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jstor.org
Blockstein، David E. (1989). "Crop milk and clutch size in mourning doves". The Wilson Bulletin. ج. 101 ع. 1: 11–25. JSTOR:4162684. The fact that none of the nearly 300 species of Columbiformes has a clutch size larger than two eggs suggests that there is limited plasticity in crop-milk production.