[skriticos2]

"Most accidents occur when the bird hits the windscreen or flies into the engines. These cause annual damages that have been estimated at $400 million within the United States of America alone and up to $1.2 billion to commercial aircraft worldwide."

http://en.wikipedia.org/wiki/Bird_strike

Relatively few fatalities for humans, but very high cost. And they are not made of metal or durable plastics. They are also less likely to fall onto your head with rapidly rotating blades due to technical failure.