Famously, the use of caged birds to alert miners to the invisible dangers of gases such as carbon monoxide gave rise to the cautionary metaphor “canary in a coal mine.”

But other than the fact that exposure to toxic gases in a confined space kills caged birds before affecting humans — providing a timely warning to miners — what do we know about the effects of air pollution on birds?

Not as much as you’d think, according to researchers from the University of Wisconsin–Madison.