The Writers Community

Thing is "Write what you know" means something a lot closer to "Know what you're writing". (Sorry. Pet peeve. I know it's a staple saying, but that doesn't make it any less faulty/misleading/ambiguous/whathaveyou.)

No matter what route you decide to take there will always be a physical aspect you need to look into. A sparrow, for example, won't be able to fly backwards. Its body isn't built for it. Since these messengers/guides/partners will be restricted to a physical body, they'll have the restrictions of those bodies. I don't really see a way around that.

While I think delving into all sorts of illnesses might be a little overkill, knowing whether they can get sick will, as ilthit points out have a big impact on their behaviour regarding sickness and the whole of society. It's the more general questions that really need answering: do they get hunted? (By what/Why not?) Do they need to eat? (What/Why not?) Do they mate? Those questions might not come up so much in the actual story, but, again as ilthit points out, they add touches of realism. More than that, they'll help the reader suspend disbelief for your animal characters and world. You don't need to know enough to survive a Mastermind specialist subject round, only enough to make the animal partners believable. Whether that's looking up the diet of a hummingbird or saying that, because they're divine in origin, they don't need food is up to you/your story. As long as you know the answers and can include them when/wherever they fit.