"An Audubon Society study to be released Tuesday found that more than half of 305 birds species in North America, a hodgepodge that includes robins, gulls, chickadees and owls, are spending the winter about 35 miles farther north than they did 40 years ago."First - 40 yrs would mean that they are only going back to the 1960's ... that just isn't far enough back to decide whether is this a normal migration or not. I seem to recall my parents & uncles/aunts talking about birds that they saw as kids that have been sticking farther south than normal.
It also has to be said that bird migrations ... or rather their boundries ... can be effected by several things -- weather, food supplies (more people are feeding the birds these days), predators, competition, magnetic field of the Earth.
It seems only natural to me that if there is an over abundance of birds in an area that some of them would start ot move out of their 'comfort zone' in order to find food supplies.
It also seems normal to me that as natural predators for these birds diminish - be it other birds, snakes, wild beasts, or simple house cats (now wearing bells) - they would be able to survive in greater numbers in 'new' areas ... well new as in the Audobon society says it hasn't seen them there before.
Then there is food supplies -- more people are feeding the birds these days ... if they weren't there wouldn't be such a large demand for so many different types of feeders - have you walked down that aisle of your local Home Depot or Menards's lately? They have feeders & birds for every type of bird you could think of -- even one's not 'normally' in your area.
Then there is the magnetic field - which is what I think is really going on. Birds use the EM field of the earth to travel from here to there and to find their way back again. If the EMF of the earth is changing from North/South and is currently in a flux state then it would make sense that the birds would be off by just that bit coming more north or south than usual.
So why are they only seemingly seeing it in the US? maybe because the US has a higher concentration of members? or perhaps because we are more 'freaked out' by any little change in things so we over report .... much more likely it.
It could also be that these birds have been simply doing it for years and we just hadn't NOTICED it -- after all they keep rediscovering species thought to be extinct -- why couldn't this be possible as well?