is that why he prescribes all the Harrison's branded supplements, like avi builder and avi-booster?
Calcivet is a very good supplement and unfortunately birds are very often deficient in it, because the best sources of calcium are those birds are not supposed to it. In nature they get it from insect shells and some wildly grown plants of course, because these are much richer in calcium and other nutrients than cultivated varieties. Cultivated and grown in greenhouses veg are often grown on depleted soils, or even worse, using hydroponics method.
I would strongly recommend using calcivet at least once a week, because head twitching, feather chewing and feather plucking are just some of the symptoms of calcium deficiency.