I think you need to define "mostly harmless". I will imagine that if a bird gets sprayed those chemicals mid-air, it will either drop to the ground or start flying erratically and collide into something mid-air which can cause death[1].
[1] I've seen a bird hit the side of my stationary car out of nowhere and it died on the spot.
Similarly these birds are attacking drones midair which may cause them to spiral to the ground and be destroyed. It seems like fair game to build anti-bird defences in to your drone given that birds are being trained to attack them. Perhaps it will make these people think twice.
That article stated, that these birds are able to bring the drone to the ground safely.
> "It seems like fair game to build anti-bird defences..."
[irony on]
Yeah totally.
[irony off]
Because it is your right to fly your drone everywhere? To potentially endanger others?
Let's say imagine these trained birds attacking your drone after it moving into the flight path of a landing aircraft. Or above a crowd at a festival, or above your neighbors garden.
What if it fails and drops? What if it crashes into something (plane) or someone (human)?
If someone would endanger me or my beloved ones (be it in a plane, at a festival or in my garden) by flying these things irresponsibly - believe me a drone would not be the only thing broken if I could get to them.
If these birds are there to protect others from people using these things for malicious reasons or in a way that is just dumb (read idiots), I would hope that the owners of these things additionally get fined a horrendously high amount. And have to pay for the deployment of these countermeasure birds.
Make the learning experience a painful one for them I would say.
> Perhaps it will make these people think twice.
I am with you on this one. OK; I think of the drone operators as "these people" but isn't this the same?
If your drone gets damage, you should file a complaint against the bird owner. Otherwise, unless the birds are threatening actual people and not just property, it might be (and should be) a crime to deliberate install mechanisms to harm them.
[1] I've seen a bird hit the side of my stationary car out of nowhere and it died on the spot.