It varies so much, it's hard to say. Some birds take to it like a duck, some take forever and some never accept it. My parrot, a small amazon, took about a year, yes year ,of training 7/365 of gradual introduction and then putting it on, and we still practice a few times a week. But Salty is 100% comfortable putting and keeping on now. He even goes with me to perform at pirate festivals in it.
NOTE .. THIS IS FOR ANAVIATOR HARNESS. I've never seen or worked with a EZ harness. Took al look, I guess it's ok. Aviators are by far, the choice of almost all parrots on here.
Watch some of the vids on YouTube to get an idea of methods to Don the harness.. If I might, a few suggestions.
1.tho they are not cheap, getting one 1x larger than needed to start off with will make putting his head Tru the head loop much easier and less traumatic. Probably the hardest part. I learned to lure Salty's head thru gradually, treat like hell when he did it and kept it thru, like a cow at a milking station.
2 He must be totally comfortable with you man handling his body and wings, cuz the next part requires you to put his wings thru the side thingys. So practice doing that aside from just harness training.
3. Once fully on, treat like hell. I used to a make the game show buzzing noise when Salty used to bite the harness, and we went thru his first one quickly, I didn't trust the half chewed thru straps. But he is calm and interested in the world when we are out and about now, ignoring the harness.
4. And maybe I should have said first, hang the harness near his cage for a week or 2 before even starting, so he doesn't think it's going to eat him . PIck a color that is not on his body or the flash colors on his wings, that can somerimes scare them.
5. Patience x 500. Parrots speed of change and acceptance is glacial, compared to our monkey brain adaptability.
Good luck and let us know how it goes.
