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RECENTLY Updated May 1, 2006

Bird Migration

Tufted Puffins - Alaska


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.

Short-billed Dowitchers - Maine

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.

Tree Swallow-Adult in breeding plumage

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

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