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Birds
and Migration, Why and How Do They Do It
By
Don Richardson
Recorded observations of migration date back 3000 years. In the
Bible in Job (39:26), "Doth the hawk fly by thy wisdom and stretch
her wings toward the south?" And in Jeremiah (8:7), "The stork in
the heavens knoweth her appointed time; and the turtledove, and
the crane, and the swallow, observe the time of their coming.
Aristotle
noted that cranes traveled from the Steppes of Scythia to the marshes
of the headwaters of the Nile.
Some
early recordings have turned out to be not so accurate. Aristotle
offered the hypothesis that birds hibernated. Ornithologist Dr.
Elliott Coues, in 1887, listed over 180 papers dealing with the
hibernation of swallows, a theory which lasted for over 2000 years.
One account stated that flocks of swallows congregated in marshes
until their accumulated weight bent the reeds into the water submerging
the birds where they fell into deep hibernation. It was even recorded
that fishermen in northern waters found a mixed catch of fish and
hibernating birds when they pulled in their nets.
 We tend to think of Neotropical Migrants as birds that
live in North America and winter in Central and South America. Try
thinking about them and their home in a new way. Ask, for example,
how the Hooded Warbler really lives and try this for an answer.
It lives in Central America spending the time from late summer to
early spring (as much as eight months) nomadically moving about
with flocks of its own and other similar species. As spring approaches,
it moves north to take a short hiatus from home to breed and raise
young. At least a month is spent in transit - to and from. This
leaves only about three months of living in the southeastern U.S.
It sounds a bit like this Hooded Warbler lives in Central America,
doesn't it?
Scientist
use words that describe plumage in a way that indicates that the
breeding part of life is not the major part of life. Winter plumage
is called "basic plumage" while breeding plumage is referred
to as "alternate plumage". "Basic" certainly describes
a more primary state than "alternate". Although breeding
may not occupy a majority of a bird's time it is certainly one of
the most important times of its life. Certainly, no species could
survive without it and there are several reasons why migration contributes
to a successful breeding event.
Southern latitudes provide considerably better winter weather than
most of us have here in North America. And during our summer, Central
America is extremely hot while the southern temperate and Antarctic
area is cold with ice and snow covering much of it. Insectivores
(insect eaters) and frugivors (fruit-eaters) can find food, by moving
south, that they could never find in a harsh winter environment.
Certainly not all, but many of the birds that remain north in the
winter eat seeds which do remain available in winter.
Northern
summers have very long days that provide many hours for gathering
food. Tropical days are only 12 hours long. Days in the north may
reach 16 hours or more. It takes a great effort and a lot of time
to gather enough food to feed three or four youngsters that will
increase to 50 times their hatching weight in just 13 days. Northward
migration expands the available nesting and food gathering area
of the world . Many migrating species occupy totally different areas
while some merely expand their range to the north in summer, with
some individuals finding nesting space to the north and others remaining
stationary. Eggs and nestlings cannot fly. Parents must be sure
there is sufficient territory around the nest to support their voracious
family.
Predation
is diminished by the fact that a predator species depending on a
particular bird species for food cannot evolve since the bird species
is not available as a food source full time.
"I got rhythm; I got rhythm; I got rhythm; Who could ask for anything
more?" That's how the song goes and that's how the birds go. Phonology
is the study of biological rhythms. Periodic activity such as flowering,
reproduction, and migration all fall within this class of study.
Words like circadian (daily), menstrual (monthly), circannual (annually)
are used to describe these phenomena. Much of migration activity
in birds is controlled by an internal clock operating on a circannual
rhythm. Each year, at a certain time, their biological clocks signal
that it's time to fly northward to breed.
Photoperiod
(periods of light) actually refers to the ratio of day length to
night length. Photoperiodicity may affect migration in some cases,
when the birds are in the temperate zones where these ratios change.
In the tropics, however, photoperiods don't have much effect. Days
and nights are always 12 hours long on the equator. The triggers
for migration are actually a complex combination of things.
We are all certainly aware that in North America the weather in
spring gets "nice" in the south before it gets "nice" in the north.
People who live in the northern U. S. and Canada migrate in droves
to the southern borders of the U. S. for the winter. I have heard
many conversations, among them, about the right time to go back
north. It always seems to hinge around the question "Is it too early"?
Migrating birds have obviously also evolved a sense for this. We
find that birds that nest in the south migrate early. Of course,
the area in which they nest is ready for habitation early. We begin
to see the Prothonotary Warbler and Louisiana Waterthrush in the
latter half of March. Birds that nest in the north, however, migrate
later since the area in which they nest is not ready for habitation
until later. Gray-cheeked Thrushes don't show up in Texas until
late April and early May. I plant most of my vegetables (Houston,
Texas) around the first of March. My dad, I remember, always put
the vegetables in on Memorial day (western New York). That's three
months later and it looks like birds have figured this out too.
The
preparation for migration includes building fat reserves for energy.
Southbound Yellow-rumped Warblers (which usually eat insects) are
observed on Block Island (Rhode Island) gorging themselves on Bayberries
(Wax Myrtle). Long distance runners and other endurance athletes
often do a "carbohydrate load" just before a big event.
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