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Winter 2013

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

February 12, 2013

House finch eye disease: an update

Biological invasions continue to sweep across the continent, one after another. A robust wave of opportunists can sometimes overwhelm an earlier one, only to be dragged down by another scourge.

Such is the case of the house finch, a West Coast native introduced to Long Island in 1940. As the birds multiplied and spread westward — reuniting with their western cousins in 1995 — they settled into an ecological niche occupied by another invasive species, the house sparrow, a bird brought from Britain in the 1850s. Where house finch numbers ballooned, house sparrows declined.

But that trend reversed in 1994, beginning in the Washington area. House finches with impaired vision and crusty, swollen eyelids started lingering at bird feeders. A new disease had emerged: Mycoplasmal conjunctivitis, a novel strain of Mycoplasma gallisepticum (MG), a parasitic bacterium known to cause chronic respiratory disease and sinusitis in pigeons and poultry.

In house finches, MG infects the conjunctiva, the membrane covering the eyes and lining the eyelids. Many finches recover from MG — if they don't starve or aren't killed by predators — but they can

Maryland’s house finches


House finch population in Maryland; Breeding Bird Survey

still carry the disease, which is transferred most effectively by direct contact with other birds while flocking in the fall and winter or when congregating at bird feeders.

"House finch eye disease" spread rapidly westward, infecting birds in every state east of the Mississippi River by 1996. It reached native house finch populations in the Pacific Northwest in 2004 and spread south along the West Coast. MG may soon enter Colorado and New Mexico, says Andre A. Dhondt, a professor at Cornell University's Laboratory of Ornithology.

The booming population of eastern house finches has been cut in half, he says, but it has stabilized, probably because the spread of the disease is dependent on the density of the finch population. The finches also seem to be developing a resistance
to MG.

Although house finches have been hit hardest by MG, other species in the finch family — American goldfinches, purple finches, evening grosbeaks — occasionally get the disease. But not house sparrows (from the weaver finch family), says Dhondt, who has injected the bacterium into the eyes of the birds: "They clear an infection very rapidly."

Having dropped many of its immune-system genes when it jumped from poultry, the MG strain that affects finches is much less hardy. The strain might be extra-vulnerable to attack by viruses that prey upon bacteria.

Feeders are a mixed blessing for birds. If not cleaned weekly, they can dish up a buffet of diseases, including salmonellosis, trichomoniasis, aspergillosis and avian pox. But even a clean feeder can crowd birds together, encouraging bird-to-bird transmission of infections. A sick bird is more likely to be drawn to a feeder, where the food might help sustain it. Otherwise, the bird might fly off to a secluded spot and die, sparing others its disease.

♦ Tips for cleaning feeders

Sources: Laboratory of Ornithology, Cornell University; Proceedings of the National Academy of Sciences; PLoS One; Journal of Animal Ecology; PLoS Genetics; Emerging Infectious Diseases; Avian Pathology; Ecology; USGS

house finch, Haemorhous mexicanus

Female house finch with MG

Male house finch

Breeding Bird Survey data