|Snowy Owl- Wikimedia|
Migration means that a bird leaves the territory it has established, exposing itself to predators and wasting its precious energy. It must have powerful reasons to make this gamble and the most common reasons are food and breeding.
One of Skalicky's examples was the snowy owl which comes down from the Arctic in search of food. They are acclimatized to cold weather and their predominate white color lets them blend in to snowy climes. Like robins and many other birds, they make the trip for food.
"Their movements are triggered by periodic population drops in one of their primary Arctic food sources - the lemming. When lemming numbers crash, snowy owls venture south in search of food. Small mammals such as mice, rats, voles and rabbits along with birds and occasionally fish replace lemmings at their food of choice."Robins and other insectivores migrate when their insect fare disappears in cold snowy weather. Other birds such as cedar waxwings are seeking winter fruits such as cedar berries.
So why does a bird leave its winter habitat when it is comfortable year round? Sibley* says that migrations northward in the spring may occur because of higher breeding success in temperate climes.
"If a northward migration in the spring results in higher reproductive success, it should benefit some birds to risk the journey. Temperate-zone nesters typically have larger clutches (four to six eggs or more) than do tropical breeders (two to three eggs). In addition, the interval between successive nestings for tropical birds is longer than for temperate species. Some tropical birds also suffer exceptionally high rates of nest predation."It is interesting that humans also migrate in search of food (jobs) and a safe place to raise the kids. Birds just fly over the artificial boundaries which we humans have created. And some birds have found like we have, that the Ozarks is a great place to raise a family.
* The Sibley Guide to Bird Life and Behavior Sibley, 2001, is an excellent source for understanding the habits of birds in general.