As I sit this morning, I have watched a number of birds come and go from the feeders. Most of the birds here are House Finches. A small North American finch, originally from the Western U.S. and northern Mexico. In 1940, however, pet stores on Long Island, New York released a few pair when it became illegal to sell U.S. native birds. In the last sixty years, those few birds have grown hugely in population and now range all over the eastern half of the continent as well.
If the birds had been released much before 1940, this probably wouldn’t have happened. The birds released in New York would have probably died after a winter or two, or perhaps they would have been successful locally, but they would not have spread.
Why? Because the habitat wasn’t right for them. House Finches are birds of open land. They do like a few trees, but the operative word there is "few." They are absent from dense forest, and over much of the eastern third of the U.S. that’s what used to be there.
In the last few hundred years, but especially since 1940s, the huge tracts of dense forest in eastern North America have been cut down to make way for suburbs, or cut through for roadways.
This new patchwork of open land among the trees has opened a huge territory for the House Finch, and they really have taken advantage of it. If our land use changes again, the House Finches in the east will change their population to respond to it.
Other birds have taken advantage of the changes humans have made to the environment in North America. House Sparrows (also called English Sparrows) were introduced to New York City around 1890. Like the House Finch after them, their population expanded westward very quickly – but in this case for a slightly different reason. House Sparrows are birds that prefer an association with humans. They’ve become common in cities, and often rival Pigeons (introduced to North America in the 1600s as a food source) in their populations.
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