Aerodynamics and weight. Both are very important for all aerospace vehicles. Something like that would also decrease engine efficiency.

Supersonic aircraft engines usually have an aerospike, which could impale a bird... that would be a bloody sight.

I wonder if anyone has looked at doing a cone-shaped grill that extends forward some distance beyond the front of the engine. It would look goofy as hell, but it would only have to be strong enough to survive a glancing impact, rather than a direct hit like a flat grill. Air resistance might be an issue though...

Are we playing 20 questions or something? Just come out with it; the name you are looking for is cow catcher.

I think that sort of geometry would encourage the air to flow around the engine, reducing pressure & density of the air actually propelling the aircraft. This would seriously impact the efficiency of the engines, and I suspect this is what the article is referring to. A flat grille would be crazy as it would just collect the birds, held in place by the extreme pressure conditions.

I was just thinking about that same thing.