Country & Coast: The mysterious ‘Devil Bird’ heads back to town and into the spotlight

May is the time when the “Devil Bird” arrives. The Common Swift (Apus apus) is one of our few birds that is absolutely a townie.

It is dependent for its survival on built structures, especially buildings in urban sites, from rural market towns to the big cities. Here, flying around 500 miles a day, it trawls the Yorkshire skyscape for insect prey.

In early to mid-May each year, the swifts arrive back from their winter in the Mediterranean and Africa to breed with us. Indeed, this is almost the only time in their lifecycle that they actually come to ground. They eat, sleep, and even breed on the wing.

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As they arrive back in Yorkshire, they congregate in large screaming flocks around their traditional haunts.

I used to watch them in Meersbrook Park in the urban heart of Sheffield, where, high on the steep slope overlooking Bramall Lane football ground, the swifts, with their dark, sickle-shaped wings, would scour the air currents for their insect prey.

Deafening high-pitched calls resonated in your ears, and the gentle touch of their wings brushed past your head.

These are magnificent birds, of mysterious habits and steeped in folklore. Their breeding sites are generally hidden safe inside old buildings and difficult to observe. Apart from that, the birds are generally on the wing, and often high in the summer skies.

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It was once believed that swifts had no legs and so, if they came to ground they couldn’t take off again. Today, there is still much to learn about swift ecology and behaviour.

A PhD researcher, Mark Walker, of Sheffield Hallam University, located a unique research site where swifts were nesting inside the box structure of a major bridge.

It has long been known that a blood-sucking insect called a louse fly parasites the Common Swift. Sometimes, adult swifts have been picked up on the ground, apparently unable to fly and swarming with these insects. It was assumed that the numbers of parasites directly affected the birds and caused distress and the difficulty in getting airborne.

Mark’s meticulous research over two field seasons indicates not. This suggests that there is more to this relationship than meets the eye. Does the louse fly actually bring some benefit to the swift, and is that why it is tolerated?

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This is relevant to an understanding not only ecology and bird behaviour, but also the wider study of parasitology. Some of the work has implications for our understanding of how hosts and parasites co-evolve to become more tolerant and less damaging.

This has enormous relevance to how many diseases spread and how their hosts, including sometimes us, might develop better resistance.

When you hear the swifts’ harsh screams and cries echo around your house or even your head, consider the louse flies that are hitching a lift too, and the complex relationships that help us understand more, even about our lives and ourselves.

Ian Rotherham is a broadcaster and Professor of Environmental Geography, Reader in Tourism & Environmental Change at Sheffield Hallam University.