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Wilson Bull., 101(1), 1989, pp. 11-25
CROP MILK AND CLUTCH SIZE INMOURNING DOVES
DAVID E. BLOCKSTEIN' ABSTRACr.-Doves unique among birds in producingcrop milk and in having a are determinateclutch size of 1-2 eggs. In North Dakota, 97.2% of 1203 MourningDove (Zenaida macroura)nests had 2 eggs. Clutchesof 3 (N = 24) and 4 (N = 4) eggs probably were laid by more than one female per nest. Experimentaladditions of a second egg to incompleteclutchesand removalsof thesecondeggfromrecentlycompletedclutchesverified that MourningDoves are determinatelayers.To examine the role of crop-milkproduction as a factor that limits clutch size, a third nestling was added to nests of MourningDoves during(earlyaddition:3-E) and after(late addition:3-L) the periodof completedependence on crop milk. All three young fledgedat 31%(3-E) and 42%(3-L) of these nests. Growthratesweresignificantly reducedfor 3-E nestlingsrelativeto normalbroodsof 2; growthrates of 3-L broods were intermediate.Crop-milkproductionis apparently importantreason an why the clutch size of doves (Columbidae) not more than two eggs.Received 7 Oct. 1987, is
accepted 19 Aug. 1988.
The most accepted hypothesis for the evolution of clutch size in nidicolous birds states that the modal clutch sizecorresponds to the maximum number of young that can, on the average, be fed and raised (Lack 1947). An offshoot of this hypothesis may explain the evolution of clutch size in the Columbiformes, a group with little or no intraspecific variation in clutch size. With the exception of Apterygiformes, Columbiformes are the only land birds in which no species has a typical clutch size of more than two eggs.With their rapid growth rates (Vandeputte-Poma 1980), columbids are a conspicuous exception to the usual avian pattern of slow growth with small clutches (Ricklefs 1968). Rapid growth appears to be facilitated by the capacity, unique among birds, to produce crop milk which is the exclusive food of 1-4-day-old nestlings (Mirarchi and Scanlon 1980, Vandeputte-Poma 1980). Crop milk is produced byboth males and females. It consists of desquamated cells sloughed off from the germinal epithelium of the crop (Beams and Meyer 1931, Patel 1936). Crop milk is rich in proteins and lipids and contains an uncharacterized growth-promoting factor (perhaps digestive microflora) (Pace et al. 1952, Hedge 1972). The two exceptional features of columbid reproductive biology, a clutch size of one or two...