The Collaborative International Dictionary of English v.0.48:

Bulbul \Bul"bul\, n. [Per.] (Zool.)
   The Persian nightingale (Pycnonotus jocosus). The name is
   also applied to several other Asiatic singing birds, of the
   family Timaliid[ae]. The green bulbuls belong to the
   Chloropsis and allied genera. [Written also buhlbuhl.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

bulbul
    n 1: nightingale spoken of in Persian poetry