[ THE OUTSIDE STORY ]
How do Birds Know When to Migrate?
On the north end of my home is a nest site favored
by eastern phoebes. Every year a pair shows up,
sets up house, and raises a family. They arrive
early in the spring, and I spend the long days
of spring and summer watching them. At some
point, the nest empties out, and then I know that
summer will soon end and the phoebes will be
on their way.
But exactly when they will be on their way is
hard to predict. Fall’s migration tends to be a more
open-ended process compared to spring’s, when
the urgency to reproduce drives birds to arrive in the
Northeast during a relatively short window of time.
There is an almost explosive quality to the arrival of
songbirds in March and April. One day we wake to
the usual quiet of winter, and then the next there is
a riot of trilling, chirping, calling, and singing.
As summer winds down, however, the volume
diminishes slowly. In August, I still wake to bird
songs, but there are fewer voices; the chorus isn’t
as frenetic and rich.
While the urge to reproduce is the primary
driver of spring migration, there are many factors that dictate when songbirds migrate south.
One of them is diet. Many species of flycatchers
and warblers are insectivores – they generally
leave first, because the bug population dwindles
as summer ends. Sparrows and other omnivores
that eat fruits and seeds as well as bugs have
more dietary choices, so they have the option to
stick around well into the fall. Phoebes belong to
a third category of birds that switch from being
insectivores during the breeding season to being
omnivores later in the year; other species that
transition include the hermit thrush, the yellow-
rumped warbler, and the red-eyed vireo.
Eating only insects during the breeding season
makes sense, because they are plentiful and
the rich protein is just what growing baby birds
need. All spring and summer I watch the phoebes
swoop, dart, and dip as they snatch bugs out of
the air and bring them to the nest. Fruits and
seeds, on the other hand, tend to be scarce when
nestlings first hatch but abundant as fall begins
and the birds prepare to migrate.
But what’s to be gained by sticking around and
changing your diet? Why not head south immediately, given that the season’s main objective
– reproduction – has been accomplished?
Because waiting has its advantages, explained
Frank La Sorte, a researcher at the Cornell Lab of
Ornithology. “There is evidence that changes in
diet for these species are associated with changes
in how fat is stored and transported to the muscles
Sorte said. Flying hundreds or thousands of miles
is an enormous task for an animal that weighs
about six-tenths of an ounce; sticking around into
the fall and feasting on seeds and fruit allows
them to store more fat and energy, which makes it
more likely that they will survive the journey.
Having a broader migration window has other
benefits, too. “Migration conditions could poten-
tially be more favorable early in the fall, but if a
migrant must depart early, the range of options is
likely to be more limited,” La Sorte said. In other
words, if phoebes had to leave as soon as the
bug population dwindled, they might not have the
option of waiting for the dry, windless conditions
that many migrating birds prefer. But because of
their adaptable digestive systems, they can wait.
As we slip into October, the mornings have grown
cool and quiet. The days no longer feel endless,
and the air no longer hums with insects. But the
phoebes remain. I don’t know when they will leave,
only that one day the bugs and seeds and fruit will
grow scarce and they will be on their way.
of lead. Consistent with other studies, lead levels varied seasonally. According to
Vince Slabe, graduate research assistant at West Virginia University, 28 percent of
the 81 eagles captured during those years between Nov. 1 and March 31 were
found to have blood lead levels of 20 µg/dL or higher. Of the 24 eagles captured
during the period ranging from April 1 to October 31, all were below 8 µg/dL.
Poisoned wildlife is just one reason for hunters to switch to non-lead bullets.
Lead also poses a human health risk, especially to children. Typically, higher
amounts of lead are found in commercially processed meat. Hunters processing their own may take more care. Even so, some states recommend that small
children and pregnant women avoid eating wild game killed with lead.
Increasingly, bullet manufacturers are offering alternatives to lead ammunition,
with highly accurate, non-lead bullets now available (and prices are going down)
for any caliber of rifle. Besides keeping lead out of the environment, non-lead bullet
performance is generally superior and many hunters switch just for better performance. If you hunt, consider the benefits of choosing non-lead bullets this season.
A veterinarian administers a subcutaneous infusion of a chelating agent to remove
lead from the blood of a young bald eagle at The Raptor Center.
The Outside Story is sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation: firstname.lastname@example.org