At feeders, birds eat tasty seeds with a side of eye disease

This article is available in Spanish through a partnership with the Institute of Ecology at the National Autonomous University of Mexico. Read in Spanish >>

As if bird lovers didn’t have enough to worry about. Earlier this year, a study was published showing that backyard bird feeders may actually be detrimental to native species, because they give invaders a boost. Now, a new study suggests that bird feeders are at least partially to blame for the transmission of diseases among a common North American songbird. That’s more than a little concerning in a country where some 55.5 million people spend more than $4 million dollars annually on backyard bird feeders and the food they fill them with. Count the number of houses on your street. One of every five of them, statistically speaking, has a bird feeder. The potential for disease transmission among wild birds is enormous.

Traditional epidemiological models treat each individual in a population in more or less the same way. A disease comes in, the animal gets sick, and then the animal transmits the pathogen to another individual. But the truth is that in most cases, a few individuals are responsible for a majority of disease transmission. Those individuals who are disproportionately responsible for acquiring or spreading those pathogens are called “super-receivers” or “super-spreaders.” Among humans, a person might be a super-spreader if he or she has poor hand hygiene, for example, and a person with a lot of sexual partners can be both a super-receiver and super-spreader when it comes to STDs.

Similarly, some of the factors that could lead an individual animal to being a super-receiver or super-spreader could be related to variations in social status, grooming, foraging, or aggression.

In North America, house finches (Haemorhous mexicanus), a common backyard songbird, commonly suffer from infection by a bacterium called Mycoplasma gallisepticum, or MG. The disease originally infected domestic poultry, but starting sometime in the mid-1990s it jumped to the wild songbirds. In house finches, MG infections lead to severe conjunctivitis (in humans this is often called “pink eye”) and can lead to substantial population declines. Ever since MG jumped into house finches, American researchers have noticed epidemics each winter.

To find out how the disease propagates through the tiny birds (an average house finch weights only a bit more than an empty soda can), Virginia Tech biologist James S. Adelman (now at Iowa State), together with colleagues from Iowa State University, the University of Oxford, UC Davis, and the Smithsonian Tropical Research Institute, observed wild house finches around the Virginia Tech campus. They hypothesized that three behaviors could have possibly been associated with super-receiving or super-spreading in the house finch-MG system: social network position, aggressive interactions, or foraging behaviors.

Over the course of five months, the researchers captured 180 birds at six sites on and around the campus. Each of the birds was outfitted with identification rings and a small radio transmitter. Before release, the birds were also assessed for MG infection. Whenever the birds returned to the bird feeders, a small RFID receiver picked up the signal emitted by the radio transmitters on the birds, then logged their visits.

They found that more aggressive birds, who interacted more frequently with other birds, were no more likely to become infected by MG than less aggressive birds. Also, contrary to their predictions, Adelman and his team discovered that it was the more peripheral individuals who were more likely to be sick, not those with the most social connections, though this trend was quite small. The most important factor that predicted illness was related to foraging behaviors. The total time that the house finches spent per day at feeders predicted the probability that they’d show signs of conjunctivitis.

The propensity to forage at feeders makes some individuals more likely to become both super-receivers and super-spreaders, though it isn’t clear whether the disease is transmitted directly between individuals or whether the bacteria can survive on the feeders themselves long enough to pass between individuals. The researchers’ observations alone were also insufficient to verify whether the infected birds were for some reason more likely to visit feeders, rather than the other way around. That’s why Adelman turned to wild-caught, captive flocks. After identifying which individuals were the most or least likely to dine at feeders, he infected one or the other and then watched house the disease spread through the group. In all, he had 11 flocks of four finches each. That’s a common wintertime flock size for these birds.

The results of the experiment were consisted with the notion that the feeders were causally related to disease transmission. In those flocks in which the individual who was most likely to forage at the feeder was infected, a second bird started showing evidence of conjunctivitis by the eighth day. While the disease eventually spread in the other flocks as well, it took twice as long: it was 16 days, on average, before a previously uninfected individual started showing any clinical signs of infection.

Does it mean that you should remove your backyard bird feeders? Well, the truth is that bird feeders aren’t actually all that helpful for birds in the first place; they’re really more for our own enjoyment than for our feathered friends’ well being. In April, I explained that birds were statistically no more likely to visit backyards with bird feeders in them than those yards without. If you really want to attract the birds in a way that’s sure to help them, the best thing you can do is add to your yard’s vertical complexity. In other words, plant trees, preferably native species. Planting native trees and bushes that that offer fruits and berries to hungry birds is also a good idea. With any luck, those plants will still be around long after your bird feeder is, continuing to feed and house generation after generation of birds, far into the future. – Jason G. Goldman | 23 September 2015

Source: Adelman JS, Moyers SC, Farine DR, & Hawley DM. (2015). Feeder use predicts both acquisition and transmission of a contagious pathogen in a North American songbird. Proceedings of the Royal Society B, 282: 20151429. DOI: 10.1098/rspb.2015.1429.

Header image: Male house finch via shutterstock.com

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