And how many "few locals" is enough for prejudice to turn into statistic?

For a dog, one bad experience can be enough. They can also spread their behaviour to other dogs that notice that they are afraid in certain situations. These dogs will "teach" other dogs and the behaviour spreads quickly.

Yep, animals learn from bad experiences quite fast. My little parrot won't go near towels anymore because we used to wrap him in one so he would hold still while trimming his nails. This doesn't hurt the bird and not doing it makes it hard for them to walk, but the sounds and struggle emmited by that pint sized green cheeked conure would make the unaware think I was torturing him.

Now, he won't even let us wrap him in a towel after he decides to dunk himself in his water bowl for a bath. No matter how wet, cold and pathetic he looks (birds smell like a wet potato after a bath), he won't let us dry him off.

how do they trim them in wild?

The other commenter sums it up--they have many more coarse/rough items to cling onto in the wild. We leave plenty of items like that in his cage as well, but he's very particular about where he perches and what he perches on. Parrots can be very set in their ways and takes a lot of work at times to introduce them to new things in their routine.

Also have to keep their beak grounded down for similar reasons. It's quite sharp and he's learned he can use it as a weapon when he's not getting his way. Imagine a 2 year old human that isn't afraid to poke you with a sharp stick when they don't get their metaphorical candy. That's pretty much my parrot.

It's kind of like sharing a space with the world's biggest (and smallest/cutest) curmudgeon at times (parrots are cranky when they don't get enough sleep). He loves me and my SO, though he loves her more than me, even though he's technically my parrot. Parrots are kind of like us in that they pick and choose who they love and make you work for it.

I can say having a parrot as a pet is a drastically different experience than I've had with cats, dogs or rodents.

Wild birds climb trees etc. more than captive birds. The reason nails grow is that they are not used as much as they would naturally.