One of the most common eastern landbirds in winter, the White-throated Sparrow breeds widely across the boreal forest and migrates south en masse in October. Most birds winter entirely south of the breeding range, in the Southeast and Midwest, and return northward in April and May.
Compare the spring and fall migrations of White-throated Sparrow. Spring migration is a quick northward pulse from mid-April to mid-May. The fall migration is longer, from early September through mid-November–almost twice as long as the spring passage. This is a pattern that is repeated for many species of birds. It can be especially pronounced in shorebirds: while the spring passage of Semipalmated Sandpiper takes about 1.5 months, the fall passage spans early July to mid-October, with distinct peaks for adults and juveniles that are at least a month apart.
Spring migration windows are short for several reasons. Since birds are highly territorial in spring, there is strong evolutionary pressure to arrive early on the breeding grounds and stake out the best territories. As flowers bloom and insects hatch, the early spring bounty can bring good food resources quickly, so birds try to time their arrival to reap the rewards. In fall, many food resources tend to persist as long as the climate will allow. Some species, like White-throated Sparrows, have a facultative migration in which some individuals will try to winter well to the north until the temperatures drop and bad weather forces them to move south. If resources persist (e.g., at a feeder) throughout the year, then these birds can usually survive without undergoing the perils of migration. Other species have noticeable staging grounds, where they may spend a month or more fattening up on abundant food at migratory way-stations in especially productive areas. As in the Semipalmated Sandpiper example discussed above, one reason that fall migrations are more protracted is that both adults and immature birds are moving over the course of the season. Often adults are the first to migrate, while immature birds remain near their natal grounds a bit longer. The later passages tend to consist of more immature birds. Watch for these patterns in other species occurrence maps as well.