Stay OR go?

Migration as an evolutionary driver

- PETER RYAN & MICHELLE VRETTOS

Birds are the most mobile organisms on the planet. Their ability to fly vast distances enables them to exploit predictable, short-term peaks in food availability. Almost one-fifth of all birds undertakes regular movements, usually tied to seasonal cycles. However, such large-scale movements also incur risks from commuting across the landscape. The balance between these costs and benefits determines who stays and who migrates. Recent studies suggest that migration promotes speciation, principally through the formation of sedentary daughter species. Here we highlight some examples and speculate how rapid global change might affect this process.

Migration has long fascinated people and considerable effort has been devoted to learning how birds migrate. We marvel at the physiological adaptations that enable Bar-tailed Godwits to fly nonstop for eight days from Alaska to New Zealand without eating or drinking. We have determined the range of cues that birds use to navigate to return to the same breeding and wintering sites year after year. We have demonstrated that the migratory urge is inherited, with genes coding for the direction and duration of migration. However, we know that the risk of being blown off course during a young bird’s initial migration can be greatly reduced by travelling with more experienced individuals and we have even used small planes to establish new migration routes for threatened species. But why do birds migrate in the first place?