Why do birds migrate north?

Avoiding competition and overcrowding is one aspect but the north - even in the artic circle is a plentiful place in the summer and the seas are rich in fish - cold waters are actually far more productive. If the food and more importantly space for breeding territory's are there it's hardly an evolutionary strategy to stay put all year round. Especially if it means you get to breed rather than having to hang around until a territory becomes available next year.