Climate Fact: Autumn Migration Timing and Temperature
In Brief: Unlike the relationship with spring migration, the relationship between climate and autumn migration is more complex. Observations from eastern North America show trends in delayed autumn migration along with rising autumn temperatures.
Different species of birds likely use a variety of different environmental cues to know when to start their spring and autumn migrations. Environmental shifts related to climate change, such as a rise in winter temperatures, can affect migration cues and the timing of annual bird arrivals and departures. A trend towards earlier springtime arrivals has been documented consistently in Europe and somewhat consistently in North America, where the rate of climate change over the past 40-50 years has been less pronounced than on the other side of the Atlantic. Trends in autumn migration are not as obvious as trends in spring migration in either hemisphere. Whereas arriving earlier at breeding grounds in the spring generally confers a much greater chance at reproductive success than arriving later, the relative advantages of beginning autumn migration earlier or later are far more complex. Migration distance, the number of broods per year, the energy required for the pre-migratory molt and the selective pressures at the wintering grounds are all factors that can affect whether a species of bird from a specific region will be motivated to begin its autumn migration earlier or later. Some evidence for later autumn migration has been documented along the East Coast. At one stopover site in Rhode Island, where autumn temperatures have increased by about 2.5 degrees Fahrenheit since 1960, 38 percent of long-distance migrants (birds wintering south of North America) and 50 percent of short-distance migrants (birds wintering in North America) observed are now arriving later in the year during their journey south, with an average 14 day delay over the study period. The long-distance migrant American Redstart and short-distance migrant Hermit Thrush are two species that appear to most closely adjust their autumn migration times based on the autumn temperature. Birds that showed the strongest trends in delayed arrival over the study period include t long-distance migrants Black-and-white Warbler and Red-eyed Vireo and short-distance migrants Eastern Towhee and Ruby-crowned Kinglet.
Source: Smith, SB and Paton, PWC. “Long-Term Shifts in Autumn Migration by Songbirds at a Coastal Eastern North American Stopover Site.” The Wilson Journal of Ornithology 123 (2011): 557-566.