It isn't difficult to make sense of this situation. After all, whether it's the Vermilion Bird, the Phoenix, or the Immortal Bird, these divine beasts might bear different names but their abilities are similar. As Fire Spirits, their combat power increases and they can heal usually hard-to-heal injuries as long as they are in a place rich in Fire Element.
Considering the state of the Vermilion Bird when it left Zhou Tian, it was hard to imagine any method that could heal it fully. Not to mention when it will die, but even maintaining its original power had become an impossible task.
Thus, considering the situation at that time, if the Vermilion Bird didn't want to lose its former strength, it would surely go to places like a volcano, rich in Fire Element, to recover. In reality, Zhou Tian's Beastmen had already found several volcanoes that seemed to have been drained of all their Fire Element by it.