So far food scarcity looks like the most plausible explanation. Someone needs to get a massive campaign together in flyover zones to get people to scatter food for them. Assuming appropriate feed for these species is even widely available in those areas. Probably too little too late in this case though... I guess we just update the wildfire documents to include care for migratory species and hope there's a next time
I want to point out the issues of 1) commenting on areas where one doesn't have expertise 2) not recognizing that and find credible sources of information. No offense, but even though you projected confidence I doubted you had any ornithology background. Since I didn't either, I recognized my lack of expertise and did some good faith basic research, informally speaking, to see if I could find out what the most likely…
We could still try scatter food.
The way this year has been we might as well help Cthulhu along and make bugs fall from the sky ourselves.