Why do birds fly to specific locations in the spring and fall?
In some cases, the original habitats were in present-day southern wintering areas. The birds developed a tendency to leave in spring to breed in territories to the north that were less crowded. Fall brought seasonal changes in weather and declining food supply in these newly settled regions. This forced the birds to migrate back to their former range for the winter. North American birds that originated in the tropics include hummingbirds, tyrant flycatchers, tanagers, wood warblers, orioles and swifts. In recent geological times these birds gradually spread northward as glacial ice receded and the continent became warmer. Other birds, such as plovers, ducks and geese, originally lived in what are now their northern breeding areas. Gradual climatic changes forced them to spend winters far to the south. Migrations appear to be the consequences of invasions or emigrations during which animals settle in new regions during part of the annual cycle, then return to the original region to comp
