The miracle and mysteries of migration

Written by Jack Sanders
Thursday, 12 November 2009 11:29

The miracle and mysteries of migration

In all of nature, one of the most remarkable phenomena is bird migration, the fall version of which is nearing its end. The concept of a creature, often weighing only an ounce or two, traveling thousands of miles, frequently at night, is astounding. Some insects, such as the Monarch Butterfly, such mammals as caribou, some bats and mice, and fishes like salmon and eels also practice migration. However, no other class of living things uses migration as extensively as do birds, the only group of creatures with the equipment, size, strength, lifespan, and intelligence to travel long distances.

It’s a formidable undertaking. Migrating birds must know where they are going and how to get there. They start out in the peak of condition, strong, fit and well fed. They must overcome many obstacles including storms, darkness, predators, shortages of food, lack of a place to land, and even skyscrapers and antennas placed in their path by man.

For many species, this can be a trip of thousands of miles, from South America to the northern regions of Canada, for example.

Typically, birds spend the winter in a warm place, their resting ground, and they migrate in spring to what had been a cold place in winter but turns warm in summer, their breeding ground. In their winter grounds, the birds must take care of only themselves, and their needs are modest. However, in order to raise a family, they require much more food. While the tropics and subtropics are rich in insect and vegetable life, there is not enough to support the food requirements of raising so many clutches of hungry chicks. Nor is there room to set up protective territories.

For a relatively small creature to undertake such a long and dangerous journey, the destination must be worth it. Indeed, they find a bounty of food — uncountable billions of insects — with few endemic birds to eat them. For the vegetarians, such as the Snow Goose, there are millions of square miles of grass, herbs, and shrubs. There are also few predators to fear and few competitors for nesting space.

Migration offers many mysteries. How could a system of summer and winter homes, often separated by thousands of miles, have evolved in many hundreds of different species of birds?  How did these birds first find and remember wintering grounds? How did they develop their own particular territories in such disparate, far-flung places? Which came first, the summer or winter territories?

Since breeding is essential to a species’ survival, the place where nesting takes place might be considered the “home port” of the bird. Certainly, this is true of non-migrant or “sedentary” birds. Thus, the earliest migrants probably set out from what would be today’s breeding grounds to find winter homes.

Scientists have developed a number of theories about the origins of migration. They include:

• The southward spread of glaciers millennia ago prompted northern birds to move southward, especially in winter, looking for food and warmth.

• Overpopulation in the hotter regions of the Earth forced tropical birds to seek less crowded territories in the temperate regions to the north or south.

• As the Earth’s climate slowly changed eons ago, seasonal environmental changes, such as periods of drought or extreme cold, gradually drove birds to find temporary feeding and breeding grounds away from their normal ranges. As the birds found these new ranges to be successful, they continued to move back and forth seasonally.

• The movement of the continents northward changed the ranges of birds, and migration is their attempt to return to ancestral ranges.

The origins of migration may lie in a combination of several of the theories — although the last, involving continental drift, has been pooh-poohed by most scientists.

Next week, we will look at how migrating birds find their way and why many prefer to migrate at night.

Coming Up

Winter Birds and Project Feederwatch, how to participate in Cornell Lab’s “citizen science” program, Saturday, Nov. 14, 1:30 to 3, Audubon Greenwich, 613 Riversville Road, RSVP 203-869-5272 at x221.

In Search of Birds of Prey,  hawks, owls, eagles and more, photographed by Jim Zipp of Hamden over 35 years, Thursday, Nov. 19,  7 p.m., free, Western Connecticut Bird Club, at Kensington Green, 655 Main Street South, Southbury, 203-426-3901.

Copyright (c) 2009 by Jack Sanders. Send sightings or comments to: jackfsanders [at sign] yahoo.com, or to Bird Notes, Box 1019, Ridgefield, CT 06877; or call 203-438-1183, extension BIRD (2473), and leave a message with your report, spelling your first and last names and telling us your town. If you need help identifying a bird, try your local nature center. If you find an injured bird, call wildlife rehabilitator Darlene Wimbrow of Redding, 203-438-0618, Wildlife in Crisis of Weston, 203-544-9913, or Wild Wings of Greenwich, 203-637-9822. The columnist’s website is www. sandersbooks. com.

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