As you yourself point out, birds outnumber drones a million to one. So obviously the available number of examples is much smaller. That doesn't, however, change the potential danger.

Perhaps not, but some of us would like to see more than "potential danger" to justify panicked, knee-jerk lawmaking. "Think of the airplanes!" is no better a rationale, in itself, than "Think of the children!" or "B...b...but terrorists!"

Or, rather, it's a great rationale if you have an agenda that needs more political support than it would otherwise be able to garner. It's not hard to imagine why governments would be utterly terrified of a civilian population with widespread drone ownership and ongoing development.

Notable this year was the two drones that crashed into the White House, and a whole string of incidents involving drones and airliners landing at JFK:

http://newyork.cbslocal.com/2015/08/03/drone-sighting-jfk-ai... http://www.cnn.com/2015/05/14/politics/white-house-drone-arr...

etc.. these are not hard to find. Now actual compendiums are being done, but I couldn't find the raw data. The FAA says it has gathered "more than 650" reports: https://www.faa.gov/news/updates/?newsId=83445

As a pilot, I do not see this as being surprising. People are doing dumb things with drones, just like pilots did dumb things with airplanes in the '20s and '30s, leading to the precursor of the FAA and the creation of the airspace system.

But the real reason for registration is so that when there is an accident involving a drone and someone on the ground, such as a person shooting down a drone, the owner of the drone can be identified.

Finally, if a drone gets involved in my airplane's propeller, it will probably be diced into chunks so small you'll never find all the pieces. However the propeller and engine will need to be rebuilt, at a cost of $20,000+, so you can bet that I'll be on the ground looking for those bits so I can stick that insurance claim.

Some airports employ hawk trainers to keep the air around the airport clear of birds. Can't do that against drones.