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The reason we notice and record such apparent fluctuations is that they are perceptible in the timespan of our experiences. That is to say that I, and you, if you're over ten, remember when kestrels were our commonest raptor and were easy to see on such journeys. For some they were so familiar that they were considered a little ordinary. But now they've gone, or at least they are going, and we feel compelled to investigate why and hopefully in future conserve them.

But how would our grand parents remember the abundance of kestrels, or their grandparents? Maybe they were once rarer, perhaps in the peak of the 1950s pesticide crisis they decreased only to recover more rapidly than other species such as the and the sparrowhawk.

Sparrowhawks are another interesting case. When I was studying kestrels as a teenager in the 1970s they were uncommon. Finding a nest was a big deal. But now I read letters from people who claim that they are reaching plague proportions, that they are responsible for the diminishing number of songbirds.

The thing is that these people, and you and me, probably became aware of birds and thus began to measure their relative abundance when the population was on its knees. Now it's just getting back to normal and thus to some they seems over-abundant.

Not that it could be. The sparrowhawk population is self-regulating and largely determined by that of its prey. No predator eats itself into extinction. So blaming it for a songbird genocide is a fundamentally flawed concept. We should in fact be celebrating the sparrowhawk's recovery.

This phenomenon is called '' and it's proving to be a real hindrance to effective conservation in some instances. Natural systems rarely function in parallel with the human lifespan and of course if we are messing those systems around they may not re-stabilise in our lifetimes. Thus maybe some of the positives and negatives will in time cancel each other out.

Kestrels mainly feed on small mammals and sparrowhawks on small birds. So it's unlikely that the increase in the latter is responsible for the decrease of the former. But what about buzzards? They do eat small mammals. Could their also significant recovery be having a negative impact on the kestrel? And now that those arch exterminators are again widespread, what effect might they be having on other raptor numbers?

The UK countryside is a dynamic and interesting place and change is not something we should necessarily fear before we fully research and understand it.