The reason robins left the north was never the cold, but lack of food. The changing landscape over the past several decades has included many more berry producing shrubs like honeysuckles and buckthorn. Mark Davis from Macalaster College in St. Paul authored an interesting paper
"Do Native Birds Care Whether Their Berries Are Native or Exotic? No."
Now the true indicators of the arrival of spring are grackles the iridescent blue/black birds with yellow eyes. This spring in Minnesota has been VERY slow to emerge and while I've seen plenty of robins we're still waiting for the grackles.

A good example of the dangers of assuming the worst of nonnative spe- cies is illustrated by Gleditsch and Carlo (2011), who described the effects of nonnative honeysuckles on native bird species.
They found that birds were eating large amounts of the non-native honeysuckle berries, but they were also eating more native berries than were eaten in areas without the "invasive" honeysuckle. Davis relates this to the car dealer effect. Car dealers often locate next to one another because they enjoy higher overall customers than if they were located in separate locations. The same is true of artist studios and art galleries. The ones grouped together generally have more visitors for open studio events and first Fridays.
The often repeated line on invasive species is that they outcompete native species. This paper shows that when studied it turns out that non-native berry producers are actually encouraging birds to distribute native berries as well. This is our new evolving landscape, and it's working just fine.