New adaptations
Migrating south requires special adaptations. Most important are an ability to navigate and a metabolism which is adapted to the heavy energy demands of long-distance flight.
No bugs...
Most of the birds that migrate live on insects, which are scarce in winter. Other birds remain throughout the winter, especially in southern Sweden when winters are mild.
...but seeds and mice!
Birds that remain during winter are seed-eaters (e.g. many smaller birds), plant-eaters (e.g. wildfowl), most owl species, and certain birds of prey that live on other birds, hares and small rodents.
Overcome winter
Non-migrating birds are adapted to winter conditions. For example, the willow grouse grows a thicker plumage and bury itself in snow in order to improve the retention of heat.
