Pigeon crop sac

Nicoll C. Bioassay of prolactin Analysis of the pigeon crop sac response to local prolactin injection by an objective and quantitative method. Endocrinology 1967 80 641-55. 

Stimulation of crop-sac milk production in pigeons and doves. 

As has been mentioned, in pigeons and doves the young are fed by a milk-like fluid regurgitated from the crop. This milk is formed by a sloughing of cells from the wall of the crop sac, a process which is stimulated by prolactin [96,97]. The hormone stimulates both an increase in proliferation of the cells which contribute to the milk and a change in the proteins and lipids contained within the cells [97,98], and these two types of response appear to be controlled independently [99]. The effect is a direct one, and prolactin can produce local effects when injected in the immediate vicinity of one side of the crop. 

Prolactin (PRL) is a hormone secreted by special cells of the adenohypophysis. It stimulates and sustains lactation in postpartum mammals, the mammary glands having been prepared by other hormones, including estrogens, progesterone, growth hormone, corticosteroids, and insulin. It also stimulates formation of milk in the crop sac of birds, such as pigeons and doves (an action formerly used for bioassay), is luteotropic m certain mammals, and has many other... 

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