Do bird feeders help spread disease?

The bird feeder in your backyard may seem like a pleasant and innocuous way to attract wildlife. But feeders could also increase the transmission of parasites among certain city birds, a study in PLOS ONE suggests.

The authors wanted to find out how city life affected the prevalence of animal disease. Other studies have yielded different conclusions depending on the species: blackbirds, for instance, seem to have fewer parasitic infections in cities, while urban woodchucks, bumblebees, and northern cardinals are more sickly. But researchers usually just compared cities and the countryside rather than studying areas with varying levels of urbanization.

To take a more nuanced look, the study authors caught 174 house finches at seven sites in the Phoenix area, with landscape characteristics ranging from cultivated plants to asphalt. Each site was scored for factors such as disturbance, density of people, and plant cover. Then the team determined whether the birds were infected with avian poxvirus, which causes lesions, or coccidian parasites that colonize the gut.

Poxvirus infections were more common and coccidian parasite infections were more severe in sites with higher disturbance and human population density, the authors report. The study suggests that “a decrease in natural land cover associated with human development is the driving force behind the increase in urban parasitism,” the team writes.

Figure 1 Do bird feeders help spread disease?

The relationship between an urbanization score (PC1) and severity of coccidian parasite infections in house finches in the Phoenix, Arizona area.

The results don’t necessarily apply to all birds or pathogens. But intestinal parasites, which can be transmitted through contact or feces, might be more likely to spread in places where birds congregate, such as feeders. Avian poxvirus also can spread directly from bird to bird.

The team checked levels of oxidative stress in the finches and didn’t find a link to the degree of urbanization or infection. But cities might still affect the animals’ physiology in other ways — for instance, by altering hormone levels or weakening immune response. Roberta Kwok | 6 February 2014

Source: Giraudeau, M. et al. 2014. Parasites in the city: Degree of urbanization predicts poxvirus and coccidian infections in house finches (Haemorhous mexicanus). PLOS ONE doi: 10.1371/journal.pone.0086747.

First image © Al Mueller | Shutterstock

Second image © Giraudeau, M. et al. 2014. Parasites in the city: Degree of urbanization predicts poxvirus and coccidian infections in house finches (Haemorhous mexicanus). PLOS ONE doi: 10.1371/journal.pone.0086747

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2 Comments

  • W. Proebsting February 12, 2014 at 11:23 am

    How does this work implicate feeders in spreading disease? The graph is based on urbanization score which you point out reflects disturbance, density of people and plant cover. What do feeders have to do with it?

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  • Roberta Kwok February 13, 2014 at 4:51 pm

    I believe the idea is that bird feeders are more common in areas with more people. Here’s what the authors say in the paper: “These results indicate that the physical presence of humans in cities and the associated altered urban landscape characteristics are associated with increased infections with both a virus and a gastrointestinal parasite in this common songbird resident of North American cities… humans may facilitate infections in these birds via bird feeders (i.e. horizontal disease transmission due to unsanitary surfaces and/or elevations in host population densities)… intestinal parasitism may increase more in urban areas because higher densities of animals in these areas (e.g. around specific sources of food, like feeders or waste) may elevate the transmission of these parasites by direct contact, an oral-fecal route, or by increasing the proportion of intermediate hosts infected.”

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