Engine Grills
Engine Grills are not fitted because they cause too much drag and loss of efficiency in the intake. They can also ice. At worst, they could detach and cause more damage than birds. In general, the incidence of damaging birdstrikes is so low, and the engines capable enough of surviving ingestion, that no grill is necessary. The only exception, I believe, is the F117 Stealth Fighter, which has them for Low Observability - a different kind of trade-off