Fall Migration in the Eastern US
The warming trend over the last thirty years in the Eastern U.S. has coincided with changes in the behavior of migratory birds. Depending on each individual specieslifestyle, birds that breed in the northern U.S. and southern Canada may either delay or advance the date at which they begin their journey south in the fall. Because springs are getting warmer earlier, and falls are getting colder later, species that can have two broods of chicks a year are doing so more frequently now than they have in the past, spending more time in their breeding grounds, and delaying their fall migration. Species that moult before migration are also tending to migrate later. On the other hand, species that are physiologically capable of having one brood, as well as species that moult at their wintering grounds, generally have an incentive to start their migration as soon as possible. Earlier breeding dates have corresponded to several species, especially birds that migrate particularly long distances, to start their fall migrations earlier. Some birds that have begun to migrate later compared to the early 1970?s and spend their winters in the Southeast are the Hermit Thrush (an average of 3.5 days later), the Yellow-rumped Warbler (9 days), the Ruby-crowned Kinglet (7.5 days) and the Brown Creeper (14.5 days). The Blackpoll Warbler (4 days) is migrating later as well but spends its winter in the tropics. Some species that are migrating particularly earlier in the year are the Yellow Warbler (an average of 7.5 days earlier) and the Common Yellowthroat (11 days).
(Source: Mills A.M. (2005) “Changes in the timing of spring and autumn migration in North American migrant passerines during a period of global warming.” Ibis 147 (2): 259.)