Common Avian Diseases
By Jon Friedman
Diseases and illnesses in wild birds are present throughout the world wherever populations of birds are found. Most backyard birding enthusiasts rarely have a need to investigate or research these conditions as the vast majority of these diseases and illnesses are not present in their backyard birds. However, backyard birders should be informed about the most common examples they are more likely to notice. Identification and prevention of the most common diseases that affect backyard birds can help slow or curtail the wide spreading of these problems into the larger population of birds. While birders have little control over the natural lives of the birds that visit their yards, we do have the ability to limit or lessen the impact of spreading diseases at our feeders and bird baths.
Occasionally, the media and press publicize extreme examples of bird diseases. When this happens, the public at large initially learns that a problem exists. Usually, when the media and press cover such events it is an indication that the human food supply is threatened or there may be a direct threat to the human population. An example of the former could be an outbreak of avian flu in China or Newcastle disease in domestic chicken production. In both of these instances, chickens are raised in overcrowded, unsanitary and unhealthy conditions – conditions that allow for the rapid and thorough infestation of the entire flock. In situations such as these, the only remedy is to destroy all the birds entirely. The emphasis on profits above health concerns in factory farming operations allows these types of conditions to persist. (As a result, I only eat free-range, pastured chickens that have been fed an organic/natural diet.)
An example of the latter could be viruses such as West Nile, malaria, equine encephalitis and dengue and yellow fevers – all potentially transmitted from mosquitoes to birds to humans. For the most part, these particular diseases and other related illnesses are actually quite limited and affect relatively few people – especially in the United States.
In short, backyard birders have little to be concerned about regarding these particular diseases and other related pathogen-induced illnesses. This article will inform our readers to what the natures of such illnesses are and their relationships to birds. But, I will focus mostly on the illnesses that birders may notice and what measures can be undertaken to possibly undermine, lessen, or potentially eliminate the spread of these problems – beginning in the backyard.
Sources of Diseases
Avian diseases and illnesses are always present and sometimes widespread in birds and other animals throughout the world. Some diseases and parasites are hosted within or on birds permanently or for shorter amounts of time. Some are fatal to the host. Sometimes birds simply become infected without dying but act as carriers of the disease, spreading it to other others within their population. Most diseases can be traced to several origins: viruses, fungal sources, bacterial, or parasites, either external or internal. Most parasites are blood-suckers or have negative effects on either/or red and white blood cells.
Parasites and Pathogens
Many organisms gain nutrients and obtain energy by feeding on birds. The largest of these organisms are big enough to kill and devour a bird – we refer to them as predators. Most readers are familiar with typical bird predators such as domestic and feral cats, other birds (hawks, roadrunners, etc.), snakes, coyotes, etc. Very small animals large enough to seen with the naked eye are usually referred to as parasites. They may be associated with a bird for a lifetime or for a relatively short time. They may live on or in the bird. Parasites may or may not be a serious threat to the bird. Many lack the ability to actually make the bird sick or cause a fatal illness. Examples of external parasites that dine on birds include, but are not limited to: bird lice, fleas, black and louse flies, Hemiptera (small bugs related to bed bugs), ticks, and mites. They are all blood suckers. Internal parasites include roundworms, tapeworms, flukes and others that live in the digestive tract or in the blood vessels of birds. More study is necessary to fully understand the impacts that parasites have on birds but ornithologists generally agree that in the overall scheme of things, parasites are more of a problem to colonially nesting species (due to the birds’ close proximity to each other) than to individuals of any given species. Micro-organisms, which include protozoans, bacteria, fungi, viruses, and rickettsiae (tiny bacteria like organisms) can be fatal to infected birds, and are commonly referred to as pathogens.
Botulism in nature is a bacterial disease born from soil and incubated by warm water and sunlight. Previously known as “western duck disease”, outbreaks can affect large numbers of birds, particularly ducks and other migrating waterfowl species found in association with larger bodies of shallow water. Two well documented cases involved large numbers of water and shore birds at the Great Salt Lake in northern Utah in 1929, where up to three million ducks and other waterfowl died and the Salton Sea in southern California, where die-offs from botulism involved hundreds of thousands of birds with outbreaks happening several times over a period of years. Some of the earliest reports of this disease in the United States date back to 1876. Botulism is also one of the most deadly diseases for humans to encounter. However, humans don’t get botulism from the environment but, rather, from faulty canned and processed foods.
Relationships to Humans
There are a great variety of bird diseases and illnesses, hundreds worldwide. Avian Malaria (different than human malaria) and West Nile are caused by protozoans carried by mosquitos, as is Aspergillosis (a fungal infection) and Tuberculosis (not the same pathogen that causes human tuberculosis). Various other viral diseases, such as Newcastle disease, fowl plague, avian pox, avian influenza (flu) and others are associated with birds. Most avian diseases are not transferable to humans, but birds can serve as hosts to pathogens that can cause serious human illnesses. For example, the rickettsia that causes psittacosis is sometimes contracted by humans who keep parrots as pets. Psittacosis may be fatal to humans that don’t get treatment. The same is true of equine encephalitis virus. The virus is usually carried to birds and horses from mosquitoes. Other blood sucking mosquitoes may prey upon an infected bird before preying on an unsuspecting human. Migrating birds have been implicated in the transport of virulent encephalitis strains between continents.
Extinction as a Result of Disease
While birds do not suffer heavily from mosquito-transmitted viruses, there are notable exceptions. A particular species of mosquito, Culex pipiens, was accidently introduced into the Hawaiian Islands in 1826. This species of mosquito was a carrier of avian pox, malarial protozoans, and other pathogens. It transmitted these pathogens to many lowland species of Hawaiian Honeyeaters. Those species were wiped out and became forever extinct. Highland Honeyeater species were spared due to the fact that the mosquito could not survive at altitudes above 2,000 feet.
Some diseases are closely associated with certain species or families of birds. While psittacosis, commonly referred to as parrot disease or parrot fever, can affect all domestic and wild parrots, it has been recorded in over 150 non-parrot and parrot species worldwide. Blackhead is another disease once thought limited to domesticated poultry but later discovered to be a serious threat to Wild Turkey populations. It is a tissue parasite that invades and attacks the liver and caeca (pouch that connects the large and small intestines), in particular. And, while it is a well-known disease in Wild Turkey populations, ornithologists now know it is a disease that is also prevalent in most other wild gallinaceous species (ground-feeding game birds like grouse, pheasant, ptarmigan, quail, partridge, etc.).
Coccidiosis is another disease which is closely associated with certain wild birds, particularly quail. Coccidia are parasitic protozoans that inhabit the digestive tracts of birds and is closely associated with the foods birds ingest. It is also a seasonal phenomenon, depending on which foods are in season at a given time of the year. Beginning in the 1940’s, much research was accomplished in regard to this disease, particularly with California Quail. The research determined that there was a direct correlation between the time of year and the foods consumed seasonally – a lower intensity of infection in summer, when quail were eating more seeds than the rest of the year, when they were eating more green plant foods. This disease is also known to affect certain species of ducks, particularly goldeneyes and canvasbacks. It has been found in pelicans, cormorants, shorebirds, and some other species. It makes sense that hawks that prey upon diseased birds can also become victims to the disease themselves.
We can see direct evidence of bird-eating hawks (accipiters like Cooper’s Hawks, for example) becoming diseased themselves in our own backyards. One of the most common bird diseases we experience here in the Sonoran Desert is trichomoniasis (trick-oh-moh-NYE-ah-sis). It is caused by Tricomonas gallinae, a flagellated protozoan that lives in the throats, saliva, and certain viscera – lungs, liver, etc. - of doves and pigeons primarily, who are the favorite prey of bird-eating hawks and other raptors (falcons and owls). Localized populations of doves and pigeons are reduced in numbers by occasional outbreaks of this disease. In fact, after hunting, it may be the major cause of death for these types of birds when outbreaks do occur. Ornithologists have suggested that besides hunting, this disease may have been a major factor in the extinction of the Passenger Pigeon; as this protozoan parasite occurs almost universally in pigeons, which are the primary hosts for the disease.
Trichomoniasis has been a problem for pigeon fanciers/breeders and falconers, as well. Pigeon breeders, who take great pride in competitively showing their birds in judged competitions, refer to this disease as canker and falconers call it frounce. For non-wild birds such as fancy pigeons, falcons and other hawks; there is a medicinal remedy, called Emtryl, to successfully treat them.
Spread of the Disease
In populations of wild birds, infected birds sometimes succumb to the disease while others survive to live longer, almost normal lives. However, they likely become carriers of the disease and inadvertently cause the spread of the disease. Usually, the disease is more often spread not-so-much through direct physical contact but via a liquid vehicle. Two common examples of how the disease is spread is by pigeons and doves are through the “pigeon milk” that parent birds feed their babies via the regurgitation process. This is, perhaps, the most direct method of transmission. Another common method is when birds drink from contaminated birdbaths. The protozoa responsible for this disease requires a liquid vehicle to remain alive and viable, so water, saliva, and regurgitated “pigeon milk” shared by birds is the usual cause for the spreading of this disease.
Backyard birders can control the spread of this disease, at least at their feeding/bathing stations, to a certain degree. Most importantly, birdbath water needs to be changed and replaced with fresh water daily. This will help prevent the heavy and quick build-up of the contaminants in the water. I suspect this is the most common method of contamination and the easiest method to control. During breeding season is when the pigeon milk method of transference becomes more widespread and there’s little one can do to prevent that other than keeping bath water fresh daily. When the protozoa come into contact with dry air, even for such short a time as 20-30 seconds, it cannot survive and dies. For this reason, most feeders are not the culprits in the spread of the disease. Most feeder designs (hopper, tube, etc.) discourage the proliferation of the disease by keeping the seed dry and preventing multiple birds from feeding at a single port simultaneously. Additionally, The Wild Bird Store specializes in designing, manufacturing, and selling many models of dove and pigeon-proof feeders that not only denies those birds the ability to feed – but in many cases, deny those birds access to the feeders themselves. An exception to this example would be providing seeds or other foodstuffs in an open platform feeder that allows all birds to mingle together and freely use it. Our dove-proof platform feeders have proven themselves some of our best-selling models over many years for exactly this reason.
Recognition and Effect
This disease, trichomoniasis, is relatively easy to spot in infected birds. In the middle and later stages of the progression of the disease, a greenish-yellow saliva-like liquid can be observed dripping from the bills of seriously infected birds. In its infancy and early stages of development, the disease can go unnoticed. This yellowish-greenish saliva coats their throats and hardens over time into a more solid like substance and slowly prevents swallowing as this build-up concentrates in the throat. This phenomenon is akin to extremely mineralized water depositing an ever thickening coat of calcium and other minerals onto the inside of water pipes. Eventually, if allowed to continually build-up, this hardened mineralization will slowly choke off the ability for water to flow through the pipe. A similar process occurs in the throats of the infected birds. This chartreuse material builds up in the throats of the birds until the swallowing passageway is ultimately completely blocked. As the throat space narrows, denying the birds the ability to freely swallow, some of this material backs up and leaks out through their mouths. I suspect most of the birds that have advanced to this stage do succumb to the disease. But those that can recover, may live a normal life span. However, as carriers of the disease, they can now pass it to other birds, through a liquid vehicle, of course. In the example of an open platform seed feeder, an infected bird may pick up a seed and finding itself unable to swallow it, drops it back into the feeder. If another bird immediately picks up that same disease-coated seed, the disease can be transmitted to that bird. If, however, that contaminated seed sits in the dry air for 20-30 seconds or more before being consumed by another bird, the protozoa die and can longer contaminate other birds. So, dove and pigeon-proof feeders go a long way in keeping healthy populations of birds and do a reasonably good job preventing the spread of this common disease in particular.
The other bird disease that backyard birders have probably experienced seeing is a viral infection called avian pox. It has also been known as bird pox, foot pox, fowl pox, and avian diphtheria. This virus is composed of submicroscopic infectious agents that rapidly multiply in living cells. Those who actually handle birds, such as ornithologists, bird banders and other naturalists, are very familiar with the symptoms of this disease, which are limited to birds only. Finch family members are the most likely birds to be infected with or be carriers of this disease. This disease can affect any members of the finch family, including cardinals and Pyrrhuloxia. It is not common among non-finches. Interestingly, and as far as my own observations and research are concerned, I have noticed this disease primarily in red finch populations, particularly House Finches. I cannot remember seeing this condition exist among the flocks of goldfinches I have fed for many years. And, while I occasionally observe buntings and grosbeaks (both in the finch family) at my feeders, I cannot recall any instances I have noticed avian pox infecting those birds.
Transference and Control
Avian pox is transferred primarily by actual physical contact between two birds. Apart from a mated pair of birds who do come into physical contact during mating and courtship rituals, and during mutual preening, most birds rarely come into such close and personal contact. The obvious exception could be serious overcrowding at the feeding station. If you suspect this is or could be a problem at your backyard feeding station, supplying multiple feeders that are distanced apart from each other will lessen the transference of this disease. This strategy also has additional benefits for the birds as it will reduce competition, and bird-killing stress, at the feeders while reducing the number of birds attempting to use a single feeder. When birds engage in a feeding frenzy, competing for a single source of food, physical contact is unavoidable. Multiple feeders, spread out geographically, will greatly reduce the likelihood of wide spreading this disease. While we cannot control to any significant extent, the spread of this disease in the wild, we can have a positive impact of reducing the expansion and effect of this disease in our own backyards to some degree.
This disease is easy to recognize. It manifests itself in exterior tumor-like growths that occur, almost exclusively, on the head, feet, and legs of finches and finch-like birds. Think of them as oversized warts. The tumors themselves are not thought to be direct causes of death. However, due to the parts of the body they inhabit, they can lead to circumstances that do cause birds to ultimately die. For example, when these tumor-like growths occupy the head they often can be found very close to the eyes, usually just off to side of the eye, sometimes above the eye. In situations such as this, the growth can obscure or completely block the bird’s vision. Without the full scope of vision birds are accustomed to, they become easier prey for predators. I have personally observed instances in which predators took full advantage of avian pox victims and swooped in from the “blind side” of the bird to make it easy pickings for a quick meal. Another example of how seriously avian pox can impact a bird’s life, without killing it, is when this tumor-like growth occurs on the bottom of their feet. I can’t attest to any pain this growth might cause for the infected bird, but I’ve noticed that when this situation happens, it makes it particularly difficult for the bird to land and perch. I remember seeing this situation come to light in real time in my backyard. I saw a finch fly into the yard, attempt to land on a branch, and, unable to do so, flew to a different branch to try again – only with the same sad result. I watched this bird attempting to perch several times until it finally flew off out of the yard. I don’t know what happened to the bird after that. But, I suspect it had a very difficult time leading a normal life, doing the things birds normally do in order to survive and thrive. As I know finches can’t hover and must perch to accomplish most tasks that don’t require flying, I assume this bird probably had a shortened lifetime. So, preventing overcrowding at any singular feeder is a good strategy to help avoid the further spread of this disease among some of our favorite songbirds.