One obvious reason for variation in migratory behaviour is that many migrants, such as Swallows, Swifts, Warblers and Flycatchers, eat insects, which are scarce at certain times of year, while less migratory species such as Sparrows or Crows often eat seeds or other foods that are available all year round. However this provides only a partial explanation. Many birds that are typically resident, such as titmice/chickadees and woodpeckers also eat insects, while there are also seed eating migrants such as the Ortolan in Europe/Africa and the Dickcissel in North/South America. 

 

The answer may lie instead with where birds get their food from, rather than what kind of food it happens to be. Migratory birds tend to rely on food resources that are out in the open and exposed both to the elements and to foraging animals. Typical examples are swarming insects and fruiting or flowering shrubs. Non-migratory birds tend to rely on food that is either underneath, behind or inside something, or is fine grained or similar in colour to the background and therefore cryptic and difficult to spot. Examples include soil invertebrates, wood-boring insects and scattered seeds. Such differences provide a clue as to how migration might have become prevalent in certain groups of birds and not in others.