"It was surprising to see how limited research on this topic is," Olivia Sanderfoot, now at the University of Washington, US, told environmentalresearchweb. "We were not expecting that, given that birds breathe more efficiently than mammals and are therefore likely more susceptible to the negative health outcomes associated with respiratory exposure to air pollution."

Birds respire more efficiently than any other terrestrial vertebrate. Coal miners used them to signal the build-up of dangerous levels of toxic gases such as carbon monoxide underground, and many see birds as "sentinel species" for environmental change, including the introduction of pesticides, insecticides, heavy metals and climate change. And they could act the same way for air pollution too.

Together with colleague Tracey Holloway of the University of Wisconsin-Madison, in November 2016 Sanderfoot searched Web of Science for articles related to ‘air pollution’ and ‘human health’ published since 1950. There were 2388 results. Searching for ‘air pollution’ and ‘birds’, on the other hand, gave just 132 articles, whilst 3507 papers covered birds and climate change.

"We know a lot about how exposure to air pollution affects human health, but not so much about how air pollution might be affecting other animals," said Sanderfoot, "and yet we are clearly capable of characterizing avian responses to other types of environmental change, such as rising average temperatures".

What’s more, the papers covered a limited geographic spread, with more assessments of the effects of air pollution on birds in Eastern Europe than in Asia or North or South America. "Much of that work is also focused on the effects of exposure to industrial emissions," said Sanderfoot. "There is significantly less known about how birds are affected by exposure to air pollution from other types of sources, such as motor vehicles."

Air pollution may affect birds even whilst they’re in the egg. Studies have shown that carbon monoxide gas can diffuse through eggshells and it’s possible that other contaminants in the atmosphere may be able to do this too.

Scientists conducting field studies face many challenges, including how to tease out direct and indirect effects of air pollution on bird communities. "It is difficult to isolate avian responses to inhalation exposure to air pollution from responses that could be due to changes in habitat, such as heavy metal accumulation in soils or shifts in forest structure, that could affect food resources and nesting sites available to birds," said Sanderfoot.

It’s much easier to look at physiological, morphological, or behavioural changes in birds when exposing birds to chemicals in a controlled laboratory setting, according to the researcher. "However, laboratory studies often do not expose birds to the mixtures or concentrations of air pollutants they would be likely to encounter in the wild, making it difficult to predict how birds might be suffering from exposure to air pollutants in natural settings."

That said, studies in the wild have tended not to measure levels of pollution directly, instead comparing the health of birds from polluted and unpolluted sites. "We need to better understand how both acute and chronic exposure to air pollution affects birds by actually measuring exposure in natural settings," Sanderfoot said. "Proxy measurements, such as concentrations of heavy metals in soil, are useful, but certainly not sufficient, in determining exposure levels and calculating risk factors for various health outcomes."

Sanderfoot says scientists must design studies that not only assess how birds are affected by air pollution, but what concentrations of specific chemicals are linked to those responses. Chemical transport models and satellite measurements could help with this task, particularly in the large areas of the globe that don’t have ground-based air pollution monitors.

Sanderfoot believes this field has immense promise. "If we don’t limit ourselves to disciplinary divides, if we work to link tools from atmospheric chemistry, toxicology, and ecology, I think we will be able to rapidly expand our knowledge of how birds are affected by air pollution," she said. "This will help inform public policy and conservation planning and open the door to new and exciting research on the effects of air pollution on non-human animal species."

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