The pine warbler is among the species with the most extreme changes in arrival date, with other short-distance migrants like the tree swallow, and species undergoing significant population growth, like the turkey vulture and red-eyed vireo. Joe Songer/al.com/TNS

The glorious spring migration is coming to an end. The arrival of black-billed cuckoos, Nelson’s sparrows, saltmarsh sparrows and blackpoll warblers will be the last trickle of the river of migrants.

We know that average temperatures have been steadily rising since the start of the Industrial Revolution in 1850. The warming leads to springs with earlier appearances of leaves, caterpillars, frogs calling, and hundreds of other biological phenomena. One would expect that migratory birds accelerate their arrivals onto their breeding grounds to keep pace with the changing ecology.

Over the past 20 years, several studies have appeared that used archival bird records over decades to see if arrival dates are getting earlier. Analysis of records from New York and Massachusetts showed significantly earlier arrivals for most migratory breeding birds. Similar data for Maine show relatively few examples of accelerated arrival dates.

A recent study headed by Lori Petranski of West Virginia University presents data on arrival dates in the Central Appalachians for 115 species using several sources of crowd-sourced data. The study is particularly notable because of its scope, with the authors presenting data for 127 years, although they do not have continuous data over that time. When using archival data, one must be satisfied with whatever data are available.

Forty-five of the 115 species showed significantly earlier arrival dates over the 127-year period. The advances were striking: 2.6 days per decade for those 45 species. Species with the most extreme changes in arrival date were short-distance migrants like the pine warbler and the tree swallow, and species undergoing significant population growth, like the turkey vulture and red-eyed vireo.

An English study led by Jennifer Gill of the University of East Anglia focused on arrival dates of a shorebird, the black-tailed godwit. At the population level, the godwits are arriving earlier. However, older, color-banded birds continue to arrive at the same time each year. Young birds are the ones arriving earlier. It will be interesting to see if songbirds show this same pattern.

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Another consequence of global warming is that the optimal temperature for different species may force the population to shift northward. It may behoove migratory birds to extend their migration northward, particularly at the southern end of the range of a species to avoid temperatures that are too warm.

Paulo Martin and colleagues from the Catholic University in Chile analyzed changes in the optimal latitude for 209 species of North American birds over the last 55 years. Seventy species showed a shift in their optimal latitude. Overall, the peak density of birds has shifted northward about 50 miles over the course of the study.

Despite an abundance of research, we still do not understand how many young birds know their migratory routes and destination. Research by Patrik Byholm and colleagues in Finland focused on the learning of migratory pathways by Caspian terns. They placed GPS-trackers on 31 related birds to track the migration between breeding grounds in northern Europe and wintering grounds in Africa.

Adult males have the main responsibility for migrating with young. Young birds always stayed close to an adult male, with the bond dissipating on the wintering grounds. Adults migrating alone were faster than those accompanying young. Four young birds that were separated from the adults all died.

In the spring, all the young birds followed the route in reverse in solo flights to attempt breeding in the area where they were born.

Shorebirds abandon their chicks soon after hatching and begin their migration weeks ahead of their self-sufficient chicks. The young have no guide for their first migration, so the direction and distance must be genetically coded. Songbirds do not migrate as families, either, so genetics must be involved in getting young birds to their wintering areas. This phenomenon is a rich subject for future research.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu


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