As the figures above show, there has been an increase in the number of fledglings per breeding attempt and a decrease in daily failure rates at the egg stage. As such, the increase in population numbers has been associated with rapidly improving nesting success, through reduced persecution, the recovery of rabbit populations from the effects of myxomatosis and release from the deleterious effects of organochlorine pesticides (Elliott & Avery 1991, Clements 2002, Sim et al. 2000, 2001a). Numbers of Buzzard were relatively stable until the late 1980s when the population size began increasing steeply. Elliott & Avery (1991) analysed data collected by the RSPB to provide good evidence that, during 1975-89, persecution was a factor in restricting the Buzzard's range. Halley (1993) found that levels of persecution in Scotland had fallen and postulated that this was a factor in the increase in Buzzard population size. In a study of two local populations in Scotland, Swann & Etheridge (1995) provided some evidence to show that persecution was a factor in restricting population density at the site which benefited from higher productivity, although they did not specifically analyse the effects of persecution. Sim et al. (2000) provide good evidence from Buzzard populations in the west Midlands that persecution levels, especially poisonings, were lower in the 1990s when the population started increasing and state that higher survival rate due to reduced persecution was likely to be one of the main factors responsible for the rapid increase in the Buzzard population in this area. Gibbons et al. (1995) found that Buzzards were less common in the uplands were grouse moors were most frequent, stating that this was due to either persecution, unsuitable habitat management or lack of food, although did not specify which was the most important driver.
There is also good evidence to support the role of changing food availability in population increases. Graham et al. (1995) showed that Buzzard breeding density was positively related to lagomorph abundance and Swann & Etheridge (1995) found that Buzzards laid larger clutches, produced bigger broods and had significantly higher productivity where rabbits were more common. Sim et al. (2000, 2001a) also provided good evidence that increased productivity coincided with an increase in rabbit abundance. Other studies have also found that breeding success is related to food availability (Kostrzewa & Kostrzewa 1991, Austin & Houston 1997, Goszczynski 1997, 2001). It is, therefore, plausible that Buzzard distribution is influenced by rabbit abundance, which has increased since increased since rabbits have overcome the effects of myxomatosis.
Habitat change may have played some role in the increases. High Buzzard breeding densities were associated with high proportions of unimproved pasture and mature woodland within estimated territories (Sim et al. 2000) and Sergio et al. (2002, 2005) found that Buzzard productivity benefited from the conversion of coppice woodland to mature forest in Italy. There is also some evidence that breeding success is related to climate, although there is little evidence for this from the UK. In Germany, Kostrzewa & Kostrzewa (1990) provide evidence to show that the number of young fledged was negatively correlated with rainfall in April and May. Although there is no evidence to support this, it is worth noting that these possible habitat/climate effects and food effects are not mutually exclusive.
