Why do birds migrate?

Migration is fundamentally a strategy to exploit seasonal abundance while avoiding harsh conditions. In spring and summer, high latitudes explode with insects and long daylight hours, offering ideal conditions to raise chicks, so many birds travel north to breed. When autumn arrives and food dwindles, they head to warmer regions where they can survive the winter. The urge to migrate is triggered by changing day length, which shifts hormone levels and prompts birds to build fat reserves for the journey. Migration is costly and dangerous, but for many species the payoff in extra food and breeding success outweighs the risks.