Complex sentence: Feduccia (1999) No New World vulture possesses a syrinx, Kemp and Newton (2003) the vocal organ of birds, therefore the voice is limited to infrequent grunts and hisses.
Split into simpler sentences: Feduccia (1999) No New World vulture possesses a syrinx, Kemp and Newton (2003) the vocal organ of birds. Therefore the voice is limited to infrequent grunts and hisses.