For the Birds Radio Program: Migration Hows and Whys
Why do birds fly north in spring? HINT: it’s probably not just because it’s too far to walk.
(Recording of a Sandhill Crane)
Last week I overheard some robins complaining about the weather and wondering who the heck’s bright idea was it anyway to head north early this year. The earliest migrants each year, like robins, Canada Geese, Killdeers, red-wings, and grackles, use weather as their cue to travel north, in spite of the fact that sometimes one day’s weather is not a good indication of what will come next. Later migrants use day-length as their primary cue to migrate, and so even when we have a warm April it doesn’t speed up the arrival of hummingbirds, orioles, and warblers.
Why do birds migrate? Although the lack of food in winter for many birds is certainly an important factor, it is clearly not the only one. Most birds leave in autumn long before their food supply is exhausted or frozen, and some return in spring before their food supply is adequate for their needs. Cold weather is another important factor, but then many birds leave the northland in July and August when our temperatures are reasonably comfortable. Another theory about the reasons for migration is that most of the birds that nest in the north are really tropical birds that leave their ancestral home for the breeding season alone, to escape the heavy competition for nest sites and food. By heading north from the equatorial 12-hour days, they also can capitalize on the extremely long days of the northern summer, which means they can raise their babies much quicker, and thus shorten the vulnerable nesting period.
Migration is fraught with peril. Birds must pass through unfamiliar areas, where unexpected dangers can crop up on any side. Most nocturnal migrants navigate by the stars, and city lights may disorient them so that they become helplessly lost. Radio and TV towers lure hundreds of thousands of migrants to their deaths. Spring and fall weather is unpredictable, and often serious storm systems kill large numbers of migrants. One 1904 snow storm in March killed literally millions of Lapland Longspurs–750,000 dead ones were picked up on 2 Minnesota lakes alone. During the unusually cold month of May, 1907, a tremendous destruction of warblers, called the Great Death by ornithologists, occurred in Wisconsin, Iowa, and Minnesota. And some raptors seem to time their migration to coincide with the movements of smaller birds so the hawks and owls can eat as they go. All in all, migration is a dangerous proposition.
Yet because most North American birds do migrate, it is clear that there are important advantages that exceed the disadvantages of this semi-annual rite.
If you want to help at least a few migrants during this vulnerable period, keep your feeders going through May. In a year like this, I suspect that many redpolls would die without artificial feeding at the end of a long, cold winter. Many birds must stop in the Northland in order to replenish their energy before they can complete their journeys north. And the typical lousy weather of early spring can kill any migrants that don’t have enough fuel in the form of food. Bird feeders are actually probably more important in spring and late fall than they are in the heart of winter, and besides, all the new migrants every day are just plain fun to watch.
(Recording of a White-crowned Sparrow)
This is Laura Erickson, and this program has been “For the Birds.”