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Population Dynamics of the House Finch


Please cite this article as:
Kammermeier, L. 1999.  Population Dynamics of the House Finch.   Birdscope, Volume 13, Number 2: 15.

A look at the big picture

Chances are you're familiar with the House Finch. This widespread species is a common resident in cities and suburbs, often breeding in developed areas with buildings, lawns, and small conifers. If you haven't seen winter flocks of this feeder bird outside your window, perhaps you've seen articles about House Finches in previous issues of Birdscope. These articles were primarily focused on the effects of the eye disease that is currently spreading through eastern House Finch populations, so we thought it was time to take a broader look at the changing distribution and abundance of this bird.

House Finches are native to the western states and provinces. from British Columbia south into Mexico and eastward to central Texas. Although House Finches originally inhabited undisturbed habitats in the West such as desert, desert grassland, chaparral, oak savannah, riparian forests, and open coniferous forests at lower elevations, they have successfully adapted to areas altered by humans. In the last 50 years, western House Finch populations have expanded into Washington, British Columbia, Montana, and Idaho, possibly because of the growth of suburban areas and increased numbers of bird feeders.

The ability of House Finches to adapt to people has not only allowed them to colonize new areas on their own, it also enabled them to make the most of opportunities provided by humans who introduced them into new areas. Distinct populations of House Finches now exist in Hawaii (where they were introduced in the late 1800s) and in eastern North America. The eastern House Finch population originated on Long Island, New York, in the 1940s. At that time, House Finches were being sold illegally as caged birds, known as Hollywood Finches. Pet traders presumably released the birds to avoid prosecution under the auspices of the Migratory Bird Treaty Act of 1918. Those few released birds thrived, and, by the 1980s, their descendants had reached the Mississippi River. Today, the eastern population is meeting its western kin in the Great Plains states. Although the evidence is inconclusive, some people believe that because House Finches are so plentiful, they have caused declines in populations of Purple Finches and American Goldfinches.

But House Finches are not unstoppable. The population explosion that occurred in the East has stopped since the eye disease epidemic (mycoplasmal conjunctivitis) began sweeping through the region, and in many areas finch numbers are now decreasing. This disease is a bacterial infection that seems to be restricted mostly to House Finches; it results in red, swollen, crusty eyelids and the birds often die from starvation, predation, or exposure. The disease was first documented in the Washington, D.C., area in the winter of 1993. 94. Reports in the East became so numerous that the Lab of Ornithology initiated the House Finch Disease Survey in November 1994 to systematically survey the spread of this disease.

Thanks to our participants, we were able to monitor the disease as it spread beyond the Appalachians in just two winters; the disease took only one more winter to spread into the Midwest. As of this issue, reports of the disease have been confirmed by epidemiologists as far west as the Great Plains. The rapid spread of this disease can be attributed to two things: long-distance dispersal of juvenile House Finches and the species' tendency to move in large, highly mobile foraging flocks. Reports become more prevalent in the fall and early winter when the birds form large foraging flocks, which increase their chances of transmitting the disease among individuals.

Compared to other diseases that strike wildlife, mycoplasmal conjunctivitis is unusually persistent. Early on, the prevalence of this disease reached epidemic proportions, and many finches died. But, whereas many diseases irrupt and then fade away, we continue to receive reports of mycoplasmal conjunctivitis five years after the initial sighting. According to Wesley Hochachka, assistant director of the Lab's Bird Population Studies, the consequence of this disease has been a steady decline in the abundance of House Finches in eastern North America. House Finch numbers are now only about 40 percent of what would have been expected had the disease not appeared.

When will the House Finch's range expansion stabilize? Will mycoplasmal conjunctivitis continue to affect the abundance of this bird? Will the disease spread to House Finches in the West? We don't know, but with your continued participation in Project FeederWatch and the House Finch Disease Survey, we should one day learn the answers.

To learn more about House Finches, call (800) 843-2473 (BIRD) or visit the Lab's web site at <>. Search under previous Bird of the Week and under House Finch Disease Survey links. Also visit <> and select PFW Birds, Maps, Graphs, and IDs.