Bird
Migration
What
would make you eat for days on end, scrambling and pecking for every possible
food calorie to store as energy and put on an immense percentage of weight?
The answer might be that somehow you are programmed to know that your
next meal was going to be several thousand miles away, and also that you
would have to use your own energy to get to that point without stopping
to take a break. And to complicate things, you only weigh a couple ounces.
Well, many of our North American birds do just that, before embarking
on an annual, or sometimes fatal trip, to locations in the South of North
America, Central America, South America and places unknown at this time
of year. Birds the size of Hummingbirds to Herons, and weights of warblers
to swans fly elsewhere while we adapt to winter.
Each individual bird does not make the decision to stay or go every year;
migration is an innate behavior that is genetically programmed and passed
down from parent to young. The juveniles have a pre-programmed mapping
system and will fly to their wintering grounds just as computer can lead
you through a pre-programmed series of tasks. In only a few species do
the young birds even maintain contact with their parents during their
perilous journey. Some well known exceptions of this are the very publicized
flights of Whooping Cranes which actually have to learn the travel route
from their parents. With dwindling populations people have intervened
to help this species by raising and retraining some juveniles to travel
first to their wintering locations and then back to the species ancestral
breeding site.
How do the birds know
when to fly south?
The biggest control factor that has been determined through many years
of studies is that birds have an internal clock that is controlled by
light. As the amount of daylight decreases, a bird starts to go through
physiological and behavioral changes. Songbirds and waterfowl that migrate
during the night to catch a tail wind turn the clocks and become active
at dusk changing temporarily to a nocturnal life style. This restless
behavior is termed Zugunruhe and will occur even in a caged migratory
bird.
Not only does a bird’s
feeding and resting behavior change but their need to defend a territory
and their mate fades. Male birds that had been battling each other, swooping
and diving, and performing elaborate displays just weeks before to win
a mate join up with “the enemy” and form flocks in order to
travel as a part of group. There can be safety in numbers when you have
more than one set of eyes looking around for a predator.
Why not stay the winter?
In some cases it is very obvious; breeding grounds are covered in snow
or there is no available food or water.
There can be more 'not so obvious' reasons in which some birds that may
specialize in certain foods can no longer find the resource, like hummingbirds
which eat nectar or insect eating birds.
Once our birds fly south for the winter – why do they bother to
make the trip back again. Again, it is not a choice they make but part
of the pre-programming. Because we have seasonal cycles and the food rich
warm season is compacted with an abudance of seeds, insects and small
prey items into only a few months, birds can take advantage of the ultimate
in rich resources and try to raise as many offspring as they can racing
to finish in a small amount of time.
Photos Top- Tufted Puffins-Alaska, Middle - Short-billed
Dowitchers- Maine, Bottom- Tree Swallow - Massachusetts, © S.French
|
