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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