Wildfire smoke impacts the body condition and capture rates of birds in California

Despite the increased frequency with which wildfire smoke now blankets portions of world, the effects of smoke on wildlife, and birds in particular, are largely unknown. We used two decades of banding data from the San Francisco Bay Bird Observatory to investigate how fine particulate matter (PM2.5) – a major component and indicator of wildfire smoke – influenced capture rates and body condition of 21 passerine or near-passerine bird species. Across all study species, we found a negative effect of acute PM2.5 exposure and a positive effect of chronic PM2.5 exposure on avian capture rates. Together, these findings are indicative of decreased bird activity or local site removal during acute periods of wildfire smoke, but increased activity or site colonization under chronic smoke conditions. Importantly, we also observed a negative relationship between chronic PM2.5 exposure and body mass change in individuals with multiple captures per season. Our results indicate that wildfire smoke likely influences the health and behavior of birds, ultimately contributing to a shift in activity and body condition, with differential short-term versus long-term impacts. Although more research is needed on the mechanisms driving these observed changes in bird health and behavior, as well as validation of these relationships in more areas, our results suggest that wildfire smoke is a potentially frequent large-scale environmental stressor to birds that deserves increasing attention and recognition.

Methods
We used bird banding data spanning 2000–2021 from CCFS, a bird banding station located in north San Jose, a city in the southern region of the California Bay Area. Banding data and metadata were used to create multiple variables for our analysis. For the primary analysis of capture trends, we calculated the number of unique captures of each species for each day of banding (i.e., excluding daily recaptures). For the analysis of changes in body condition, we restricted the analysis to individually marked birds that were captured at least twice between the months of July–November in at least one year. We restricted our analysis to bird species common to California and excluded migratory species that are known to generally be absent from the banding station vicinity during the months of October and November (generally, ‘peak’ fire season along the California coast). For the second analysis of changes in body mass, we began by reducing the data down to individuals that were captured at least twice during the fire season in single year. To estimate exposure to smoke, we used data from ground-based air quality monitors included in the EPA Air Quality System (AQS) near the CCFS banding station. See Methods in published manuscript for full explanation and processing steps.