How a House Finch Disease Reshaped What We Know About Epidemics

By Andrea Alfano
December 2, 2013
house finches Healthy House Finches. Photo by Jeanette Tasey/Cornell Lab.

Microbes may be tiny, but that doesn’t stop some of them from causing major destruction. Even supervillains like Lex Luthor or Doctor Doom were never able to pull off as much devastation as the bugs that cause bubonic plague or malaria. Comic book supervillains come with a backstory, but what is it that turns some microbes into monsters? Until recently, all scientists could do was make educated guesses.

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It turns out it’s hard to find a good opportunity to study harmful, contagious diseases. It’s just not safe. One option is to study an epidemic that doesn’t affect humans—but then it’s hard to track the disease, since animals don’t check into hospitals. Fortunately, some scientists have an extensive network of dedicated citizen scientists to help them.

That’s how one team of researchers was able to study a highly virulent disease in House Finches. Their recent paper in PLOS Biology sheds light on what makes some disease-causing microbes, or pathogens, more harmful than others.

Citizen-scientists helped track the spread of House Finch eye disease across North America during the 1990s and 2000s. Map courtesy Project FeederWatch.

In February of 1994, Project FeederWatch participants in the Washington, D.C., area began reporting that House Finches at their feeders had swollen, red, crusty eyes. Lab tests revealed that the birds had Mycoplasma gallisepticum, a pathogenic bacterium that causes a form of conjunctivitis known as House Finch eye disease. The disease spread rapidly across the Eastern Seaboard, leaving House Finches listless, mostly blind, and vulnerable to predators and bad weather. (Where did the disease come from? See Jumping the Species Barrier.)

“At that time, there was a big panic that this disease would spread to migratory birds that would go to South America, and then all the birds there would get eye infections,” says André Dhondt, who is director of Bird Population Studies at the Cornell Lab and an author of the study.

Dhondt and his colleagues realized that if they were to have any chance of preventing such a disaster, they needed to learn more about this microbial menace. So they called upon their network of FeederWatchers and created a new citizen-science project called the House Finch Disease Survey entirely devoted to tracking the disease. (The survey has now merged with FeederWatch—participants are urged to take a second look for symptoms in any House Finches at their feeders.)

Using both citizen-science data and laboratory experiments that tested virulence levels, researchers found patterns they didn’t expect. They saw that the disease had become less virulent as the birds carried the disease westward and into areas with sparser finch populations. But after it became established on the West Coast, where human and House Finch populations are denser, the disease rapidly became more harmful again—just like it had in the East. The pattern was in direct conflict with a long-standing hypothesis that a pathogen would tend to decrease in virulence over time, in order to avoid killing its host before it could be transmitted.

But to understand what drives the evolution of virulence, you have to look at things from the perspective of the bacteria, says Dhondt. “If I’m a bacterium, what do I need to do in order to be successful? If I become too virulent and kill my host before I transmit, that’s not good. But if I don’t make my host sufficiently sick, I won’t be transmitted, and that’s not good either.” That’s the idea behind the newer “trade-off” hypothesis of virulence that was developed in the 1980s and 1990s, but has remained largely untested until now. In contrast to the earlier hypothesis, this one posits that virulence may increase or decrease over time in response to conditions that affect transmission.

In other words, it’s not that pathogens are truly evil or bent on destruction of their hosts. “Any harm that a pathogen does to its host is basically a byproduct of the need to transmit,” says Dana Hawley of Virginia Tech, the study’s lead author. Virulence and transmission are inextricably linked for pathogens because both increase as the pathogen makes more copies of itself within its host.

A healthy male House Finch.

The pattern in virulence levels observed in House Finches make sense in light of the trade-off hypothesis. In regions where the finches were more sparsely distributed, any given finch is less likely to come into contact with another finch. A very sick, weak finch is even less likely to come across another finch, because it won’t be moving around as much.

But in order for the pathogen to get transmitted, the sick finch must come into contact with another potential host. As a result, in sparse populations only pathogens with lower virulence get transmitted. In evolutionary terms, sparse host populations select for lower virulence. The inverse of this is true as well, as shown by the fact that the bacteria became more virulent not only initially in the East, but once again in the denser populations of the West Coast.

House Finch eye disease ended up killing off half of all House Finches in eastern North America, a testament to the destructive power of epidemics. A true understanding of how pathogens like M. gallisepticum come to be such powerful forces is crucial to preventing future epidemics among humans. This new research is a major step toward achieving such an understanding, and citizen-science data was a key part of the solution. If citizen scientists can help us combat some of the world’s most powerful villains, one has to ask: What else are they capable of?

In addition to Hawley and Dhondt, the authors of this study include Erik Osnas and Andrew Dobson of Princeton, Wesley Hochachka of the Cornell Lab, and David Ley of North Carolina State University.

(This post was written by Cornell University senior Andrea Alfano. Images: map courtesy of Project FeederWatch; healthy finch on green background by Gary Tyson.)


  • Thom Jones

    Could you supply more pictures of infected eyes of the finches? The one picture included is difficult to make out.

  • Hugh

    Hi Thom – You can learn much more about the disease at this page:
    And specifically, this section of the page contains more photos of infected birds:

    Thanks for your comment! – Hugh

  • Barry West

    During the winter/spring of 2013 I photographed several Evening Grossbeaks with eye disease at my feeders. One was almost blind. Is this a variation of the Finch problem? How many other species are being affected?

  • dvd

    Living in NJ — and in an urban area, where HF are prevalent — and also a Project Feeder Watch volunteer in the late 90’s, I was surprised to read of the devastation claimed in this article. I did see some infected HF then (still do on rare occasions), but not a serious problem — as this article implied. I certainly had not noticed a 50% reduction in the HF population. I think there has been more of a reduced number of HF in the last few years – based on my feeder activity, but not so much in the late 90s and turn of the century. I did notice a few American Goldfinch with the conjunctivitis during the 90s — but not since.

  • Lesley

    I regularly see (sadly) several House Finches in my area of Coastal Georgia with ‘eye disease’ to a point that even with regular cleaning of my feeders I was tempted to take my feeders down thinking I could effect the numbers. I currently have a Mourning Dove with what appears to be ‘finch eye disease’ as a regular visitor to my feeders.

  • Char Birgy

    Four years ago I had several house finches wit h the disease. The population declined. This year I have lot, probably 50 or more finches & have not seen any diseased birds.

  • David H

    Noticed a great number here in GA over the last year as well. Saw it in several species on my feeders. Is there anything that can be done to treat the disease on the feeder? Any antibiotic or other treatment available that could be added to the feeder itself. Saw a lot with the Pine Siskins as well last spring. My wife is a veterinarian and had asked the same question when we tried to save a few who eventually died. Has Cornell attempted any experiments with the diseased birds?

  • Patrick Bray

    North of Atlanta in Suwanee, we have had the eye disease with the Gold Finches only that I was able to see. We have a lot of birds come through and we clean the feeders regularly. It is disturbing to see a little bird that cannot see through the crust on their eyes.

  • James Cook

    As far as I know, MG was first seen in house finches in January, 1994 — because I was apparently the first one to notice them. (Some years ago, I sent Dr. Dhondt a copy of an article that appeared in the Washington Post in May of 1994 that reported the January sightings.) From that first day (the date Jan. 20, 1994 is I remember, but I’d have to double check) and for the next three years, swollen-eyed house finches took over much of my life. I and a wildlife veterinarian in Virginia (who’s name escapes me at the moment) were the first to report the disease to wildlife authorities a month or so later that year, when my photos and the specimens he had collected arrived at the National Wildlife Health Center in Madison, WI, on or about the same day. Some of the house finches from which M.G. was first confirmed came from my yard. Some of my original photos taken back then can be found on a website I made (see below) about the outbreak, a few years or so before the CLO House Finch Disease page appeared. There was nothing online at the time to help all those bird feeders and wildlife rehabilitators desperate for information about what was happening to their house finches — and there I was at ground zero, dealing with the NWHC, my state DNR, the Southeastern Cooperative Wildlife Disease Study at the University of Georgia, and others. I found myself in a unique position to share what I was learning. So I made my web page to fill that void. Once the CLO House Finch Disease site went online, I stopped updating mine and simply referred people to that. I was just a novice bird feeder back then, with no training in any of this, who found himself in the center of the MG storm. It was time to let the pros take over — and I was more than happy to do so. But I still feel very much like the patron saint of house finches I once was. And even now, almost exactly 20 years later, I am still so sad to see this heartbreaking disease just doesn’t seem to want to go away.

    My original House Finch Disease web site, (originally an AOL member site):

    And my failed (and now abandoned) attempt to have the House Finch named as the official bird of Nassau County (NY)

    I’m still so disappointed it wasn’t chosen. What a missed opportunity.

  • Hugh

    Hi James – thanks so much for filling the rest of us in on early history of House Finch eye disease. I’ll pass your message on to Dr. Dhondt. It’s great to know that you’re still out there pulling for House Finches. – Hugh

  • George Tomasevich

    Greenwood Wildlife Rehabilitation Center, near Longmont, CO, routinely gets HOFI
    with Conjunctivitis. Last spring we put Tylosin in the H2O of all HOFI patients
    as a precaution. I am not aware of conjunctivitis in any other species. The normal treatment includes Tylosin for 21 days after symptoms subside. Birds,
    especially restless males, are released immediately after that, because they
    start banging into the sides of their enclosures.

  • Thanks everyone for such great observations and questions! There are answers to lots of FAQs (such as number of species affected, what to do if you see a sick finch) at:

    If you have house finches at your feeders and you would like to help us monitor their health, consider signing up for Project FeederWatch ( This is the only way to contribute data to our study at this time. We especially need participants in western states such as Arizona, Nevada, Colorado and Utah, where the disease has recently arrived!

    -Emma Greig
    Project Leader
    Project FeederWatch

  • Glen Porter

    During 1999 and 2000 I noticed several house finches apparently with this disease, in my yard in the greater Vancouver area, British Columbia. I haven’t noticed it for several years now.

  • “sparse host populations select for lower virulence” – that does not bode well for our ever-growing agglomerations of humans around the world. AND it also presages what could happen if a disease struck at the basis of our foodstuffs, e.g. those highly-densely populated grains like rice, wheat, maze that most of humankind either directly or through their livestock depend upon. It also helps explain though why the bubonic plague would eventually ebb – after it makes the population less dense it may getv less virulent.

  • Karen Quackenbush

    Fort Worth Texas. Just realized that a number of the house finches at my feeders have conjunctivitis. I have cleaned the feeders. How long must I keep them down? It took me almost a year to finally get a pair of titmice and some downies. I would be SO disappointed to lose them.

  • Starla smith

    We recently observed a finch with infected eye at our San Antonio suburban feeder. Will follow your prescribed protocol & continue observing. Thx for info & direction on how to proceed.

  • JM

    I recently pulled my bird feeders after seeing an infected bird and doing a little research on the internet. A couple of winters ago during a very cold spell I had a little blind goldfinch at my feeders; I now know what he was suffering from.

    Are you sure about the picture of the infected bird above? It looks more like the pictures of pox on house finches that I found when researching this. The 2 birds I have seen irl had their eyes crusted shut but not the big growth shown in the picture.

  • Rachel P

    I’ve noticed two song sparrows so far at our feeders with weepy crusted eyes. One blind and the other nearly blind. We would like to do whatever we can for these birds. We will take down and clean our feeders but Starla Smith mentioned a protocol in her comments. What is the “protocol”? Could a wildlife rehab center treat/cure the birds if I take them in? Thanks.

  • Rachel P

    Sorry–they are female house finches, NOT song sparrows!!

  • Michele Crain

    i have discovered one for sure and possible two baby/ immature finch (not sure if HF or purple. Parents look like purple finch) that have “masses” on them. One has it on the left shoulder and right side of face/chin. The other has it on its head. In the past we have had the conjunctivitis HF. Have you heard of this before? Any idea of problem? I wanted to get it reported. Thank you’ve listening and hope to receive acknowledgement you received this. Thank you!!

    [This comment has been migrated from an earlier post version by Cornell Lab staff.]

  • Sayyeda Fatima Raza

    Is this disease still common among house finches? Because this disease was first identified a problem in 1994, so was the problem solved?