Nature 2021 August

One Swallow Does Not a Summer Make

It is still one of the most enjoyable sights of summer to see swallows sweep across our skies or swoop down to take insects from near the water surface. Unfortunately, their numbers are not what they were. Swallows are presumed to be one of our better indicators of forthcoming weather. Apparently, when seen flying high expect fine weather and when spotted low to the ground they are a portent that wet weather will soon be on the way.

This is one of many fragments of country lore that has found its way into English verse. In The Shepherd’s Week, written in 1719, John Gay recorded of his character, Cloddipole, the eponymous shepherd:

When swallows fleet soar high and sport in air.
He told us that the welkin would be clear…

(NB. Welkin = sky)

Whether this rubric is true has, over the years, been a matter of some contention amongst naturalists. Those scientists in support say the swallows fly at whatever height their insect prey is found. The long-held belief was that in fine weather insects rise uncontrollably due to localised thermals that start at ground level. In humid conditions insects have more difficulty flying because the moisture in the air condenses on their bodies, making them too heavy to be taken on warm air currents to higher altitudes. So, they can only take short distance low-level flights and shelter in vegetation.

The debate amongst naturalists came to a head when The Daily Telegraph’s Nature Notes columnist, Robert Burton, published an article recording his observations on swallows, martins and swifts. Unlike these latter two which feed on smaller prey, swallows which have larger mouths, prefer the heavier-bodied insects such as hoverflies and bluebottles. To catch these, swallows prefer to swoop lower down and will also be seen over open water taking newly emergent freshwater insects, such as mayflies. By contrast, swifts and martins prefer to feed at much higher levels from the clouds of small flies, swarms of aphids, flying ants and spiderlings attached to silk lines all driven skywards by the warm air currents.

Back in 4th century BC the Greek natural philosophers, Pliny and Aristotle, concluded that swallows and their relatives that disappeared in winter, hibernated underwater, in mud, much as some amphibians, like toads and newts had been found to do. 2,200 years later Linnaeus (1707-78), the 18th century Swedish taxonomist certainly believed swallows hibernated. However, this theory began to be questioned by the burgeoning numbers of amateur naturalists drawn from the ranks of gentlemen and reverends. We know this from Gilbert White (1720-93), a curate and author of A Natural History and Antiquities of Selborne.

White pioneered the importance of the meticulous observation of behaviour and recording of the natural world. His initial belief was that swallows hibernated somewhere local to where they were in summer. However, his subsequent investigations led him to dismiss Linnaeus’ theory that swallows hibernated underwater. He had observed swallows and martins congregating in trees close to water for which they had an affinity due to their natural prey. Through his meticulous observations, White concluded at best only some might lay up in a torpid state in river banks, caves or abandoned farm buildings over the winter, but most moved somewhere away from their summer homes. His brother John lived in Gibraltar and had seen swallows crossing the Straits. White did speculate that some swallows might travel even further afield but was  unable to associate this long-distance travel to what he observed around Selborne. His progressive ideas led to much heated correspondence with his fellow naturalists up until the time of his death.

The debate over hibernation versus migration continued throughout the 19th Century. It was not until 1912 that the mystery was finally put to rest when birds ringed in England were recovered in South Africa.

Researching the newspapers from the first decades of the 20th Century it soon becomes obvious that readers had a growing enthusiasm to write to their local paper with their wildlife observations. In many of the regional newspapers there were regular ‘Nature Notes’ sections, the forerunners of the previously mentioned one in The Daily Telegraph. With the start of the Great War in 1914 it seems many amateur naturalists were not discouraged when they had to swap their Norfolk Jackets and plus-fours for khaki uniforms. In the months that followed there started to be accounts from servicemen who reported their wildlife observations whilst stationed on the Western Front.

I found one such example in The Scotsman from 15 September 1917. It is entitled Swallows in the Trenches:

“While making my way up to the front line yesterday, I was much interested in a family of swallows that followed us for a considerable distance. The weather was cool, and a slight drizzle of rain was falling. We decided to leave the muddy trench and go overland through a wilderness of thistles and long grass and weeds. The swallows attached themselves to us immediately we left the trench and for a minute or two I could not understand the strange tameness of these dainty birds. They circled round my batman and myself, flying just about 3½ feet from the ground, now a yard in front and sweeping within a foot or two of us. They followed until we reached the dugout that was our destination. Evidently the explanation was that, as we walked through the tangle of weeds, we were disturbing insects and the instincts of those pretty migrants told them how to find their evening meal. I have never noticed these birds following anybody at home, but just here the only vegetation is the weeds that cover the ground, the trees having all been killed by bombardment. The cold and wet probably kept all flying insects undercover of the weeds until disturbed by our walking. GNB.”

It was Aristotle who coined the phrase ‘One Swallow Does Not a Summer Make’.

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