A top predator
As a top predator, sparrowhawks can only thrive if their prey is present in good numbers and has a healthy population.
Over the centuries the natural balance between sparrowhawks and their prey had meant that hawk numbers remained stable until the 1800s, when they suffered persecution at the hands of Victorian trophy hunters or landowners and gamekeepers.
Reduced illegal killing during the Second World War allowed sparrowhawk numbers to recover, but numbers were declining again by 1950 as a result of the widespread use of organochlorine pesticides, such as DDT. These accumulated in the birds and resulted in a number of problems, including thinning of eggshells that reduced breeding success. By the late 1950s sparrowhawk numbers crashed across the UK, and they almost disappeared from eastern England where the use of these pesticides was heaviest.
The population only started to increase after the chemicals were banned following a public outcry against their poisoning effects. But breeding did not recommence in eastern regions until the early 1980s. By around 1990, the national population was estimated to be about 32,000 breeding pairs. During the 1990s, numbers again declined in some regions - perhaps in response to reduced food availability. A further survey showed there had also been a decline in numbers between 2005 and 2008.
Some people are worried that sparrowhawks eat too many small birds and cause their population to fall or even become extinct. Emotions can cloud the fact that the scientific research points to the contrary. Long-term scientific studies have shown that sparrowhawks generally have no or little impact on songbird populations.
A number of previous studies found that songbirds were no more common when sparrowhawks were absent than when they were present. When pesticides entering the food chain decimated the sparrowhawk population in the 1960s and 70s, songbird numbers remained unchanged.
It is also worth remembering that sparrowhawks and songbirds have existed side by side for thousands of years without any detrimental effect on songbird numbers. Food availability and the number of suitable nesting sites naturally restrict the number of sparrowhawks in an area.
Small birds can rear between five and 15 young in a season.
In the absence of predators such as sparrowhawks, the vast majority of these would die anyway, of starvation or disease. The reason that small birds raise so many young is an evolutionary adaptation because so many will die.
Only one or two of these 5-15 young need to survive to breed themselves in order to keep the songbird population stable. If they all survived to breed there wouldn't be enough nest holes, caterpillars or territories to support such numbers.
Survival of the fittest
Sparrowhawks remove the most vulnerable individuals, so those with the best escape tactics survive. This brings immense stability to the system, as the fittest and healthiest individuals survive. These are much more likely to breed successfully themselves and produce a greater number of fitter young birds that have a better chance of survival.
If habitat is diverse and contains plenty of food and cover for small birds, the balance is tipped further in favour of the prey. In contrast, reducing the quality of the habitat can make songbirds more vulnerable to predators such as sparrowhawks.
The sparrowhawk has no serious predators itself, although its chicks and fledglings are taken by pine martens and goshawks. This threat is, however, not significant as both of these are scarce in the UK.
An important scientific study conducted by the British Trust for Ornithology (BTO) and funded by Songbird Survival was published in March 2010 and looked at the relationship between populations of sparrowhawk and prey species over a period of 33 years.
This study concluded that for the majority of songbird species, there is no evidence that increases in sparrowhawks are associated with songbird population declines. It is also clear that for the majority of declining species with unfavourable conservation status, population declines are due to factors other than predation.
For three songbird species (bullfinches, tree sparrows and reed buntings) there was evidence of a negative correlation between hawk presence and songbird numbers. But a correlation does not necessarily mean that sparrowhawks are the cause of the decline. This is valuable research but the RSPB would welcome further investigations in order to understand fully the effects of predation on songbird populations.