Radagast the Brown's Quenya name, it means "Lover of birds".

When the Istari were sent to Middle-earth to overcome Sauron, Yavanna begged Curumo (Saruman) to take Aiwendil with him, as he had a love for all things of her making.

Radagast's neglect for his Istar duties in Middle-earth seemed to be the reason why he incurred Saruman's scorn, with his fellow Istar calling him names like 'Radagast the Bird-tamer', 'Radagast the Simple' and 'Radagast the Fool'.
Encyclopedia entry originally written by Ainulindale