Host parasite interactions in White-crowned sparrows

Parasites and pathogens have a ubiquitous but hidden presence in wildlife populations; because of their obscured nature, their true impact on the health and the population dynamics of wildlife has been difficult to evaluate. Although impact of pathogens in wild animals appears to be exacerbated by secondary stressors like food shortages or a changing climate, researchers have still only a poor understanding of the nature and magnitude of these interactions. This study aims to improve this understanding of the relationship between parasitism and stress stemming from food shortage in wildlife by studying wild birds and their blood parasites at a study site in the Rocky Mountains of Colorado.

In this study population, we experimentally manipulate both parasite numbers and food availability by selectively administering antimalarial drugs and by supplementing food. We then try to measure immune function, overall bird body condition, growth and reproductive success of each individual. This allows us to understand: 

1. the relative importance of these factors on wildlife health, survival and reproduction, 

2. the mechanisms through which these impacts are expressed - e.g. do they involve changes in behavior, hormones or parts of the immune system? 

Ultimately, insights gained from this study can help us understand how habitat degradation and the resulting food scarcity can cause wildlife disease epidemics, and can help prevent disease outbreaks in wildlife. Such knowledge in turn can help sustainably manage wildlife populations and help maintain stable and healthy ecosystems.

Presence of malarial parasitism

Our parasite surveys revealed that the White-crowned sparrow study populations are naturally infected with at least 6 species of malaria-like parasites, belonging to the genera Leycocytozoon, Plasmodium, Haemoproteus and Trypanosoma. 

It is not unusual to find individual birds, outwardly healthy-looking, that were simultaneously infected with at least 3 species of haemoparasites. While conventional ecological wisdom has been that this demonstrates the lack of virulence of these pathogens our results indicate otherwise.  

Effects of malaria infection on sparrow reproduction 

- White crowned sparrows infected with malarial parasites pay a high price when it comes to reproduction. The chances of successfully rearing a clutch were diminished by at least 15-20% for each blood parasite species a parent was infected with. These failures were not distributed equally across the different breeding stages. While infected females tented to produce similar-sized clutches as healthy females, the number of nestlings that actually fledged was significantly smaller, with infected parents apparently not being able to provision their young with enough food.

 Effects of infection on bird song 

Theoretical considerations suggest that bird song should communicate ‘honest’ information about the condition of the singer. It turns out that in white-crowned sparrows this is exactly what happens. When male sparrows were treated with antimalarial drugs their malarial infections largely disappeared. Surveying their song activity and song structure revealed that healthy birds also sing differently than their haemoparasite-infected counterparts, with both song rate and song variability being affected. Interestingly, different species of blood parasites (e.g. Leucocytozoon versus Plasmodium) affected songs in different manners, thus suggesting that females may not only be able to evaluate whether a prospective mate is healthy, but also what kind of haemoparasite infection he is carrying (Gilman et al. 2007).

More recent research (Munoz et al. 2010) revealed that just mounting an immune response (without an infection) is sufficient to temporarily disrupt the song of a white-crowned sparrow (see comparison of baseline song (L) against truncated sparrow song (R) sung after treatment with an immunostimulant). This confirms the idea that bird song reflects in a sensitive fashion the condition of a male.