Concerning Istaris

[akhtene]

There is no hint of an explanation of why Yavanna's evident desire that the Istari should include in their number one with a particular love of the things of her making could only be achieved by imposing Radagast's company on Saruman;

Sorry, I don't know where the quotation is taken from, but it seems "The Silmarilion" (Of the Rings...) gives kind of explanation. The Istari were sent

... to move Elves and Men and all living things of good will to valiant deeds.

While Gandalf and Saruman were busy with Elves, Men and other 'thinking' (??) races, it was Radagast's task to deal with animals, birds and plants.