What Is Trichomonosis?

Trichomonosis (sometimes written as trichomoniasis) is an infectious disease caused by a microscopic single-celled parasite called Trichomonas gallinae. The parasite lives in the upper digestive tract of birds — primarily in the mouth, throat, crop, and oesophagus — where it causes tissue damage that prevents the bird from swallowing food properly.

The disease is not new. It has been documented in pigeons and doves for centuries, and falconers have long known it as “canker.” However, it was not until the early 2000s that researchers at the British Trust for Ornithology (BTO) and the Zoological Society of London (ZSL) identified a new, highly virulent strain spreading rapidly through finch populations in Britain. That strain has since had devastating consequences for the greenfinch.

The parasite itself cannot survive for long outside a host. It does not form spores and is killed by drying and by many disinfectants. This makes it theoretically controllable — but the way birds interact at garden feeders creates ideal conditions for its transmission.

The Impact on UK Greenfinches

Before trichomonosis took hold, the greenfinch (Chloris chloris) was one of Britain’s most familiar garden visitors. Plump, yellow-green, and gregarious, these finches were a staple at sunflower heart feeders across England, Scotland, Wales, and Northern Ireland.

BTO data from the Garden Bird Feeding Survey and the Breeding Bird Survey paint a grim picture. Since the first major trichomonosis outbreak was confirmed in British finches in 2005 and 2006, greenfinch populations have fallen by approximately 65 per cent. That is roughly two-thirds of the entire UK breeding population lost within two decades. The species is now on the UK’s Red List of Birds of Conservation Concern, having been moved from Green to Amber to Red in successive assessments by the RSPB, BTO, and JNCC (Joint Nature Conservation Committee).

The first outbreak was centred in Wales and south-west England. From there, the disease spread rapidly northward through England and into Scotland. Today, trichomonosis is considered endemic in British finch populations, meaning it circulates year-round rather than appearing only in seasonal outbreaks, though mortality events tend to peak in late summer and autumn when birds congregate most heavily at garden feeding stations.

It is worth noting that this decline is not solely attributable to disease. Habitat loss, changes in agricultural practice, and the reduction of weed seed availability in the countryside have all played a role. However, trichomonosis is considered the primary driver of the sharp and sustained post-2005 decline.

How Trichomonosis Spreads at Garden Feeders

Understanding the transmission route is the single most important step in taking effective action. Trichomonas gallinae spreads primarily through direct contact with infected saliva or crop secretions. In practical terms, this happens in several ways at garden feeding stations.

Contaminated food and water

An infected bird that regurgitates partially digested food — which greenfinches and other finches do occasionally do, particularly when feeding young — deposits live parasites directly onto feeder surfaces, seed trays, and into water containers. The next bird to feed or drink from the same spot can ingest those parasites and become infected. Because the parasite dies quickly when dry, wet conditions — such as rain-soaked seed trays or dirty water bowls — maintain a hospitable environment for transmission.

Bill-to-bill contact

Birds that jostle and compete at feeders make brief but repeated contact with one another. Even a momentary touch of beaks at a perch or seed port is sufficient to transfer the parasite.

Pigeons and doves as reservoirs

Woodpigeons, stock doves, feral pigeons, and collared doves are natural reservoir hosts for T. gallinae. They carry the parasite without always showing severe symptoms themselves, but they shed parasites consistently in their saliva. When these species share feeding areas with finches — which is extremely common in UK gardens — they act as a persistent source of infection. This is one reason why managing pigeon access to garden feeders can be a meaningful protective measure.

Seasonal peaks

Transmission risk rises significantly in late summer and early autumn. Juvenile birds, newly independent and with no prior exposure, arrive at gardens in large numbers. Birds that fed independently through summer begin gathering at feeders as natural food sources decline. Both the density of birds and the proportion of immunologically naive individuals increase simultaneously — creating conditions that favour rapid spread.

Recognising the Symptoms

Knowing what to look for allows you to act quickly — removing sick birds from the feeding area reduces the risk of further transmission. The symptoms of trichomonosis follow a recognisable progression.

Early signs

• Lethargy: Affected birds appear reluctant to fly and may remain at feeders or on the ground even when approached.
• Fluffed feathers: Sick birds often sit with their feathers puffed out, a general indicator of illness in many species.
• Repeated swallowing: Watch for birds that make repeated swallowing or head-bobbing motions, apparently struggling to get food down.
• Wet plumage around the bill: Salivation and regurgitation can leave the feathers around the face and chin matted or damp-looking.

Advanced signs

• Visible swelling: The parasite causes lesions and cheesy yellow-white deposits inside the throat and crop. In severe cases, swelling of the neck or lower face becomes visible from outside.
• Weight loss: Because affected birds cannot swallow properly, they lose condition rapidly. A bird that looks thin with a prominent keel (breastbone) visible or palpable despite appearing to feed is likely suffering from nutritional failure caused by the blockage.
• Open-mouthed breathing: In very advanced cases, the lesions in the throat can obstruct the airway, causing a bird to breathe through an open bill.
• Death: Without intervention or recovery, affected birds typically die within days to weeks of showing symptoms. You may find dead birds beneath feeders or on the ground nearby.

It is important to note that not every sick bird in your garden has trichomonosis. Salmonellosis (common in house sparrows and chaffinches), avian pox, and other conditions can produce overlapping symptoms. If you are unsure, the safest course of action is to treat the situation as a potential disease event regardless of the precise cause.

Other Garden Birds at Risk

While the greenfinch has suffered most visibly, trichomonosis affects a range of other species that visit UK gardens.

Chaffinch

The chaffinch (Fringilla coelebs) has also experienced notable population declines linked to trichomonosis, though the impact has been less severe than in the greenfinch. BTO monitoring continues to track chaffinch numbers closely.

Bullfinch

Bullfinches have been recorded with trichomonosis infections in the UK. Given that the bullfinch is already an amber-listed species with a long-term population decline, any additional mortality from disease is a concern.

House sparrow

House sparrows can carry and contract T. gallinae, though they are somewhat less susceptible than greenfinches. They remain of conservation concern in the UK for other reasons, so minimising disease pressure is still worthwhile.

Woodpigeon and collared dove

As mentioned above, these species are reservoir hosts. They can and do suffer mortality from trichomonosis, particularly in warm, wet summers when parasite loads increase, but they do not crash in population terms the way greenfinches have.

Raptors

Sparrowhawks, peregrines, and other birds of prey that feed on infected prey birds can contract trichomonosis themselves. This is a particular concern for peregrine populations in some areas, though it is not currently considered a major driver of raptor population trends.

What to Do If You Find a Sick Bird

Finding a lethargic or visibly unwell bird in your garden requires prompt and careful action. Follow these steps in order.

Step 1: Do not handle the bird unnecessarily

Your first instinct may be to pick the bird up. Resist this unless necessary. Handling causes additional stress to an already compromised animal. Additionally, while T. gallinae does not infect humans, other pathogens present on sick birds — such as Salmonella — can. If you must handle the bird, wear disposable gloves or use a plastic bag over your hand and wash thoroughly afterwards.

Step 2: Remove other birds from the area

Take down all feeders and remove water sources immediately. This sounds drastic, but it is the single most effective action you can take to prevent further spread. A sick bird at an active feeder is a direct and ongoing transmission risk to every other bird that visits.

Step 3: Contain the sick bird if possible

If the bird appears unable to fly and is at risk of predation or further injury, you can place it gently in a ventilated cardboard box lined with kitchen paper. Keep it in a quiet, warm (but not hot) place. Do not attempt to give it food or water — an animal with an obstructed throat cannot safely swallow.

Moving Forward

Once you have the fundamentals in place, the possibilities open up considerably. The UK offers fantastic opportunities for anyone interested in this hobby, and with the right foundation you will be well placed to make the most of them.