by museums during the past 135 years. The team’s results reveal the most complete and detailed picture to date of historic air pollution levels in America’s Manufacturing Belt, and provides us with a more complete understanding of historic climate change. This research also lays the groundwork for a rigorous new assay that can be used to independently verify other, mostly indirect, methods for assessing air quality over specific regions during the past century-and-a-half.

Midwestern museums are filled with dirty birds

“If you look at Chicago today, the skies are blue. But when you look at pictures of Beijing and Dehli, you get a sense for what US cities like Chicago and Pittsburgh were once like,” said Shane DuBay, a graduate student who studies interactions between organisms and their environment at the University of Chicago.

Mr. DuBay and his collaborator, photographer Carl Fuldner, a graduate student in the department of Art History at the University of Chicago, knew from anecdotal reports that urban air pollution was a serious issue for many midwestern cities in the Manufacturing Belt, but there was no quantitative method to measure precisely how bad this historic problem actually was. Undaunted, they persisted.

“Using museum collections, we were able to reconstruct that history.”

Photographs of museum bird specimens with soiled plumage side-by-side to those with cleaner plumage are startling. These birds had been collected over the same time periods but from different regions of the United States, or they were collected from the same areas but from different time periods.

“The photographs give the project a visceral dimension — you make a connection to the images,” Mr. Fuldner said.

Wild birds collected soot on their plumage as they flew through smoky skies

As early as the 1930s, it had been noted in the scientific literature that wild birds were becoming darker. This observation led to a discussion at the time as to the reason for this coloration: were dark birds the result of plumage soiling or genetic changes?

This simple question inspired Mr. DuBay and Mr. Fuldner to investigate.

Museum collections include everything from whole specimens, skeletons and fossils to tissue samples, cell lines and DNA libraries. These collections provide the essential framework for our collective knowledge of taxonomy and systematics as well as environmental changes, but they also contribute to divergent issues such as public health and safety and even to national security. Basically, museum collections are vast libraries of biological information, and each specimen is a single volume within that library, waiting for someone to come along with a question and a sensible method for how to investigate it. Yet paradoxically, these collections are overlooked and undervalued by policymakers, the public and even many academics, and thus, are provided inadequate funding to maintain and support them.

Mr. DuBay and Mr. Fuldner sought to highlight the value of natural history museum collections for addressing novel research questions, and they particularly wanted to emphasize the importance of time-series collections, such as birds collected from the same area over long periods of time.

“In this case, we started with a conceptual premise: we wanted to restore the link between these specimens and the historical landscape — the particular time and place — from which they were taken,” Mr. Fuldner writes in email.

Mr. DuBay and Mr. Fuldner designed a study to examine airborne soot created by local manufacturing, domestic heating, and railway transportation that soiled local bird specimens’ plumage.

“When you touch these birds, you get traces of soot on your hands,” Mr. DuBay said, an observation that helped answer the “genetics versus soiling” question. “We’d wear white gloves while handling them, and the gloves would come away stained, like when you get ink on your fingertips reading a newspaper.”

“These birds were acting as air filters moving through the environment,” Mr. DuBay elaborated.

Museum bird specimens had captured airborne soot on their plumage

Due to their bright white undersides that clearly show plumage soiling, one of five the bird species included in this study was the horned lark, Eremophila alpestris.

One of the five species of study birds, the horned lark (Eremophila alpestris), is a common North American song bird. (Credit: Francesco Veronesi / CC BY-SA 2.0)

These small brownish songbirds occur in wide open spaces throughout most of the northern hemisphere. But the bright white bellies and brilliant yellow throats of the horned larks collected in the midwest over the decades are a dingy grey. This discoloration comes from airborne soot.

Horned Larks (Eremophila alpestris pratensis) at The Field Museum, collected 100 years apart in the Manufacturing Belt of the American midwest. (Credit: Shane G. DuBay and Carl C. Fuldner | doi:10.1073/pnas.1710239114)

One component of soot is black carbon. It consists of airborne particles of pure carbon created by incomplete combustion of fossil fuels, coal and wood. As a direct result of the industrial revolution that was well underway by the mid-1800s, cities within America’s Manufacturing Belt, such as Chicago, Detroit, and Pittsburgh, suddenly found themselves enshrouded in clouds of atmospheric soot generated by coal burnt for manufacturing, domestic heating, and railway transportation.

These dark clouds were more than just unsightly, they were noxious; driving a sharp increase in a plethora of respiratory illnesses in city dwellers, along with increased mortalities. Eventually, this growing urban smog problem finally motivated civic-minded reformers to enact the first city-wide efforts to mitigate soot, a move that also helped launch the modern environmental movement.

These efforts were successful: American cities no longer experience the levels of atmospheric black carbon that peaked in the early 1900s. However, cities in emerging economies, such as China and India, now are blanketed by huge clouds of particle pollution, which threatens the health and well-being of their residents.

Smog over Beijing, China in 2010, via CCTV (Credit: William Veerbeek / CC BY-NC-SA 2.0)

More recently, we’ve established that black carbon is a major contributor to human-caused (anthropogenic) climate change. For this reason, it is important to get a more complete understanding of black carbon’s effects on climate, both past and future, and its effects upon environmental history and policy.

To do this, scientists rely upon a variety of methods to estimate historical black carbon emissions. An indirect method that is often used is to substantiate past fuel consumption for the region being studied. But the burning efficiency of fuels varies, so it’s difficult to work out real-world black carbon emissions over time.

A more direct method is to examine Greenland ice cores. This is an extensively referenced source of North American black carbon emission data extending back before the 1950s. But airborne black carbon was primarily the product of urban centers and highly industrialized regions, so air pollution levels varied tremendously across the landscape: how might scientists determine the airborne levels of black carbon experienced by, say, residents of a particular city at a particular time?

“We found that the soot, as a kind of literal trace of the historical landscape, would be a perfect way to explore that concept. The soiling on the birds was known to the collections managers and curators, but it’s also immediately apparent just by opening a drawer,” Mr. Fuldner said, adding that more recently collected specimens were much cleaner than specimens that had been collected decades earlier (Figure 1).

Using scanning electron microscopy (SEM), the team of researchers started by confirming that darkened feathers from bird specimens were indeed soiled with black carbon particles (Figure 1):

Fig. 1. Comparison of two Field Sparrows (Spizella pusilla pusilla), one from 1906 and one from 1996. (Lower) SEM micrographs of belly feathers plucked from specimens in Upper. SEM micrographs of the Field Sparrow from 1906 show black carbon aggregate composed of small spericals. The feather from the 1996 specimen lacks black carbon deposition. Both specimens were collected during spring months in the vicinity of Chicago. SEM images were made with a Tescan LYRA3 field emission microscope with secondary electron (SE) detection and an acceleration voltage (HV) of 3.0 kV. Feather samples were carbon-coated before imaging.
(Credit: Shane G. DuBay and Carl C. Fuldner | doi:

Plumage of many bird specimens was soiled by black carbon granules

After establishing that darkened plumage resulted from accumulations of black carbon granules, Mr. DuBay and Mr. Fuldner expanded their study to include more than 1,300 specimens of five species of songbirds and woodpeckers with prominent white areas in their plumage. These specimens represent an important time series because birds replace all of their feathers every year and these specimens had been collected entirely within the Manufacturing Belt during the previous 135 years. The Manufacturing Belt comprises the states of Pennsylvania, Ohio, Indiana, Michigan, Illinois, and Wisconsin — states where industries burned large quantities of soft and sticky bituminous coal, which is a semi-solid form of petroleum. When burned, bituminous coal emits greater quantities of black carbon granules than the harder, anthracitic coal that was more commonly used in the Eastern United States.

Mr. DuBay and Mr. Fuldner started by photographing each specimen. They quantified soot accumulation by measuring the proportion of light that white breast and belly feathers reflected. Reflectance ranges from 0% (pure black) to 100% (pure white). Black carbon, which is the primary light-absorbing component of soot, has low reflectance, so this was a nifty way to easily quantify the relative “sootiness” of each specimen.

The amount of carbon that accumulated on the plumage of individual bird specimens (black dots; Figure 2) was charted by the year when the specimen was collected and averaged with 95% confidence (solid black line with grey shaded area around it; Figure 2). These data were compared to the amounts of coal burned (orange line; Figure 2) and to black coal emissions (purple line; Figure 2):

Fig. 2. Black carbon deposition on specimens of five bird species from the US Manufacturing Belt, collected between 1880 and 2015. Each point represents the z score for an individual specimen (n = 1,097) based on the inverse raw reflectance value taken from its breast and belly feathers. The black line is a GAM (k = 20) with 95% confidence limits (indicated by the shaded area), determined from the individual specimens. The orange line is consumption for coal in the United States expressed in British thermal units (BTUs) (US Energy Information Administration). Before 1950, coal consumption data are available in 5-y intervals. After 1950, coal consumption data are yearly. The purple line shows estimates of total US black carbon (BC) emissions (from this), which uses fuel consumption data and emission factor data to generate a historical emission inventory. The dashed line at 1910 denotes the progressive shift in cities within the US Manufacturing Belt from prosecuting to educating emissions violators. The dashed line at 1960 denotes the approximate moment after which black carbon emissions become decoupled from coal consumption. (Credit: Shane G. DuBay and Carl C. Fuldner | doi:10.1073/pnas.1710239114)

“We were surprised by the precision we were able to achieve,” Mr. DuBay said. “The soot on the birds closely tracks the use of coal over time. During the Great Depression, there’s a sharp drop in black carbon on the birds because coal consumption dropped — once we saw that, it clicked.”

Birds collected later during World War II were sootier due to a sharp increase in manufacturing and concomitant coal use, but after that war ended, bird specimens became cleaner because local residents began heating their homes with natural gas instead of coal.

“The soot on these birds’ feathers allowed us to trace the amount of black carbon in the air over time, and we found that the air at the turn of the century was even more polluted than scientists previously thought,” Mr. DuBay pointed out.

Sooty museum birds provide a vital time series of historic air quality

This study develops an assay that lays the foundation for a wide variety of research projects, starting with using museum collections to establish local atmospheric pollution levels for other parts of the world — and these data are even useful for estimating health impacts faced by our grandparents and great-grandparents. This assay is easy and direct; contrasting with Greenland ice-core data, which require interpretation of where atmospheric pollution originated based upon a variety of parameters.

But air pollution affects more than just people: this research also opens the way to understanding how black carbon pollution historically impacted avian and wildlife health and population dynamics.

“As we note in the paper, we know that particle pollution has already been associated with increases in heritable DNA mutations in bird and rodent populations, but there is currently a dearth of research on the effects of black carbon on non-human animal populations, so that is definitely something we plan to pursue,” said Mr. Fuldner in email.

This study highlights the usefulness and importance of museum time series collections to science and to society. For example, museum time series collections of eggshells provided the necessary data that allowed scientists to track the increasing levels of the pesticide, DDT, in the environment (ref). Another time series study of bird eggshells documented the levels of heavy metals, such as mercury, in the environment (ref).

It’s also worth pointing out that, although one hundred or so years ago no one knew whether preserving internal organs from collected birds might be informative in the future, some progressive and thoughtful scientists still made the effort — and Mr. DuBay and Mr. Fuldner’s research will benefit.

“Another rich resource that could play into future study, for example, is a collection of pickled bird lungs, which could presumably shed light on how the black carbon affected the birds’ health,” Mr. Fuldner said in email.

Genetics will also provide valuable glimpses into DNA mutations and population dynamics.

“We have discussed looking at population dynamics via DNA with reference to heritable mutations,” Mr. Fuldner added. DNA studies could illuminate historic changes in behavior: because birds are highly visual animals, darkened plumage could have dramatic effects upon mate selection and population dynamics amongst birds.

“In general, there is a lot of work left to be done on various scales in various parts of the collections.”

Mr. DuBay and Mr. Fuldner expect that their study serves as a valuable reminder that museum collections hold a vast treasure trove of knowledge and discovery for present and future scholars that could never have been imagined when specimens were being collected. Additionally, other countries, particularly Britain, have a much longer history of collecting specimen time series than does the United States, so scientists can use this method to take an even longer look back in time.

“In the early 1900s, there was no way a collector could have known two graduate students in Chicago would use that bird to paint a picture of atmospheric conditions for that time,” Mr. DuBay said.


Shane G. DuBay and Carl C. Fuldner (2017). Bird specimens track 135 years of atmospheric black carbon and environmental policy, Proceedings of the National Academy of Sciences, published online on 9 October 2017 ahead of print | doi:10.1073/pnas.1710239114

Also cited:

Joseph J. Hickey and Daniel W. Anderson (1968). Chlorinated Hydrocarbons and Eggshell Changes in Raptorial and Fish-Eating Birds, Science 162(3850):271–273 | doi:10.1126/science.162.3850.271

Lawrence J. Blus, Charles J. Henny, Allen Anderson and Richard E. Fitzner (1986). Reproduction, Mortality, and Heavy Metal Concentrations in Great Blue Herons from Three Colonies in Washington and Idaho, Colonial Waterbirds 8(2):110–116 | doi:10.2307/1521060

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Originally published at Forbes on 31 October 2017.