The
question of how birds find their way between breeding and
wintering grounds has puzzled people for as long as they
have been aware of the phenomenon of migration. Today we
know many more parts of the puzzle's solution than we did
even twenty-five years ago. Some would argue that there are
really two puzzles: (1) how birds navigate over thousands of
miles to find their way between breeding and wintering
sites, and (2) how birds find their way back to precise
nesting or roosting sites (homing behavior). To do either,
birds must be able to orient (that is, determine compass
direction) and to navigate (judge their position while
traveling). The short explanation of
these complex phenomena is that birds find their way by
using a variety of cues in a hierarchical fashion. Different
species may use these cues in different orders of priority,
and some cues may always be used in preference to others.
Birds acquire directional information from five primary
sources: (1) topographic features, including wind direction
which can be influenced by major land forms, (2) stars, (3)
sun, (4) Earth's magnetic field, and (5) odors. Some of the most convincing
experiments demonstrating the navigational abilities of
birds were performed by behavioral ecologist Stephen Emlen.
He took advantage of the "migratory restlessness" of caged
migrant birds -- fluttering and hopping that tend to be
oriented in the direction of migration. Using Indigo
Buntings in a planetarium, Emlen found that the birds
oriented in the proper migratory direction using the stellar
cues projected onto the planetarium ceiling. When Emlen
shifted the position of the planetarium's stars, the birds
shifted their orientation as well. The buntings were shown
to learn a "sky map" as they watch the rotation of the stars
while they grow up. The young birds learn to recognize the
area of least apparent movement around the pole; if maturing
buntings were exposed to a false sky rotating around the
star Betelgeuse (in the constellation Orion), they acted as
if Betelgeuse were the North Star. But how do birds find their
way on overcast nights? Apparently, they are able to set
course by the setting sun, unless this too is obscured by
cloud cover. Lacking either stars or sun for information,
birds will orient by wind direction, although not always
correctly. Ornithologist Kenneth Able used radar and
portable ceilometers (electronic devices for measuring the
altitude of overcasts) to track nocturnal migrants and
reported that birds frequently flew in the wrong direction
by using wind as a cue when stars were unavailable.
Interestingly, while most ornithologists believe that birds
employ topographic features like mountains, rivers, tall
buildings, etc., to navigate in the vicinity of the home
site, there is little evidence of the use of such
cues. Recent experiments have
revealed that pigeons are capable of detecting the Earth's
magnetic field and can use it to orient and possibly to
navigate. We still do not understand just how pigeons manage
to sense such weak electromagnetic fields, but birds are far
more sensitive to them than are human beings. We have yet to
learn how widespread this ability may be among other species
of birds. Homing pigeons, in addition
to sensing magnetic fields, recently have been shown to use
smell for at least short-distance orientation in returning
to their loft. Earlier studies indicated that Leach's
Storm-Petrels (and probably other "tubenoses") might
possibly use their sense of smell to locate their nest
burrows, although they rely on other cues to find the
colony. Shearwaters, like pigeons, are capable of
spectacular homing feats. Many Shearwaters were taken from
their nest burrows off the coast of England, transported
across the Atlantic Ocean, and released near Boston. Twelve
and a half days later some of these individuals were back in
their nest burrows. How they accomplished the feat is
unknown, but they certainly didn't sniff their way
home. One clear message emerges
from studies of avian orientation and navigation: birds do
not rely on a single source of information to guide them on
their travels. Instead, they possess the ability, shaped
over evolutionary time, to use redundant cues from a variety
of sources. Such a system enables birds to find their way
under most conditions that they routinely
encounter. SEE: Migration;
The
Avian Sense of Smell;
How
Do We Find Out About Bird Biology? Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.