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Instinct ?
by: Mike Lushington
How do the birds know when it is time to migrate?
I mentioned in last week's column that the robins appeared this year
about two weeks earlier than they used to do. Other people have remarked
that they seem to be seeing certain species of birds earlier now than they
used to when they were younger. Is there some basis for this,or simply a
question of the memory playing tricks? Part of the answer lies in knowing
something about the mysteries and the magic of migration.
There is no "one-pattern-fits-all-birds" kind of migration. Rather,
different species migrate in different ways, according to their particular
needs. Robins, crows, grackles, starlings,and other birds migrate only far
as they have to ensure their winter sources of food. That is why, some
winters, people are surprised to find them around even in January or
February. If there is an adequate supply of Mountain Ash berries, for
example, a few robins will stay around, while many others will retreat
only as far as they have to to find their own patches. Such birds are in
good position to start moving north as soon as conditions permit. For
these birds, spring comes when the ground starts to emerge from under the
snow. In a winter such as the one we have just had, with little snow and
an early melt, it really wasn't too much of a surprise to see them arrive
earlier than usual.
Other birds depend largely or entirely upon insects for their food.
These birds, the swallows, warblers, flycatchers, and vireos migrate much
greater distances in the fall because they have to be sure of getting far
enough south so that insect populations will not be killed off by the
frosts that can occur even in northern Florida. That is why very few of
these species will remain anywhere in North America during the winter, and
those that do on occasion, such as the Tree swallow or the Yellow-rumped
warbler, can do so because they can make use of small fruits or seed for
short periods of time. That is why these species are among the earliest of
the swallows and warblers to make it back this way as well.
Much the same applies to hummingbirds, except that, in this instance,
the preferred food is nectar that can only be found in flowers (or handy
nectar feeders). Just as the small insectivores have to delay their spring
migration to be sure of finding their food when they arrive, hummingbirds
have to be able to count on finding blossoms producing nectar when they
land here. Unfortunately, in a late spring, or one that has a sudden late
winter storm, many individuals of these species will be trapped, usually
with fatal consequences.
Ducks and geese generally don't go too far in the winter either. They
need only to be able to find open fields, or mud flats in order to keep
finding the food they need. The small shorebirds, on the other hand, are,
for the most part, true long distance travellers. Many of them live on
small shrimp or other tiny arthropods that flourish in summer but that die
back in winter in northern waters. Like the insect eaters, these birds
have to travel south of the climate line that will affect their food
supply.
We think of weather in terms of warmth or cold, but most birds respond
to it only in terms of the effect that temperature will have on the stuff
they most need to eat. Thus, while I may lament over a raw, cold, cranky
April day, those birds that have arrived are content if they can find open
fields, thawing patches in the woods, or ice-free mudflats. Given those
conditions, they could care less about my yearning for a touch of heat and
sunshine.
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