Wildlife: Kestrels are a rarer sight now, but not if you know where to look

Picture: Adrian Brooker, City of London Corporation

Picture: Adrian Brooker, City of London Corporation - Credit: Archant

Bob Gilbert looks at how these beautiful, slim birds of prey have adapted over the years.

I have been watching kestrels. Twenty or thirty years ago these beautiful, slim birds of prey seemed much more numerous in our city; I would see them almost daily and could name seven or eight school or church towers where they were nesting.

Perhaps our changing approach to building styles has reduced their nesting numbers, compounded by an excess of anti-pigeon measures preventing their access to potential sites. I am reminded of one occasion years ago when I was called upon to rescue a kestrel caught in the pigeon netting surrounding one of the buildings of what was then our local Adult Education Institute. Adult education, those glorious old London School Board buildings and nesting kestrels, are all, it seems, declining assets.

But still, I see a kestrel occasionally flying overhead or sitting atop a tower block or on a roof ridge, watching the world beneath. Along our motorways, and in the less chemical parts of our countryside, they are common still. On a recent walk along a hillside, we found ourselves at eye level with one. Face on to the prevailing wind it had gauged its force so well that it did not have to hover but hung there perfectly still, as though suspended by invisible threads. It is impossible to see them like this and not remember Gerard Manley Hopkin’s ‘Windhover’ poem and his description of the:

dapple-dawn-drawn Falcon, in his riding Of the rolling level underneath him steady air’.

So we stood, equally still, until the bird plummeted downwards to something only he could see in the grass beneath him. This is a remarkable ability of the kestrel, to spot the distant movement of prey amongst foliage, but what it not generally realised is that it has a secret weapon for the kestrel, unlike ourselves, can detect ultraviolet radiation. It is this ability that enables it to see the scent trails of voles and other small creatures, trails that are made of urine which has a high ultraviolet reflection.

The role of UV light in the evolution of life on earth is a complex and fascinating story. UV light is, in fact, inimical to life and before the planet was protected by the ozone layer, the only place that life could survive was in deep vents at the bottom of the oceans where the UV rays could not penetrate. The great evolutionary leap forward was the development by these early organisms of a layer of molecules that protected them against the UV radiation by absorbing it and then releasing it into the ocean as heat or fluorescent light. It was further development of these molecules that eventually led to photosynthesis, to a supply of oxygen to the atmosphere and eventually to the creation of the ozone layer itself. Thus an evolutionary device that came into being to protect individual organisms from ultraviolet radiation, eventually produced the layer that was to extend that protection to the entire planet.

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I may seem to have wandered too far from my original subject, but this is how I bring it back to the kestrel. The protective molecule formed in those early organisms is now known as retinal. And, as you might guess from the name, it it is the same molecule that was eventually to become an essential part of the eye, gathering in light information to enable the brain to construct a visual picture of the surrounding world. What the kestrel has retained, alongside a range of other species from bees to birds, is the ability to absorb the UV alongside other wavelengths of visible light. My diving kestrel, ‘morning’s minion, kingdom of daylight’s dauphin’, was therefore demonstrating a link between the very earliest forms of life on earth and those which share it with us today.

Bob Gilbert