birds
Screens are a non-starter. A 4lb+ bird at 250kts is a cannon round basically. Since the encounter could be at any altitude perhaps the best defense is a good offense. Some way to
1) detect the object on approach to the engine
2) ensure it does not get to the engine(s).
The idea would be some sort of directed energy release from the side of the a/c linear with the axis from the spool. Well timed, it would remove the threat from entering the engine. I think 1) is achievable and testable but 2) is tougher given the thin sides of a/c and the extra weight needed for such a rarely needed system.
(obviously this system is only for a close to body engine mounts, #1 and #4 would be on their own in a 4 engine application).