There was a strong correlation between the observations of dead birds and wildfires and the toxic gases they produced, but not with the early winter storms.
A new report based on necropsies conducted by the USGS's National Wildlife Health Center in Wisconsin suggests that the die-off was caused by long-term starvation and was likely exacerbated by a spate of freakishly cold weather.
Flycatchers, swallows and warblers are among the species “in a mass die-off across New Mexico, Colorado, Texas, Arizona and farther north into Nebraska.