The significance of clutch size, egg coloration, and other reproductive traits of mourning doves
The mourning dove (Zenaida macroura) is one of the most successful avian species in North America. In this study, I identified adaptations that promote multiple brooding, one characteristic of the unique reproductive strategy of mourning doves. I also investigated how clutch size and egg coloration affect reproductive success;Mourning doves are capable of multiple brooding because they produce food (crop milk) for nestlings in vivo and feed older nestlings a diverse granivorous diet. This allows extended breeding seasons. Mourning doves also have adaptations that enhance the number of broods that can be produced in a season. By constructing small nests and reusing old nests, they initiate nesting cycles quickly. Small clutch size, constant incubation, fast nestling growth, and early fledging serve to shorten the nesting cycle;Mourning doves are unusual in that they always lay two eggs per nest. To find out why, I exchanged eggs between nests to create clutches of one and three, and compared these to natural clutches. The parents incubated three eggs and brooded three nestlings at normal temperatures. Because adults produced a limited amount of crop milk, however, nestlings in enlarged broods grew more slowly, took longer to fledge, and fledged at 83% normal weight. Parents of enlarged broods fledged 23% more offspring than those of control broods, but low survival of young fledglings in the former treatment probably negated this positive effect on reproductive success;Mourning doves are also unusual because they lay white (noncryptic) eggs in open nests. To determine whether this depresses reproductive success, I compared the survival of white clutches to that of clutches spattered with brown paint, simulating cryptic coloration. These treatments were compared under two incubation regimes: (1) constant incubation, which is typical of mourning doves, and (2) interrupted incubation, in which the attending adult was flushed from the nest once every 3 days. Cryptic clutches had greater survival when incubation was interrupted, but did not have greater survival when incubated constantly. I argue that white egg coloration in pigeons and doves was a selective pressure for development of constant incubation.