The strategy to survive primary malaria infection: An experimental study on behavioural changes in parasitized birds

Malaria is caused by protozoan parasites of the genus Plasmodium. Avian malaria parasites are prevalent worldwide, and they infect the great majority of terrestrial bird species. Malaria is thought to have reduced the populations or even caused the extinction of some endemic bird species in Hawaii. However, fitness effects of the parasite on mainland birds are still poorly understood. Also it is unclear whether we get a full picture about the numbers of diseased wild birds using the method of mistnetting.

Here, we evaluate experimentally the behavioural changes of siskins caused by a malaria parasite during predator attack, and evaluate the impact of disease on birds’ activity. These parameters are important in nature for survival. We found that infected birds do not differ in their reaction to aerial predators until they are on terminal stage but markedly reduced their mobility during primary infection. Together with mortality caused by infection, these factors are responsible for the considerable underrepresentation of infected birds in mist nets and traps in wildlife projects.

Underestimation of parasite burden might mislead our understanding of the true pathogenicity and disease epidemiology in wildlife. These results should be recognized in studies addressing parasite virulence and demography of birds, particularly when estimating mortality rates in wildlife populations.