There are likely a number of factors, some species specific for determine migration times. At a lecture a few years ago by Pete Mara of the Smithsonian Migratory Bird Center, he spoke of a competitive advantage of adult male American Redstarts of arriving to breeding and wintering ground earlier to gain higher quality territory. This may account for smaller, incremental shifts.  Last year, his article was published about drought in wintering grounds and migration. As weather patterns shift during El Nino, La Nina, and increase climate change pressures, I would seem likely that traditional migratory windows would be altered. 

http://www.physorg.com/news/2011-03-declining-rainfall-major-migrating-birds.html

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