DDT and dieldrin caused a decline in the kestrel along with many other raptors. Once these were banned the kestrel increased in numbers. Now the stronghold for Kestrels is more in the east than in the west. In cold uplands the Kestrels migrate to warmer and more hospitable lowland areas. In the east of England the kestrels are more sedentary.
Limiting factors are nest sites and lack of high vole numbers in the spring.
Kestrels seem to know when it is liable to be a good vole year, even before the voles start to breed. It appears that voles have urine that glows and the Kestrels can see it. Richard Hill then gave us a talk on imprints, mainly he was discussing imprints for breeding rather than flying and hunting with. He pointed out that birds can imprint at almost any age - from a baby to a bird that is several years old. All you have to do with the older bird is what many falconers do anyway - spend hours of time with the bird and feed it and generally replace any normal mate with yourself.