Once upon a time, House Finches (Haemorhous mexicanus) only lived west of the Rocky Mountains. Then, in 1940, a group of captive birds flew to freedom from their New York cages. Their numbers slowly grew until there was a population explosion. Today, House Finches reside throughout the U.S. and Mexico.

There’s a downside to dense populations though – disease. In the 1990s, a bacterium (Mycoplasma gallisepticum) started swirling among groups of House Finches. The infection causes conjunctivitis (like “pinkeye”) in the birds. If you’re a bird with swollen eyelids and crusty build up, you’re not going to be very good at flying or avoiding the neighbor’s cat.

Today, roughly half as many House Finches live in the East compared to 20 years ago.

 

Sources:

Learn more about House Finch diseases (with gross pictures)

See the growth and decline of House Finch populations in your state (1)

1. Pardieck, K.L., D.J. Ziolkowski Jr., M.-A.R. Hudson. 2014. North American Breeding Bird Survey Dataset 1966 – 2013, version 2013.0. U.S. Geological Survey, Patuxent Wildlife Research Center.