“If you’re in a flock, decisions about what to do next are improved if you exchange information with those around you. We can speak to each other. Swifts have no voices, but what they can do is pay attention to what other swifts are doing. And in the end it can be as simple as this: they follow each other.”
– Helen Macdonald
Vesper flights are the journey that great flocks of swifts take at dusk and dawn, ascending to a height of 6,000 feet, into the earth’s connective boundary layer, where they disappear into the clouds and the darkness and can no longer be seen. Up there, high above the noise of the earth and in full view of the stars, they are able to recalibrate their inner compasses and ‘quietly, perfectly, orient themselves.’ Naturalist and author Helen Macdonald’s essay on the vesper flights of swifts is an enlightening and engrossing account of the way in which swifts come together to make decisions, driven by her own fascination for the birds, and the image of their twilight excursions, which echo the solemnity of certain spiritual rituals which take place at the start and at the end of the day, when the earth is quiet enough that we can relocate our own bearings in relation to the world. You can read the full story here.
In the summer of 1979 an aviator, ecologist and expert in the science of aircraft bird-strikes called Luit Buurma began making radar observations in the Netherlands for flight safety purposes. His plots showed vast flocks of birds over the wide waters of the IJsselmeer that turned out to be swifts from Amsterdam and the surrounding region. Every evening in June and July they flew towards the lake, and between nine and ten o’clock they hawked low over the water to feed upon swarms of freshwater midges. Just after ten they began to rise, until fifteen minutes later all were more than six hundred feet high, gathered together in dense, wheeling flocks. Then the ascent began: five minutes later they were out of sight, and their vesper flights took them to heights of up to eight thousand feet. Using a special data processor linked to a large military air-defence radar in the north of Friesland to more closely study their movements, Buurma discovered that swifts weren’t staying up there to sleep. In the hours after midnight they came down once again to feed over the water. It turns out that swifts, beloved genii locorum of bright summer streets, are just as much nocturnal creatures of thick summer darkness.
“Stars, wind, polarised light, magnetic cues, the distant rubble of clouds a hundred miles out, clear cold air, and below them the hush of a world tilting towards sleep or waking towards dawn.”
But he made another discovery: swifts weren’t just making vesper flights in the evenings. They made them again just before dawn. Twice a day, when light levels exactly mirror each other, swifts rise and reach the apex of their flights at nautical twilight.
Since Buurma’s observations, other scientists have studied these ascents and speculated on their purpose. Adriaan Dokter, an ecologist with a background in physics, has used Doppler weather radar to find out more about this phenomenon. He and his co-workers have written that swifts might be profiling the air as they rise through it, gathering information on air temperature and the speed and direction of the wind. Their vesper flights take them to the top of what is called the convective boundary layer. The CBL is the humid, hazy part of the atmosphere where the ground’s heating by the sun produces rising and falling convective currents, blossoming thermals of hot air; it’s the zone of fairweather cumulus clouds and everyday life for swifts. Once swifts crest the top of this layer, they are exposed to a flow of wind that’s unaffected by the landscape below but is determined instead by the movements of large-scale weather systems. By flying to these heights, swifts cannot only see the distant clouds of oncoming frontal systems on the twilit horizon, but use the wind itself to assess the possible future courses of these systems. What they are doing is forecasting the weather.
“What they are doing is flying so high they can work out exactly where they are, to know what they should do next. They’re quietly, perfectly, orienting themselves.”
And they are doing more. As Dokter writes, migratory birds orient themselves through a complex of interacting compass mechanisms. During vesper flights, swifts have access to them all. At this panoptic height they can see the scattered patterns of the stars overhead, and at the same time they can calibrate their magnetic compasses, getting their bearings according to the light polarisation patterns that are strongest and clearest in twilit skies. Stars, wind, polarised light, magnetic cues, the distant rubble of clouds a hundred miles out, clear cold air, and below them the hush of a world tilting towards sleep or waking towards dawn. What they are doing is flying so high they can work out exactly where they are, to know what they should do next. They’re quietly, perfectly, orienting themselves.
Cecilia Nilsson of the Cornell Laboratory of Ornithology and her team have discovered that swifts don’t make these flights alone. They ascend as flocks every evening before singly drifting down, while in the morning they fly up alone and return to earth together. To orient themselves correctly, to make the right decisions, they need to pay attention not only to the cues of the world around them, but also to each other. Nilsson writes that it’s likely that swifts on their vesper flights are working according to what is called the many-wrongs principle. That is, they’re averaging all their individual assessments in order to reach the best navigational decision. If you’re in a flock, decisions about what to do next are improved if you exchange information with those around you. We can speak to each other. Swifts have no voices, but what they can do is pay attention to what other swifts are doing. And in the end it can be as simple as this: they follow each other.
From: Vesper Flights