Some species wander more regularly than others. Many winter finches, such as crossbills and Pine Grosbeaks, may arrive en masse in response to scarce food in their northern homes. Snowy Owls may wander south in search of food, especially after successful breeding years when the population of owls has increased. Irregular migrations such as these are termed irruptions.
Migrating birds may be blown off course by the strong winds of hurricanes and other violent storms, or grounded by fog, heavy rain, or other adverse weather conditions.
Some birds, often juveniles, disperse northward after the breeding season in what is referred to as post-breeding or vagrant wandering. This is especially common with some herons and ibises.
Occasionally birds appear in new areas by migrating in a direction opposite to that expected, referred to as reverse migration. One theory to explain this is that their internal navigational system is malfunctioning. This may explain sporadic appearances of Fork-tailed Flycatchers in North America.
Individuals of some species, particularly western hummingbirds, wander east/southeast during autumn. No one has teased out exactly why this seems to be happening more often in recent decades than in the past.
Remember that range is a dynamic concept, and species’ ranges change over time, albeit usually quite slowly. Tufted Titmice and Northern Cardinals, for instance, live much further north than they did 100 years ago.
To find out where species are during the year, check out our bird maps in eBird. Type in the name of a species and a map comes up of where eBirders have recorded the species. You can also put in a date range to see where a species has been seen during a particular time of year. For example, between December and February, Pine Grosbeaks have been seen across the middle latitudes of North America. Compare this to the map of sightings of Pine Grosbeaks in June through July.
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