As lesser goldfinches continue to hang out on our Littleton farm, almost three months after their usual migration to the south, I have considered how human activity affects the natural range of bird species. In the case of these finches, suburban landscaping and feeder handouts augment natural food sources and likely diminish migration pressure as winter settles in.
Perhaps the best example of human impact on North American bird ranges is the settlement of the Great Plains. As towns and cities appeared along the primary rivers, which flow from the Rocky Mountains to the Mississippi Valley, the ranges of various Eastern bird species (northern cardinals, eastern bluebirds, eastern phoebes, indigo buntings and others) have been spreading westward. While some woodlands have always lined these rivers, human settlements greatly increase the diversity of vegetation; reservoirs along the streams also spawn the development of backwater wetlands and other riparian ecosystems which attract birds that would otherwise avoid the vast fields and grasslands of the Great Plains.
Indeed, while I often call attention to the human destruction of natural habitat in this blog, I must acknowledge that some species benefit from our activity. Unfortunately, some of the birds most capable of adapting to human settlements are those that were introduced by man: European starlings, house sparrows, cattle egrets and Eurasian collared doves come to mind. In the end, it's still best to protect natural habitat, avoid the introduction of non-native species and let nature take control. See also: Settlers and Cardinals and Feeding Birds.