It is explained in this topic:
But I do not remember exactly on which page. Also, when you render animations of animals that use secretary bird´s animations, quite often some of them (especially sitting, lying, flying and eating animations) render into the surface, ending in animal that looks like some parts of it are missing during such animation (like your brachiosarus during sleeping animation). You can counter it by simply rendering all animations, then positioning/tilting animal in such a position/place that renders animation whole, and render only those animations again. Also, if you put another model below the surface, you will have much wider scene, and be able to make quite high flying birds (like my falcons).
And advice to scale animals: start APE, load animals you want to compare your animal (I use animals it is compatible with + its relatives) with into it, and then render animation of the new animal you´re making into some folder. Load sprites into APE, and you can compare it with another animals.