Hi Monoglypha, you should give serious thought as to what sort of bird photography you want to do, and then select a suitable solution for it, before you spend a small fortune on tripod and head.
for example
1.If you frequently shot from hides then a tripod is of very little use, a bean bag is far more useful/practical, and it can be used from a car, and costs next to nothing.
2. Most birds can be shot off a monopod, very cheap (in comparision to a tripod) and easy to carry.
I shoot with a 600/4vr off a monopod using all 3 (not at same time) nikon telecons with no problems and use a bean bag even more often.
I have a a gitzo 1548 with wim mk2 and only use it when I want to shot birds in flight.You should also consider the weight implications of a large tripod and gimbal head, will this suit you if you are a walk about birder?
Some other thoughts for you to consider, a waterproof sleeve in case you get caught out in the rain ( you won't be able to put a 500 in you coat pocket)is very useful.
A bag to carry or transport/carry the lens.
A replacement foot is a must, existing Nikon one is too tall and flexible.
Just a few thoughts/suggestions for you to consider, before you spend another small fortune.
regards