Study: Mass bird deaths in Colorado due to toxic gases from wildfires iNaturalist Massive bird die off in Salida on the morning of Sept. 9. Fifteen dead birds were found next to house next to Arkansas River. and last updated 2021-04-22 08:44:27-04 DENVER â A new study suggests a mass death of birds last summer was likely due to toxic gases produced from wildfires. The study, published in the journal "GeoHealth," reviewed pictures of dead birds that were uploaded to a crowdsourced science platform called iNaturalist. Various users across the western United States, including Colorado, took the pictures after noticing the birds in their backyards and even on hikes.