Timing a warbler's dash
Researchers see Kirtland’s Warblers up north after banding them in the Bahamas
Published: June 22, 2012
Researchers working with the endangered Kirtland’s Warbler accomplished
something in 2012 that has long remained a tantalizing dream of
scientists and birdwatchers alike: They estimated the duration of a
bird’s migration from its wintering areas to breeding grounds based on
sightings of color-banded individuals in both areas.
Conservancy scientists David N. Ewert and Kimberly R. Hall and other
investigators described the breakthrough in the March 2012 issue of the
Wilson Journal of Ornithology.
Between 2002 and 2010,
they captured 232 Kirtland’s Warblers on southern Eleuthera, in the
Bahamas, and gave each one a unique set of colored leg bands. Then,
3-15 times a season from mid-April to early May, they visited sites
where overwintering warblers congregated to document a date as close to
departure as possible.
Focusing on a subset of banded males
whose breeding areas had been mapped in previous years, the researchers
subsequently monitored territories in Michigan every 1-3 days from
early May through May 30 or until the warblers were found. From 2005 to
2010, they were able to relocate at least 7 and as many as 23 uniquely
banded warblers each spring.
Based on dates of last
observation on Eleuthera and first observation in Michigan for five
birds, Ewert estimated the average duration of migration to be no more
than 15.8 days and the average distance flown to be 89.8 miles (144.5
km) per day.
The five birds traveled between 1,362 and 1,381
miles (2,192-2,222 km) in all, completing their journeys between 13 and
The results are valuable because comprehensive
conservation planning requires knowledge of not only where a species
makes its nest and spends the winter, but also what happens as it
migrates between the two locations. The migratory period is typically
the most poorly understood aspect of a bird’s life history.
devices known as light-level geolocators have begun to shed light on
the migrations of Purple Martin, Wood Thrush, Black Swift, and other
species, but the geolocators in use today are still too big for
passerines as small as a Kirtland’s Warbler, which tips the scales at
0.48 ounce (13.8 g). A Purple Martin or Wood Thrush is three times as