I suspected it... I discovered fly does'nt work in Ysgrath. I hoped there might be some cursed area where it would work but obviously I will have to test it on a mob before trying a player.
Since no one said it's bad, I've decided to give a go with the Hummingbird (i really like the fact that it's tanky and it can fly above the world when fighting) and try the 'relentless chaser' playing style.
If you have any other situations (besides the cursed area) where you've seen a solo hummingbird (playing it or facing it) shine, please let know: I'm so far from being good at PK... Actually, I decided to try my first shifter because I remembered a post from THE_JOKER explaining why an Air shifter is one of the great class to learn how to pk. (because there's only a few commands available and because of 'flyto')