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The Feast Of Optical IllusionAre swallows as punctual as we like to think? From coast to coast across America, swallows on migration are celebrated as models of punctuality by legend and local lore. In California, as all birders know, the Cliff Swallows of San Juan Capistrano are said to return to their nests at the old mission each year on the very same day, March 19, the Feast of St. Joseph. Crowds gather to watch and cheer their arrival. In my little town in New Jersey, it's Barn Swallows that are celebrated and the crowds are not quite as large, but the legend of the birds' timely return to the Port Republic Bridge has spread as far as I have been able to push it. "They come back on April 15 every year, just like clockwork," I've told anyone who would listen."They're traveling north for weeks all the way from Brazil and Argentina, thousands of miles, and they arrive back here at their colony under the bridge on the very same day each year—the Feast of Uncle Sam. Isn't that mind-boggling?" Some doubting Thomases have tried to shake my faith, of course."Are you sure it's always the exact same day?" they ask. "Listen," I have explained coolly."Every year when I spot the first one, I write the date down on the perpetual calendar in our kitchen. I've been recording the return for 17 years in a row. Two or three times they've come back on the 14th and a couple of times on the 16th, but almost always it's the 15th—right on the dot. Why don't you join me this year? I'll call you up at dawn, and we can walk down to the bridge together, so you can see the show for yourself." For some reason, this comment generally sends my listeners off on a search for another hot dog or a second beer. I can't seem to build up that crowd of observers waiting the re-arrival in California style. Nevertheless, my swallow data seemed rock solid—until the other day when I actually stopped to look at it. My wife and I have been recording notes about local birds, plants, and butterflies on the calendar since we moved here in 1989. April has become the most cluttered month. Notes such as "swallows at bridge," "first Barn Swallow," or just "BASW" appear 12 times on the April page, as follows:
Even a cursory glance indicates problems with my story. Only twice have the swallows returned on April 15th, only once on the 14th, and never (that I noted) on the 16th. In 2002 they seem to have come back more than a week ahead of schedule, and in 1997 they were 10 days "late." Look closer and the story grows messier still. Nearly a third of the returns, five of seventeen—1994, '96, '99, '00, and '04—I missed entirely. In 1993 I recorded their return as April 12th, then 48 hours later, scribbled it down again as the 14th. And my other notes are not consistent. Does the abbreviation"BASW" without detail on April 7, 2002, indicate a swallow back at the bridge—or simply the first individual I came upon anywhere in town, which could have been, of course, a bird nesting elsewhere or a migrant headed farther north? Also, I now wonder how much my actions have distorted the record. I don't visit the bridge every day, even in mid-April when the birds are due, and I seem to remember several years looking at the calendar, realizing the date was near or past, and hurrying down to the bridge. But if they had returned by then, how could I know they had only just arrived? Should an arrival date marked April 15th actually be interpreted as a return that might have happened on any day over the previous two weeks? Take a look at accounts of the Cliff Swallows at San Juan Capistrano and you'll see fudging seems to go on there also: any early-arriving birds, for example, that are spotted back at the mission before March 19 are dismissed as "scouts." What started me thinking about all this is an inspiring and iconoclastic article you can find on the Internet with a couple of clicks:"The spring migration pattern at Fortine, Montana" by Winton Weydemeyer, originally published in The Condor 75:400-413, 1973. Weydemeyer—rancher, tree farmer, state senator, photographer, naturalist, and lifelong conservationist—tracked the return of migratory birds to his family's farm in the Whitefish Range in extreme northwest Montana, 15 miles south of the Canada border, for 50 years. Yes, 50. Weydemeyer's records detail the arrival of 138 species of birds, Common Loon to Chestnut-collared Longspur, each spring from 1920–1970, with only a half-dozen years missed, when his brother Donald kept the records. See the article for Weydemeyer's discussion of the accuracy of his record keeping and the inevitable difficulties of noting true "first arrivals." In brief, he was a far more careful compiler than most of us, and he continued the data gathering his entire adult life. Is there an account anywhere else of such a thorough record of migration assembled by a single individual over such an extensive length of time? What makes Weydemeyer's analysis especially intriguing, however, is his tactful questioning of several "principles" of migration that are still widely believed today. He quotes Alexander Wetmore's 1930 passage in The Migration of Birds, for example, " . . . birds come and go with surprising regularity on their appointed date. Arrival in spring is particularly punctual . . . and unusual is the season when the first of the travelers fail to put in their appearance within a few days of the average date." It's a sentiment many of us birders have repeated countless times. But, notes Weydemeyer in his understated tone, his data "at least in the locality studied," stand" in marked contrast to this principle." His delightful, hand-drawn graphs reveal a very different picture than the one most of us keep in our heads. In truth, he claims, "the arrival dates of birds cannot be neatly catalogued nor can the order of their appearance be forecast." The average arrival dates of the 99 species for which he compiled the most complete records varies over 36.6 days! Red-eyed Vireo and Wilson's Phalarope proved the most punctual migrants in Fortine, with arrival dates spread over 16 days; at the other extreme, the Pine Siskin's return was spread over 94 days. And those "punctual" swallows? Both Cliff and Barn swallows proved themselves barely average against the field. Their spring arrival dates were spread over 37 and 32 days respectively. The Rough-winged Swallow's range was 46 days and the Tree Swallow's 45. Weydemeyer uses the Tree Swallow, whose first arrival date varied from February 29 to April 15, as evidence that even the sequence of arrivals cannot be predicted. In other words, a "late" spring for one species was not necessarily late for another. The Tree Swallow's position in the spring sequence of all 138 species varied from 4th to 28th, a difference of 24 spots, which is a typical variation for all species. Space prevents me from doing Weydemeyer's analysis justice (he compares weather from year to year, sorts species by food sources, and punctures several other standard explanations of migration timing), and I urge any interested reader to Google to the original article. But perhaps out there somewhere lives a reader interested in proving Weydemeyer wrong. Perhaps you'd like to demonstrate that what is true at Fortine, Montana, is not the case at your hometown site. If you plan to repeat his methods, you have embarked on what could be a lifelong adventure. I hope you are determined, diligent, and—at the moment—very young. Good luck!
For permission to reprint all or part of this article, please contact Tim Gallagher, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-2443. email: twg3@cornell.edu |
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