Because these migrants are seasonal, they may have fewer predators. Specialized predators would be less able to survive the winter in the absence of their migrant prey. In summer the days are longer at higher latitudes. The further north a bird travels, the more hours of daylight it will have for gathering food, ie it can rear more young. For birds, with their high mobility, migration is the rule rather than the exception.
Most birds move somewhere as the seasons change.
For some, such as Swallows or the Arctic Tern, the move is dramatic involving thousands of miles. Others, like Starlings or Robins, may move a few hundred miles into an area where some of their species already live. They look just like the 'residents' and their arrival may go largely unnoticed.