Pigeon is an important economic poultry species in many countries. As an altricial bird, its growth and development are largely reliant on pigeon milk produced by the crop tissue in the first week. During the breeding cycle, pigeons undergo a series of behavioral changes. Pigeon milk is generally characterized by having high concentrations of proteins and lipids, and a complicated regulatory network is involved in the milk formation. Hormones, especially prolactin, could promote the proliferation of crop epidermal cells and nutrient accumulation. The expression of target genes associated with these important biological processes in the crop epidermis is affected by non-coding RNAs. Meanwhile, signaling pathways, such as target of rapamycin (TOR), Janus kinase/signal transducer and activator of transcription proteins (JAK/STAT), protein kinase B (Akt), etc., influence the production of crop milk by either enhancing protein synthesis in crop cells or inducing apoptosis of crop epidermal cells. In order to adapt to the different breeding periods, pigeons are physiologically changed in their intestinal morphology and function and liver metabolism. This paper reviews the behaviors and physiological adaptations of pigeon during the breeding cycle, the composition of pigeon crop milk, and the mechanism of its formation, which is important for a better understanding of the physiology of altricial birds and the development of artificial crop milk.

The mechanism of sex determination in chickens, especially the molecular mechanism of female ovarian development, has not yet been fully elucidated. Previous studies have shown that RSPO1, which is associated with ovarian development in mammals, might have a conserved role in chickens. In this study, we systematically investigated the spatiotemporal expression pattern of RSPO1 in various tissues, especially gonads, of male and female chicken embryos using qPCR and Western blotting, and we explored its correlation with the expression of key genes in the estrogen pathway using drug treatment or gene overexpression in vivo and in vitro. Our results reveal that RSPO1 was widely expressed in all examined tissues of chicken embryos, showing a female bias in gonadal tissues at both the mRNA and protein levels. Surprisingly, RSPO1 was not differentially expressed between male and female gonadal cells with fadrozole-induced estrogen pathway blockades, and furthermore, estradiol-induced estrogen stimulation altered the expression of RSPO1. In addition, overexpression of RSPO1 in gonadal cells induced the mRNA expression of its downstream target genes, Wnt family member 4 (WNT4) and Catenin beta 1 (CTNNB1), and that of estrogen receptor α (ERα), an estrogen pathway gene. In summary, this study provided new evidence for elucidating the role of RSPO1 in ovarian development in poultry.