Birdseed, cowbirds, and disease: The unintended consequences of bird feeding

By Shane Hanlon, Department of Biological Sciences, University of Memphis

Birding is the baseball of nature appreciation; it’s an American pastime. Usually when you ask birders, “Do you feed birds?” many will say yes. Pressed further, if you ask them why, the usual responses are, “They’re pretty,” “It’s calming,” or “It helps the birds.” In 2011 alone, over 53 million Americans fed birds and other wildlife, racking up a $3.8 million dollar bill in the process. However, the idea that bird feeding “helps” birds may not be true. The surprising fact is that no one really knows the effects of feeding on the health of birds. This question spurred an unlikely collaboration between the Wild Bird Industry and a research group from Millikin University.

Example of bird feeders used in the study. Photo by Dr. Travis Wilcoxen.

The Wild Bird Feeding Industry is a group “promoting responsible feeding…and enhancing the experience of the consumer.” The organization includes members from birdseed packagers and processors, bird feeder manufacturers, and wild bird specialty stores. In accordance with the goal of responsible feeding, they approached Dr. Travis Wilcoxen and Dr. David Horn, both assistant professors of biology at Millikin University. Dr. Wilcoxen’s lab studies vertebrate ecophysiology, with a specific focus on avian endocrinology. While the topic of responsible feeding was a new direction for the lab, Dr. Wilcoxen had thought about this question before as a graduate student at the University of Memphis. Every spring, Dr. Wilcoxen would move to Florida to conduct research on the Florida Scrub Jay, a threatened species that is endemic to the state. Part of his research involved the implementation of automated feeders to supplement the birds’ diets. The diets were fixed and only behavior was observed, but he always wanted to determine the physiological effects of supplemental feeding on the jays. While he never got that chance as a graduate student, the new opportunity proposed by the Wild Bird Feeding Industry seemed like a match made by fate.

It should be noted that Millikin University is a solely undergraduate institution. It is impossible to talk to Dr. Wilcoxen about his research without him touting the accomplishments of his “undergraduate army.” So, in order to get this project off the ground, he set this army to work.  Bird feeders were installed at nine forested sites with little to no bird feeding history in four counties in central Illinois. The sites were randomly assigned to either a feeder or no-feeder control site, with a maximum distance between sites of forty-two miles and a minimum of three. Because the team was concerned about how feeders affect community composition, population density, and subsequent measures of health, they conducted point counts, monitored birds at the feedings, and trapped as many birds as possible using stationary nets. The project was so demanding that Drs. Wilcoxen and Horn employed a full-time field technician to oversee much of the field work, in addition to eleven undergraduate students.

To measure the health of the birds, birds were caught and taken to the lab for testing. The team assessed twelve species for external differences in body and feather condition, fat content and visible disease. Blood samples were also collected and tested for reproductive hormones, stress physiology, immune function, and various additional indicators. They hypothesized that the presence of feeders would alter the bird community’s composition and benefit individual health. And, for the most part, they were correct. The feeders improved feather quality, increased immune function, reduced stress, and improved antioxidant capacity. But it was not all good news.

Even though feeders increased immune function, there was an increased incidence of disease in birds that used feeders (10.3%) versus those that did not (1.2%). Of the diseases that were observed, 79% were avian pox, 9% conjunctivitis (pink eye), 7% cloacal infections, and 5% were fungal skin diseases. Notably, while most of these diseases were also observed at control sites, albeit at much lower prevalence, no birds at control sites had conjunctivitis. However, because these diseases were confirmed through visual means only, the actual prevalence of disease in the birds may be much higher. To address this potential shortcoming, his group and collaborators have recently submitted a grant proposal to use molecular techniques to obtain a more accurate assessment of how prevalent disease is in these populations.

The other striking negative effect of the feeders was a disproportionate increase in brown-headed cowbirds at the feeder sites. Cowbirds are brood-parasites that lay their eggs in the nests of other species that then raise the cowbird chick as their own. Another mouth to feed means lower reproductive success for the parents because fewer of their own chicks survive. So, a greater number of cowbirds could lead to greater brood-parasitization. “We’re basically creating a singles bar for the cowbirds,” recalled Wilcoxen, which raised the question, “How do we get rid of cowbirds without a pellet gun?” His team will direct future studies that focus on specific types of birdseed that should be less palatable to cowbirds and functionally exclude their presence.

What does this mean for birding community? Should there be a ban on all bird feeders because they may increase the incidence of disease? In short, no. Dr. Wilcoxen and his team recognize that this is the first of what will likely be many studies that examine the effects of bird feeding on avian health, good and bad.  In addition to determining if there is a type of birdseed that would deter cowbirds, the team is also interested in whether different formulations that vary in fat or protein content, or even contain anti-oxidants, may alter the health of birds. “We can put out birdfeed anytime we want” said Wilcoxen. “We want to make sure we’re not causing problems to the birds we’re trying to help.”

Shane Hanlon earned his B.S. at the University of Pittsburgh, where he studied amphibian toxicology with Dr. Rick Relyea. He is now a graduate candidate at the University of Memphis under the advisement of Dr. Matt Parris, where he studies how agricultural contaminants alter host-pathogen interactions in amphibians. In his free time (i.e., not during field season) he blogs about ecology and policy at

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