Ruby-throated Hummingbirds are well-studied indicators of how climate influences migration.
That the birds are arriving on their breeding grounds earlier today than in previous periods has been demonstrated repeatedly. Now investigators have learned that differences in the hummingbirds’ first-arrival dates vary by latitude.
The researchers analyzed weather conditions in the eastern United States and Central America between 1895 and 2010, and plotted the latitude of more than 36,000 sightings submitted to the North American Bird Phenology Program from 1880 to 1969 and to hummingbirds.net and Journey North from 2001 to 2010.
In addition to showing that winters and springs are warming, a conclusion consistent with a growing body of scientific evidence, and that hummingbird arrival dates were advanced at all latitudes, the analysis revealed that Ruby-throats are arriving 15 days earlier at lower and middle latitudes (between 33°N and 41°N) but only 11.5 days earlier at higher latitudes (42-44°N).
Even more surprising, the Ruby-throats’ rate of migration appears to be slowing. In recent times, the birds have been taking an average of 38.0 days to travel from 33°N to 45°N, more than four days longer than the average trip required during the historical period, 33.8 days. An increase in the availability of hummingbird feeders along migration routes may partially explain the delay, write the researchers.
They published their study in the January 2013 issue of The Auk, the quarterly journal of the American Ornithologists’ Union.
Read the paper:
Jason R. Courter, Ron J. Johnson, William C. Bridges, and Kenneth G. Hubbard (January 2013), Assessing migration of Ruby-throated Hummingbirds (Archilochus colubris) at broad spatial and temporal scales (Evaluación de la Migración de Archilochus colubris a Escalas Amplias de Tiempo y Espacio). The Auk, 130 (1): 107-117 (pdf).
A version of this article appeared in the August 2013 issue of BirdWatching magazine.