Obviously weather is a variable that affects bird populations (and wildlife populations overall), and I don't think anyone would disagree with that. Floods in June during peak nesting or severe drought or an ice storm during winter can severerly impact things but are out of our control.
Bird flu is not going to ravage wild pheasant populations, ever. It may kill a few, but it will never impact them signficantly because wild pheasants do not hang out inside barns amongst thousands of other pheasants (unless we're talking flare nares, which aren't wild birds anyways). Its the constant proximity of birds close to each other indoors that wreaks havoc, which is why its devastating for domesticated commerical poultry.
The biggest variable is habitat. To have naturally-reproducing wild upland bird populations, you need habitat. Big Ag has a say in that, and they will continue to have a say in that.