The Mountain Plover (Charadrius montanus) has an uncommon parental care system in which males and females tend separate nests. To improve their fitness in this system, females have the opportunity to allocate their reproductive investment to male-tended nests and their own nests differently. To examine investment we measured dimensions of plover eggs in Phillips County, Montana, and calculated their volumes. We modeled possible differences in egg volume in male- and female-incubated nests in relation to the effects of sex of the incubating adult, Julian day of nest initiation, and drought conditions. We measured >1000 eggs from 194 nests tended by 131 females and from 213 nests of 148 males. Male- and female-incubated eggs had similar mean volumes (13.20 versus 13.17 cm3, respectively) but differed significantly across the breeding season. The eggs in female-incubated nests tended to be larger than those in male-incubated nests early in the breeding season but were smaller as the season progressed, while the volume of male-incubated eggs peaked in the middle of the season. Egg volumes were affected by drought conditions, being larger during the driest periods of this study. Volumes within a nest were similar and were not influenced by the age of the incubating adult. Larger eggs tended to produce larger chicks. The similarity in the size of Mountain Plover eggs, even between male- and female-incubated nests and under different environmental conditions, provides evidence for stability of this uncommon system of parental care.