Interactive effects of avian malaria and food supplementation in a natural population of mountain white-crowned sparrows (Zonotrichia leucophrys oriantha) in the Rocky Mountains

Background/Question/Methods

Reproductive success of an organism is essential for the continued existence of the species; however, reproduction is very energetically intensive and a parasite’s consumption of a host’s resources can make reproduction for the host even more difficult, whether acting either alone or in conjunction with reduced food availability. The goal of this study is to evaluate the interactive effects of avian malaria and early season food limitation on the reproductive success of a free-living population of mountain white-crowned sparrows (Zonotrichia leucophrys oriantha) in Colorado at the Rocky Mountain Biological Laboratory. To expound the effects of avian malaria and food availability on host reproduction, food availability and parasite loads were manipulated. Early in the breeding season, when food availability was low due to lingering winter snowpack and new snowstorms, supplemental seed was provided on select territories. Simultaneously, parasite loads were reduced on a subset of territories with selective administration of antiparasitic agents. Over ten field seasons at the Rocky Mountain Biological Laboratory, all resident mountain white-crowned sparrows were captured and banded. Data was recorded that measured total parasitemia, and reproductive success. Reproductive success was measured by documenting clutch size, proportion of eggs that hatch, and proportion of hatchlings that fledge.

Results/Conclusions

Interestingly, there was no significant effect of avian malaria infection or food supplementation on the clutch size of the female. When the proportion of eggs that hatch was evaluated, there was a significant interaction between avian malaria and food supplementation for both males and females. In individuals harboring an avian malaria infection, increased rates of food visitation resulted in a significantly higher proportion of eggs that hatch. In healthy individuals, increased rates of food visitation resulted in a marginal increase in proportion of eggs that hatch. When evaluating proportion of hatchlings that fledge there was a significant interaction effect between avian malaria and food supplementation for males. Males that received food supplementation did not experience a significant decline in the proportion of hatchlings that fledge with an increase in total avian malaria parasite burden. Contrastingly, males that did not receive food supplementation experienced a significant decline in the proportion of hatchlings that fledge with an increase in total avian malaria parasite burden. These data demonstrate the importance of avian malaria in conjunction with food availability on the reproductive success of mountain white-crowned sparrows.