Causes of change

There is good evidence that improved breeding success due to a decline in organochlorine pesticide use is the most likely cause of the increase in this species, but that reduced survival, especially of young birds, may be driving the decline in Scottish populations.

Further information on causes of change

Sparrowhawks suffered a severe population crash caused by organochlorine pesticides in the 1950s and 1960s, when the species was extinguished from large areas of lowland Britain (Newton 1986, 2013). Studies of this species in eastern England confirmed this, and the recovery of the Sparrowhawk in this area was primarily dependent on declining organochlorine contamination which resulted in an improvement of breeding success mainly due to an increase in hatching success, itself associated with improved eggshell thickness and reduced egg breakage (Newton & Wyllie 1992). The figures above support this, showing improving numbers of fledglings per breeding attempt, a fall in failure rates at the egg stage and increases in brood size. Integrated population modelling supports the importance of productivity, as well as the survival of first-year birds, in determining population change (Robinson et al. 2014). This also suggested that an unknown factor, perhaps the availability of good-quality territories (and hence the number of individuals that can breed each year), also influences the annual population change.

Comparison of an increasing population in east-central England with stable and decreasing populations in southern Scotland showed that differences in population trend were associated mainly with differences in the recruitment of new breeders (greatest in the increasing and lowest in the decreasing population) and in age of first breeding (earliest in the increasing and latest in the decreasing population). There were also differences in the annual survival of breeders (greater in the increasing population) while differences in breeding success between areas were slight and non-significant (Wyllie & Newton 1991). A comprehensive long-running study of Sparrowhawks in Scotland during 1972-86 provides further detailed evidence. Overwinter loss operating in the period between the fledging of young and subsequent recruitment to the breeding population was identified as the key factor, explaining 77% of the variance in total annual loss, and largely accounting for the pattern of change in breeding numbers (Newton 1988). Work by Newton & Marquiss (1986) found that annual survival of established breeders and breeding performance was the same in both a declining and increasing population, but that recruitment of incoming breeders was lower in the declining population and state that this was the main proximate cause of decline. A more recent study in Scotland during 2009-12 found that nest failure rates were significantly higher in rural populations than in urban areas, with further work required to understand the causes behind this difference (Thornton et al. 2017).

The population has stabilised since the mid 1990s and, possibly through the effects of intraspecific competition, average brood size has begun to fall again (see above) and a recent moderate decline in abundance has occurred. Trichomonosis, the disease which has caused the recent very steep decline of Greenfinch, has also been recorded in Sparrowhawks (Chi et al. 2013); however evidence of any link between the disease and the recent Sparrowhawk decline is yet to be shown.

Information about conservation actions

The drivers behind the recent declines of this species are unclear and further research is needed to understand the causes. If specific reasons can be identified to explain why recruitment is lower in some populations and why nest failure rates are higher in rural populations, then appropriate targeted conservation actions can be proposed.

Similarly, raptors are known to suffer from trichomonosis, so if there is evidence that this is contributing to the decline it would be useful to confirm this. If so, there are currently no obvious conservation actions that can be taken apart from promoting improved hygiene at feeding stations, assuming that the disease is passed to sparrowhawks when they feed on infected birds.

Anticoagulant rodenticides (rat poisons) were found in 39 out of 42 UK Sparrowhawk carcasses examined between 2010 and 2012, in most cases at low concentrations; two birds had slighter higher levels, although with no clear evidence that rodenticides were a contributory cause of death (Walker et al. 2014). Nevertheless, precautions such as prompt removal and safe disposal of poisoned rates, as well as continued monitoring, would be prudent.