The Dodo (Raphus cucullatus) of Mauritius and the Solitaire (Pezophaps solitaria) of Rodrigues were approximately the same maximum size (∼22.5 kg; Livezey 1993). Worthy (2001) recently estimated the weight of the extinct, flightless pigeon (Natunaornis gigura) of Fiji as “slightly smaller“ than the Dodo’s, which suggests that there might be some innate feature of these birds that limits the size they can attain. This limitation may have been the ability of the adults to produce sufficient crop milk to carry the young through the crucial early stages of their growth.

Like other columbids, Dodos and Solitaires presumably fed their young crop milk. Through the course of evolution from a relatively small, flying bird to a far larger, flightless one, the food requirements for a growing young (a cubic function) must have increased relatively more rapidly than the area of the lining of the crop (a square function). Besides reducing clutch size to the minimum of one, which the Dodo and Solitaire had done, the size of the crop itself can be enlarged, which is shown by the large bulge in the region of that organ in the detailed contemporary paintings of Dodos, or by increasing the area of the lining that produces the “milk“ by means of folds or other inward projections of it. The Dodo’s very large crop probably had a dual function: (1) adding space for food storage, so the Dodo could take a maximum amount of a large fruit; and (2) producing crop milk. The case of the Solitaire was quite different. Leguat’s (1708) figure of the female—the only picture of a Solitaire by a contemporary who knew those birds well—does not show any indication of a very large crop or a partial separation of the stomach and the crop as in the Dodo. The picture shows instead two feather-covered “risings,“ one over each side of the crop. Those “risings“ may have contained parts of the crop in which the lining was folded or otherwise increased in area. If that is true, they may have been the precursors of glands—in which case, Leguat’s likening them to the beautiful bosom of a woman was more appropriate than he imagined. Leguat stated that those “risings“ were found only in the female; because of the unusually great degree of sexual dimorphism in this species (Livezey 1993), Leguat was probably correct. If these (potential) glands of the female provided enough crop milk to bring the young beyond the stage during which crop milk was needed, it may have made possible a division of labor, whereby the male supplied food to his mate and to himself during that period. That may have been accomplished by the male’s making tours of the pair’s territory (or even beyond it), collecting food items in his crop, bringing them back to the female at the nest, and regurgitating them before her, a method that may have been used by both parents to feed the young after the crop-milk stage.

The Passenger Pigeon (Ectopistes migratoria) of North America, and presumably the Nicobar Pigeon (Caloenas nicobarica) of islands in the Indo- Australasian region, also laid clutches of one egg and moved in large flocks to areas of abundant food to breed. It was therefore vital for the young to be fledged and able to move with the flocks when the food supply was exhausted. Presumably, the crop milk from both parents accelerated the growth to that point. On the other hand, the single clutch of some of the large pigeons, including the largest, crowned pigeons (Goura), is consistent with the crop-milk hypothesis.

Although we are not likely ever to prove or disprove a connection between crop milk and size in these pigeons, examination of the linings of crops of a variety of living pigeons for projections that might increase their surface area may offer clues as to how such areas evolved or which pigeons were ancestral to the Solitaire.