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California's Hummingbirds Have Changed Their Beaks in Response to Backyard Feeders, Study Finds
(www.smithsonianmag.com)
Without the feeders, birds with larger beaks might not be able to find food as well than birds with slimmer beaks, which would make them less likely to live long enough to reproduce. The feeders have allowed birds that have DNA to code for bigger beaks to survive and make it to adult hood, have offspring, and pass those genes along. Evolution at it's finest.