Between one-third and three-fourths of Anglosphere households, depending on the study, sometimes feed wild birds (Jones & James Reynolds, 2008, p. 2). Some people feed birds for the educational value or because of the pleasure they get from viewing them; others feel empathy for the birds or want to make up for the suffering humans have caused birds by destroying their habitats. None of them want to hurt birds. But bird feeding, done improperly, can spread serious diseases.

Studies show that backyard bird feeding may lead to disease transmission (Jones & James Reynolds, 2008; Robb, McDonald, Chamberlain, & Bearhop, 2008, p. 481). Over two years of a study, fed birds were more likely to experience transmissible diseases than unfed birds were (Wilcoxen et al., 2015). Why? When you feed birds, they come into close contact with each other at the feeder. Because you provide more food in one place than any natural source, birds are far more likely to come into contact with each other at a feeder than they are in the wild.

House finch conjunctivitis is a model system for studying the ecology of wildlife diseases, in part because infection with conjunctivitis is visible (Hotchkiss, Davis, Cherry, & Altizer, 2005, p. 2). So information about house finch conjunctivitis is likely to apply to other diseases as well. And the results aren’t good.

Congregation at bird feeders caused a conjunctivitis epidemic among house finches (Dhondt, Tessaglia, & Slothower, 1998). Mycoplasmal conjunctivitis spreads more among finches in seasons with more heavy reliance on bird feeders (Hartup, Mohammed, Kollias, & Dhondt, 1998). Finches that spend more time on feeders have a higher risk of conjunctivitis (Adelman, Moyers, Farine, & Hawley, 2015).

These transmission rates aren’t just due to random contact. Finches with conjunctivitis spend more time at feeders (Hotchkiss et al., 2005, p. 6). Male finches prefer to feed near conspecifics with conjunctivitis, because the conspecifics are more lethargic and thus less likely to engage in aggressive behavior (Bouwman & Hawley, 2010). (Female finches show no such pattern (ibid).) It is possible, however, that the conjunctivitis birds get from feeders is typically milder than the kind they get from other places, and still gives them immunity (Hartup et al., 1998).

If you weren’t aware of this, you aren’t alone. Studies show that the vast majority of people who feed birds are unaware that disease transmission is a risk and, because of this, rarely use best practices to prevent disease transmission (Galbraith et al., 2014).

The best practices are as follows:

Clean and disinfect the bird feeder with a weak bleach solution at least once a year, and more often if it is a platform feeder (Brittingham & Temple, 1988, p. 201; Wilcoxen et al., 2015) Do not use a tube-type feeder, which has a limited number of perches and greater crowding potential (Hartup et al., 1998, p. 287) Store seed in a dry place. Do not use moldy seed. If seed in the feeder becomes moldy, throw it out (Brittingham & Temple, 1988, p. 201) Avoid feeding on the ground (Brittingham & Temple, 1988, p. 201) Do not feed in the summer if mourning doves or rock doves use your feeder (Brittingham & Temple, 1988, p. 201) Consider leaving feeder empty between feedings, although this intervention has been understudied (Wilcoxen et al., 2015) If you see a dead or dying bird (Brittingham & Temple, 1988, p. 201): Wearing gloves, pick up all carcasses and either bury them or wrap them in plastic bags and dispose of them Clean and disinfect the feeder Sweep up and dispose of seeds spilled on the ground Move the feeder to a new location but continue to feed to avoid infected individuals introducing disease at someone else’s feeder

Works Cited

Adelman, J. S., Moyers, S. C., Farine, D. R., & Hawley, D. M. (2015). Feeder use predicts both acquisition and transmission of a contagious pathogen in a North American songbird. Proc. R. Soc. B, 282(1815), 20151429. https://doi.org/10.1098/rspb.2015.1429

Bouwman, K. M., & Hawley, D. M. (2010). Sickness behaviour acting as an evolutionary trap? Male house finches preferentially feed near diseased conspecifics. Biology Letters, 6(4), 462–465. https://doi.org/10.1098/rsbl.2010.0020

Brittingham, M. C., & Temple, S. A. (1988). Avian disease and winter bird feeding. The Passenger Pigeon, 50(3), 195–203. Retrieved from http://images.library.wisc.edu/EcoNatRes/EFacs/PassPigeon/ppv50no03/reference/econatres.pp50n03.mbrittingham.pdf

Dhondt, A. A., Tessaglia, D. L., & Slothower, R. L. (1998). Epidemic mycoplasmal conjunctivitis in house finches from eastern North America. Journal of Wildlife Diseases, 34(2), 265–280. https://doi.org/10.7589/0090-3558-34.2.265

Galbraith, J. A., Beggs, J. R., Jones, D. N., McNaughton, E. J., Krull, C. R., & Stanley, M. C. (2014). Risks and drivers of wild bird feeding in urban areas of New Zealand. Biological Conservation, 180, 64–74. https://doi.org/10.1016/j.biocon.2014.09.038

Hartup, B. K., Mohammed, H. O., Kollias, G. V., & Dhondt, A. A. (1998). Risk factors associated with mycoplasmal conjunctivitis in house finches. Journal of Wildlife Diseases, 34(2), 281–288. https://doi.org/10.7589/0090-3558-34.2.281

Hotchkiss, E. R., Davis, A. K., Cherry, J. J., & Altizer, S. (2005). Mycoplasmal conjunctivitis and the behavior of wild house finches (Carpodacus mexicanus) at bird feeders. Bird Behavior, 17(1), 1–8. Retrieved from http://www.ingentaconnect.com/content/cog/bb/2005/00000017/00000001/art00001

Jones, D. N., & James Reynolds, S. (2008). Feeding birds in our towns and cities: a global research opportunity. Journal of Avian Biology, 39(3), 265–271. https://doi.org/10.1111/j.0908-8857.2008.04271.x

Robb, G. N., McDonald, R. A., Chamberlain, D. E., & Bearhop, S. (2008). Food for thought: supplementary feeding as a driver of ecological change in avian populations. Frontiers in Ecology and the Environment, 6(9), 476–484. https://doi.org/10.1890/060152

Wilcoxen, T. E., Horn, D. J., Hogan, B. M., Hubble, C. N., Huber, S. J., Flamm, J., … Wrobel, E. R. (2015). Effects of bird-feeding activities on the health of wild birds. Conservation Physiology, 3(1), cov058. https://doi.org/10.1093/conphys/cov058