Swallows & Martins
In the final week of September a circuit round the quiet backroads through Stainburn and Braythorn took us past barns and outbuildings that during the summer had been alive with swooping swallows as well as past one wide-eaved house festooned with the mud nests of house martins. Even a few weeks before, the wires had been lined with swallows, the young ones (pictured) distinguished by their shorter tail streamers and the paler rufous of their throats and foreheads.
Now, the buildings and wires were silent with every last swallow and martin departed on their long southward migration. Covering about 200 miles a day they fly at low altitude, finding insect food along the way. Once they reach the continent, most British swallows eventually concentrate along the Mediterranean coast of Spain before crossing at its narrowest point near Gibraltar. Then comes the most stressful part of their journey as they face the ever expanding Sahara desert where for several days they will have little food, rest or water.
Swallow and martin migration is now an established fact but it is fascinating to realise that for several thousand years, going back to the days of Aristotle when their autumn disappearance had been noted, most observers and scientists believed that, like bears, bats, reptiles and amphibians, they hibernated. In the eighteenth century there were documented reports of swallows and martins being discovered in an unmoving, torpid condition, in caves, hollow trees, mine shafts and under the thatch of houses.
Some “hibernationists” thought they excavated burrows, others that they hibernated under water. Even the great field naturalist Gilbert White, in his “Natural History of Selborne”, published in 1788, could not quite make up his mind, believing that most swallows migrated but that perhaps those raised in later broods would not have the strength for the long flight south and might hibernate. The controversy continued into the nineteenth century before the migration theory became generally accepted.
However, it is now known that some species, notably hummingbirds found at high altitudes in the Andes or the southern latitudes of Patagonia, do go into a state of torpor overnight, lowering their metabolism in a process similar to hibernation, to survive the very low night-time temperatures. Poorwills, a species of nightjar, have been found in a torpid state in the American Rockies. The doubts of the “hibernationists” have been partly vindicated although not with swallows.