eating habits

So i've had this little bit that came with my cage for sometime now, and it's a small natural perch maker, where one takes a branch from outside and uses it in the bird's cage. I picked one from a tree out back, set it up, and Hermes loves it, though... maybe a little too much? He nibbles at...

When I purchased Hermes from his breeder, she told me that I could occasionally introduce some small bits of fruit into his diet as treats or even rewards when training him, and I have seen other tiel owners feed their birds from time to time, though I am not too sure what fruits and I should...