I dont think there is any unbalance in PvP. In fact, the only class I've felt uncomfortable playing are STR Bear and Pally Mage (to some extent). Above level 50, birds have a fair chance of winning. Its all about knowing what ur opponent is going to do, and quick reactions. This is my opinion, I haven't reached level 75/76/77 yet so correct me if I'm wrong.

Gear:
Always wear bow set not talon.

Stats:
Full dex

For example:
1. Bear
First thing bear is gonna do is beckon / HS
1. Bird
First thing bird is gonna do is Repulse, to make the bear miss, in addition to causing 100 to 150 damage.
2. Bear
Now that the bear has missed its beckon, its gonna stomp and start its slashes. Stomp is going to stun the bird and in the mean time, the bear can use its slashes.
2. Bird
Bird knows its going to stomp, it uses Blinding shot / Thorn wall followed by Break armour (always keep a distance). Totally if none are dodged, It will cause about 250 - 300 damage. Bird will let bear stomp, take some damage, and use avian (4 or 5 avian, have to get it confirmed) to get free of bears stun. By now the bear has taken significant damage and has not regenerated enough during the time the bird was stunned because bird used avian. Also a full dex bird has so much hit that it can overcome dodge of bear. It will doge one or two skills at max, if you are not debuffed.
3. Bear
Bear has low health and will start using slashes. By this time beckon would have restored and bear would try combo again
3. Bird
Bird roots the bear, constantly moving back, uses bird combo. Bear is almost dead by now. Avian would have restored by now.
4. Bear
Bear tries to pull bird, or stomps to get out of root. Lets say the bird does not dodge.
4. Bird
If bear stomps, bird uses thorn wall, blinding, break, combo, BEAR DEAD. If bear successfully pulls bird, bird uses avian, repulse, break, bird combo, BEAR DEAD.