Heavy wildfire smoke shows up as 3% fewer bird species the following breeding season

Separating smoke from the weather that makes it is the hard part of this problem. Fires cluster in heat and drought, and heat and drought press on birds on their own. Meier and Strobl compared the same routes against themselves over many years, holding weather, land use and differences in how the counts were conducted ^[10]. Then they took on the two explanations that would have made the smoke result trivial. If burnt habitat were the cause, the effect should disappear where nothing burnt nearby; on routes with no fire inside five kilometres it persisted ^[11]. If birds were simply moving a valley over, netting out local shifts should erase it; it persisted there too ^[12].

Neither test identifies a pathway, and the phys.org account of the work describes a correlation evident across the United States as a whole ^[22]. The finding is also lagged: heavier smoke in one year, shorter species lists in the next breeding season ^[6]. That ordering rules out one nuisance. A volunteer straining to hear birds through haze is a detection problem, and a detection problem does not wait a year to appear.

The percentage is a route-level average ^[7]. On a route logging 55 species, 0.03 times 55 is 1.65 species ^[20]. At the most heavily smoke-affected monitoring sites, the count fell much more sharply than that ^[9]. Behind the estimate sit at most about 42,000 route-years, if all 2,800 routes were run in each of the 15 seasons ^[21]. Species number was one of three measures: the pair also examined how evenly birds were distributed across species, and how far apart those species sit in evolutionary terms ^[5].

"I wasn't sure whether we would see any effect at all across such a large and ecologically diverse area," Meier said ^[15]. She is plain about the size of what came back. "The effect isn't huge, but it is measurable. And we never know which species will disappear and which of these are particularly important for an ecosystem. Therefore, even a small decline can, in the worst-case scenario, cause an ecosystem to become unbalanced," she said ^[16].

Route counts cannot show what happened to individual birds, and the study does not establish what that is. Other research points to smoke weakening birds, possibly through weight loss and respiratory damage, and to altered behaviour during the breeding season ^[13]. The pair also left valuation out by design, choosing to demonstrate single impacts as reliably as possible first ^[14].

"In the case of burned-out buildings, we can relatively easily quantify the cost of the damage. With biodiversity, things are far more complex," Meier said ^[17]. Birds disperse seeds, pollinate plants and help control pests, and a decline in their diversity reaches other animals and plants and eventually sectors such as agriculture and forestry ^[19]. "There are other consequences of forest fires that we have barely taken into account to date, even though they have an impact on society," Meier said ^[18].