Vasoactive intestinal peptide (VIP) is a PRL-releasing
factor in birds (El Halawani et al., 1990, 1997). Passive
immunization with VIP antibodies terminated incubation
behavior and reduced circulating PRL in incubating chickens
(Sharp et al., 1989). Active immunization of turkey
hens against VIP resulted in inhibiting incubation behavior
and increasing egg production (El Halawani et al., 1995,
2000).