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16 december 2015
The swallows I know best live under the bridges that cross the Trinity River system in Fort Worth, Texas. The city has built a linear park along its heavy-managed main water artery, and the bridges that carry various highways and streets over the park and the river are ideal nesting places for swallows. I believe the ones I've seen there are cliff swallows, but they could belong to any or all of several other related species.
Swallows are famous migrators. Much of Angela Turner's engaging and knowledgeable Swallow is devoted to their travels. I reflected that I don't know whether the swallows of North Texas migrate south or north from here, or maybe don't travel far at all. Ours is a hot climate, and getting hotter by the year as the globe warms, but we still have winters and annual hard frosts that tamp down the insect population. It would be hard for swallows to find enough to eat here in January, though the rivers that course through Dallas / Ft. Worth might provide enough food, and the bridges over them enough shelter, for the swallows to survive. As it is, I see them mostly in the spring and fall, when it gets either warm or cool enough to urban-hike along the rivers.
For much of European history, observers were confused about what swallows did in the winter. One prevailing urban (rural?) legend had it that swallows hibernated. Not in their nests, of course, which clung to the eaves of houses and barns and were visibly empty – but at the bottom of ponds. Turner notes insightfully that every actual report of swallows hibernating underwater, from the proto-scientific 17th and 18th centuries, is of the variety "my cousin told me he used to see this as a Shropshire lad." But no less a skeptic than Dr. Johnson believed the story.
Turner describes early horrid experiments where swallows were drowned or frozen in order to test their aptitude for suspended animation. Then in the mid-18th century, observers began to tag birds in hopes of finding where they really went. Turner describes an ingenious experiment by a certain Johann Leonhard Frisch. Frisch tied threads around the legs of swallows – threads dipped in watercolors. When the swallows returned with the spring, their threads were still colored; they hadn't been underwater. At the same time, Frisch proved to science what folks had surmised all along: the same swallows will return to the same nests year after year (49).
Turner's book consists of an initial chapter on the natural history of swallows (and their close cousins martins). Then follows a sort of history of that natural history (including things like the Frisch experiment). Then there are four chapters that are all fairly similar to one another. Swallow legends, swallow lore, swallow superstitions, swallows in art, swallows as totems and omens – these are not divided into their own chapters but share space in several passes over the wealth of cultural and environmental knowledge about swallows and martins.
One thing most Americans have heard about the purple martin is that it can eat its weight in mosquitoes. I spent parts of a few summers, 10-15 years ago, in the North Woods of Wisconsin, where martin houses are common. You can also get eaten alive by mosquitoes up there, so the correlation between martins and mosquito control was weak, though widely believed. Anyway, as Turner notes, mosquitoes weigh nothing and martins cannot survive on a mosquito diet. But as insectivores who rarely turn to any food crop as a supplement, swallows and martins are beneficial to farmers and usually welcome.
Turner presents some paradoxes in swallow conservation. Hit hard during the European settlement of North America by habitat clearance, changes in land use, and just plain indiscriminate shooting, swallows have been serendipitously helped by the even greater development of the 20th century – which has provided those ideal bridges and overpasses for them to roost in. Threats to the swallow have now shifted southwards. Many European swallows winter in Africa, where they are an important food source (and where development akin to that of 19th-century North America is putting pressure on their habitats). Swallows are perhaps resourceful enough to ride out our depredations, though. Some evidence shows that they are already adjusting their migratory patterns to fit climate change.
Turner observes that swallows are portents not only of spring but of many other good things. They are also portents of death and disaster. She explains that as close co-residents of human settlements ever since there have been human settlements, swallows have shown up just before good events and bad events with equal predictability throughout history. They are so ubiquitous that they presage just about anything.
Turner, Angela. Swallow. London: Reaktion, 2015.