Most birds migrate north to south, not the other way around

Whether birds migrate from the south to the north or vice versa seems like a matter of scientific semantics. But the origin and order of things has implications as to why birds started migrating in the first place. A new study suggests birds move mainly from the north to the south, undermining traditional theories.
Scientists have long assumed most birds that split time between North America and the tropics of South America began doing so as a way to avoid the competition for food and mating partners in warmer regions -- moving from the biologically diverse and overpopulated south to the quiet of the north.
But new research suggests this notion is backwards, and that most birds move from northern climes southward to escape harsh winters.
In studying the evolutionary history and family trees of American songbirds -- analysis that encompassed some 823 species of sparrows, warblers and blackbirds -- researchers found that most migrating species evolved from ancestors that originated in temperate regions. Long-distance migration was twice as likely to be a characteristic of ancestors from a temperate climate than it was from older species of the tropics.
"We're not saying it only happens one way," study author Ben Winger, a researcher at the University of Chicago in Illinois, told New Scientist. "But it happens more often by moving winter ranges toward the south."