its migration cycle (plate 15- 162). The bird is given
a geolocator on its breeding grounds and that device
(which weighs very little and does not impede the bird)
is removed when the bird returns to its nesting area
in the following spring (many bird species are highly
site faithful to their breeding grounds). Analysis of the
geolocator data will show the researcher exactly where
and for how long the bird wintered and will track its
migratory routes south and north. This technique
represents a major breakthrough in establishing the
connectivity between breeding and wintering areas
and the routes of travel to and from.
The majority of passerine migrant bird species are
nocturnal migrants. Their call notes may be heard on
clear nights both in spring and fall as they make their
journeys. Most migrant bird species move south to
the Gulf states in late summer and autumn and then
cross the Gulf of Mexico, a journey of about 965 km
(600 mi) over water, which is subsequently repeated as
they come north in the spring. Some species, however,
migrate mostly overland, moving into northern Central
America and continuing south. This route is typical of
migrants that breed in the central and western United
States and also of groups such as swallows, which are
diurnal migrants and feed while on the wing.
One unique migratory route is that of the Blackpoll
Warbler (Setophaga striata; plate 15- 163). In the fall,
on its boreal North American breeding grounds, it
responds to cold fronts that bring strong northwesterly
winds and migrates southeast across the Atlantic
Ocean to eventually encounter the equatorial trade
winds that help move it southwest toward its wintering
grounds in Amazonia. This flight, which usually is
accomplished nonstop, requires about four days on the
wing to complete. In spring Blackpolls use an entirely
different route, migrating across the Gulf of Mexico
and overland to their breeding grounds in boreal
forests. If it tried to repeat the route of its fall migration
it would fly into head winds for its entire journey.
Many North American migrant species may be
subject to ongoing human- caused reduction in
wintering sites, an increasing concern. For example, the
Veery (Catharus fuscescens; plate 15- 164) has, based on
specimen records, been thought to winter in northern
Amazonia, where much forest remains. However,
most of those specimens were taken during the active
migratory period. It appears that Veeries actually winter
farther south, in south- central and southeastern Brazil,
areas that have been subject to intensive habitat loss.
Thus the species could be in decline.
Some migrant species are now rare in North
America, possibly due to a combination of nesting site
specialization and loss of their Neotropical wintering
areas. The Kirtland’s Warbler (Setophaga kirtlandii;
plate 15- 165) nests only in successional Jack Pine forests
in Michigan. The bird was once probably more widely
spread and could possibly occupy a larger nesting
area today, but loss of winter habitat in the Bahamas
may have restricted its population. The Bachman’s
Warbler (Vermivora bachmanii) is now considered
to be extinct but was once an inhabitant of dense
canebrake areas in southern hardwood swamps. Loss
of cane habitat in Cuba, where the bird wintered, as
well as habitat reduction in North America is thought
to have been responsible for its population demise.
On the other hand, increase in second- growth habitat
could actually favor such species as Chestnut- sided
Warbler (Setophaga pensylvanica; plate 15- 166) and
Indigo Bunting (Passerina cyanea). As with virtually
all tropical ecology puzzles, questions about the effects
of tropical deforestation on migrants are complicated
and elude simple answers.318 chapter 15