One constellation most people can recognize is the Big Dipper. Did you
know that the two stars farthest from the handle of the Big Dipper point
toward the Little Dipper, and that the last star in the handle of the Little
Dipper is the North Star? Did you also know that the North Star is the star
around which all other stars rotate, and that it is a reliable reference
point for due north? This is the kind of information that indigo buntings
learn and rely on to get their bearings for migration. Read on to find out
how this was discovered...
In the summertime, the high-pitched, persistent song of the dazzling male
indigo bunting is a common roadside sound throughout the eastern United
States and southeastern Canada. Because of an increase in this species'
preferred habitat of abandoned fields, forest clearings, and brushy woodland
edges (such as along roads and power line clearings), it is more abundant
now than when the Mayflower docked at Plymouth Rock. Since the 1940's, its
range has also been expanding into the southwestern United States.
As is typical in the bird world, the male indigo bunting is the more dapper
and conspicuous of the sexes. Whereas it is not easy to catch a glimpse
of the secretive, dull-brown female, it's hard to miss the cobalt blue male
that often belts out his strident song from an exposed perch high in the
tree tops. Interestingly, the brilliant blue of male indigo buntings results
not from pigment but rather from the diffraction of light through the structure
of the feathers. This accounts for why in some lighting the males appear
exquisitely turquoise blue, while in others they appear to be almost black.
Older male buntings are first to arrive on their North American breeding
grounds in late April to mid May. They will have already staked out their
territories by the time the females arrive about two weeks later. Within
a day or two, each female will select a mate and settle on a particular
territory where she will most likely remain throughout the season. Together
each pair will raise as many as three broods.
Despite the appearances of monogamy, recent advances in genetic analyses
have revealed that 20-40% of the buntings born in a season are fathered
by males other than the holder of the territory in which the young are born.
Another fly in the pudding of nuclear family life is that about 15% of breeding
males will have as many as four females on their territory, either simultaneously
or over a season. The females do most if not all of the feeding and caring
for the young for the 9 to 12 days they are in the nest, although the male
defends the nest against intruders. Once the young have fledged the males
will tend to them, particularly if the female is busy building a new nest
for the next brood.
If you've ever tried to learn to identify birds by song, you know that there
is a lot of variation between the songs of individual birds within a species.
You may never hear a bird in the wild that sounds exactly like the song
recording you listened to repeatedly on your store-bought CD or tape. The
individual differences in birds songs enable male birds within some species
to distinguish one another. For example, experiments have shown that territorial
males respond differently to hearing tape recordings of a neighbor's song
versus that of a stranger's song. In addition to individual variation there
are also regional dialects which are detectable differences in songs within
a species in different geographical locations.
In the case of indigo buntings, 80% of males learn their particular song
by imitating an older male with an established territory during their first
spring. Alluding to the adaptive value of this pattern of song learning,
males that match an older, territorial neighbor's song their first year
are more successful in attracting mates and fledging young. Most likely
this is because they are able to trick their competitors (other first year
males) into mistaking them for the older, established male. Combined with
the fact that males usually return to the same breeding territory each year,
this song mimicry gives rise to local dialects in which the sequence of
notes in the songs of neighbors is similar, and different from that of birds
in other areas.
Between mid-August and November, with their breeding duties over for the
year, indigo buntings group together in large flocks and abandon their northern
quarters for the tropics. Traveling up to 2,000 miles, most indigo buntings
settle for the winter anywhere from southern Mexico and the West Indies
to northern South America. A smaller number will spend the winter in southernmost
Florida. At this time of year, the indigo buntings' alter-ego takes over.
The males give their syrinxes (equivalent to human vocal chords) a few months
hiatus, and they no longer sport their namesake shock of blue feathers.
Instead they assume an overall brown plumage with few streaks of blue remaining,
thus resembling the females. Whereas their social life on the breeding grounds
revolves around defended territories occupied by a single male and one or
more females, loose flocks composed of hundreds or thousands of birds roost
together by night and forage together by day while in the tropics. The main
food of choice becomes seeds and buds, rather than insects and spiders which
are the staple during the breeding season.
Studies of indigo buntings have shed light on one of the many mysteries
surrounding bird migration-- how birds select the appropriate direction
in which to fly. In the 1960's, Stephen Emlen cleverly demonstrated in experiments
conducted outdoors and in a planetarium that indigo buntings, like sailors
of yore, use the pattern of the stars as a compass.
In his experiments, Emlen took advantage of a behavior pattern shown by
migratory birds in captivity during the periods of time when their normal
spring and fall migrations occur. Termed Zugunruhe, a German word meaning
migratory restlessness, this behavior is characterized by increased activity,
mainly hopping and fluttering of the wings. By placing captive buntings
in cages made out of a funnel of blotting paper with an ink pad as a floor
and a see-through top, Emlen was able to determine that buntings with a
view of the night sky oriented their Zugunruhe activity in the appropriate
direction in which they would be migrating, i.e. northward under a spring
sky and southward under a fall sky. Without a view of the night sky, the
birds activity became randomly oriented. This orientation of activity was
apparent in the pattern of inked footprints left on the blotting paper sides
of the cages.
By systematically covering up parts of the night sky within a planetarium,
Emlen went on to show that the birds gauge direction using the pattern of
stars nearest the North Star, the pivot around which all the stars rotate
and a reliable reference for due north. He also was able to conclude that
young buntings learn this pattern during their first summer.
Experiments done since then on a variety of species have revealed that the
stars are but one of several cues that birds rely on for orientation. Other
sources of information include the position of the setting sun and the pattern
of polarized light it creates, the earth's magnetic field, odors, wind,
and topographic features such as mountains and bodies of water.
One of the last birds to stop singing as the summer months draw to a close,
the indigo bunting is one of our best known songbirds. As the subject of
an impressive amount of scientific research, this stellar migrant has enabled
us to advance our limited understanding of the mind-boggling capabilities
of migratory birds, and allowed us to enhance our view into the curious
world of avian social life.