This is a pretty good story to teach very young kids about love and caring for one another.
It starts with a bird being interested in the changing colors of the lion's tail and the bird caring for the lion more and more as she watches him play and enjoy his environment, but as the bird sticks around, the lion also starts to care for the bird as well. There's no reason given in the book for why the lion cares for the bird, other than the fact that the bird is following the lion and chirping and singing around him, which the lion finds beautiful, leading to their friendship. The relationship is set up somewhat superficially, which is my only major complaint about the book. Why not add a bit more interaction between the two other than "bird finds lion interesting, lion likes bird's singing"?
It's similar to Jan Brett's "Mossy" in teaching young kids about love through simple connections and admiration for one another, but it is a little more complicated for two reasons. First, there is the bigger theme that love can happen between two very different creatures, a bird and a lion, and it can cross language barriers (the book repeatedly reminds the reader that the lion cannot understand the bird's chirping, while the lion himself remains silent through the whole story). Second, late into the story there is a thunderstorm that the lion needs to rescue the bird from which might be seen as a bit scary or startling for younger children. These details might make the book a little more mature than "Mossy".
It's excellent for even very young children, I would say 3-3.5 year olds can understand what's going on, but it might be best for 4 or even 5 year olds to understand the larger points I just mentioned.
The illustrations are excellent.