A bird, or indeed any object, entering a jet engine is a very undesirable situation and may cause engine failure. There are also fears we're not far away from seeing the first incident of a drone entering a jet engine.

Why do we not see grilles or cages on the front of the jet engines to prevent this happening?

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    $\begingroup$ Any grill that is closed enough to keep bird/drone parts from entering the engine is going to seriously impede the airflow into the engine itself. $\endgroup$ – Ron Beyer Apr 18 '16 at 20:55
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    $\begingroup$ While the other question's answers state that this is not possible due to aerodynamic losses, weight, ice collection, etc. yet nobody provides figures to sustain this statement. So the answers could be improved, or this question reformulated into asking for the feasibility (it seems possible) and consequences on the cost of such device. $\endgroup$ – mins Apr 19 '16 at 8:15
  • $\begingroup$ A grille over the front of an engine will not stop a bird from entering the engine of a plane going at a few hundred knots. It will just stop a complete bird from entering, it will however now be in manageable chunks. If the aircraft is going slow enough not to mince the bird you will now have a bird sized blockage in the airflow in addition to the surface area of the grille. At least if the bird enters the engine there is still a chance that the turbine can mash the bird into manageable mush. $\endgroup$ – AndyW Feb 19 '19 at 16:15