British Wildlife of the Week:

Kestrel

A common kestrel (Falco tinnunculus) hovering in mid-air
Image Source: Andreas Trepte

When I was young, the first bird of prey that I was able to easily identify was the kestrel. I suspect the same is true of many other people. The kestrel’s silhouette, suspended in the sky as if by wire over some heathland or motorway verge, became instantly recognisable to me, for no other British bird of prey can hover in place with such utmost precision.

To maintain its position in mid-air, a kestrel must fly into, and at the same speed as, the oncoming wind – the current of air passing over its wings provides the lift it needs. The wind must be at the right speed: between 21 and 42 km per hour. Any slower and the kestrel finds it difficult to hang in the air; any faster and the bird has to use much more energy not to lose ground. With its long tail spread wide, the kestrel rapidly beats its wings, about six times a second, almost looking as though it is furiously attempting to escape its stationary position in the sky. But although its engines are revving, its brakes are on – the kestrel wants to remain exactly where it is. Its superb hovering capabilities means that it is known as the ‘windhover’ in some areas, while its tendency to hover above roadside verges looking for prey has earned it another, slightly less glamorous nickname: motorway hawk.

Hovering requires extraordinary agility and a great deal of effort. A kestrel’s wing and tail feathers are in constant motion, making minute adjustments to prevent itself from being blown backwards. But although the rest of its body may be knocked about by the wind, its head barely moves more than a few millimetres out of position, as if it were held in place by invisible clamps. This allows it to keep its gaze fixed on the ground below as it searches for movement within the grass.

Unlike many other falcons, which feed mainly on small birds, the kestrel’s diet consists primarily of field voles – probably Britain’s most common mammal, with a population of over 75 million. To find small, fast-moving prey that spends most of their time hidden away in the undergrowth, the kestrel has excellent vision. And it has another ace up its sleeve: its eyesight is sensitive to ultraviolet light.

Rodents habitually mark their territories with their urine to declare their presence to rivals and mates, and this urine just so happens to reflect ultraviolet light. The urine may be invisible to our eyes, but to a kestrel, it is the equivalent of a luminous signpost. This means that a kestrel can easily locate areas where voles, mice and other rodents are numerous. It can even differentiate between a fresh and stale trail of urine, so it can pinpoint locations where its prey has been recently active.

A male kestrel (Falco tinnunculus) perching on a stump
In medieval England, the only bird a knave was legally allowed to keep was a kestrel because it was small, widespread and not particularly impressive or useful.
Image Source: Charles Sharp

The kestrel was once our commonest bird of prey. I can still remember how the sight of them hovering over roadside verges when I was young broke the tedium of many long motorway journeys, and I would crane my head back to see them out of the rear window as I whizzed by below, marvelling at their breathtaking aerial control. But over time, my sightings of them grew less frequent. The kestrel population in our country has declined substantially over the past few decades and it has now been comfortably overtaken by the buzzard, which was once comparatively rare and hard to spot. Between 1995 and 2008, the UK lost one-fifth of its kestrels, while buzzards increased by 63%. Today, my car and train journeys commonly yield sightings of buzzards, but rarely kestrels.

What has caused this steep decline? The bioaccumulation of rodenticides has likely had an effect, but the main issue is probably the intensification of farming. The conversion of rough grazing to improved pasture means fewer meadows and field margins; this has reduced the abundance of the kestrel’s prey and the availability of hunting habitat. As we constantly strive to make the world around us tidier and easier to manage, we no longer allow nature to grow wild – usually to its detriment.

Next time on British Wildlife of the Week, we’ll be looking at the world’s smallest member of the carnivore family – the weasel.

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