Causes of change

There are few specific studies providing evidence for the causes of the increase in this species, although evidence presented here shows that increases in breeding success have been important. Ecological causes of this could be increases in food availability and the increasing suitability of urban areas (driving the species' expansion there), although specific evidence supporting these hypotheses is limited.

Further information on causes of change

The demographic trends shown here reveal that there was a strong increase in the number of fledglings produced per breeding attempt between 1968 and the late 1990s, reflecting a decline in daily failure rate of nests at the egg and chick stages. Clutch size is currently at a similar level to 1968, but brood size has decreased. The number of fledglings per breeding attempt has dropped slightly since the late 1990s, perhaps due to density dependent effects. This suggests that the increase in Carrion Crow numbers is related to increases in breeding success although, as there are no estimates of survival, it is not possible to say what part this has played.

This species is omnivorous and highly adaptable and is thus able to exploit changing habitats and the ephemeral food resources in intensive agriculture, from ploughed fields to grazed pasture, allowing breeding pairs to hold territories year-round. It is also able to exploit the varied food sources found in towns and cities. Richner (1992) provided good evidence that food-supplemented pairs had a higher nesting success and produced more and heavier fledglings, demonstrating that food limitation can cause low fitness for individuals and thus could potentially restrict population-level reproductive success. In a local study, Yom-Tov (1974) showed that provision of excess food improved chick survival, and concluded that the distribution pattern of food was the ultimate factor limiting breeding success, perhaps because this affects levels of intraspecific nest predation. Although the impact on population size was not considered in these studies, it is possible that food availability for Carrion Crows has increased and so helped support the population increase. O'Connor & Shrubb (1986) suggest that the general increase in density of sheep in upland areas, and the increase in carrion resulting from this, may be responsible for the expansion of Carrion Crow populations, although evidence for this was not given and this is clearly not relevant to lowland areas (where sheep numbers have decreased).

A second hypothesis to explain this species' increase is that control by gamekeepers has reduced, but evidence supporting this is limited. Tharme et al. (2001) stated that the control of Carrion Crows by gamekeepers was the most probable cause of the low densities on grouse moors, although they found no significant relationship between the number of gamekeepers and Carrion Crow density. Furthermore, bag returns have shown no overall change in the number of Carrion Crows killed since 1961 (Tapper 1992, Tapper & France 1992).

Information about conservation actions

Numbers have increased consistently from at least the 1970s onwards, hence the Carrion Crow is not a species of concern and no conservation actions are currently required.

On the contrary, legal control of carrion crows occurs on many shooting estates, and crows have been suggested as possible drivers in the declines of other species. In some cases it is possible that crows can have local effects on breeding productivity of songbirds (Stoate & Szczur 2010), although it is uncertain whether they can also have population level effects and hence whether controlling crows would be an effective method to help to conserve songbird populations. In the case of waders, nesting success for three out of six waders (curlew, redshank and lapwing) was significantly higher when avian predators (both Carrion Crows and Common Gulls) were controlled in one study (Parr 1993). However, another moorland study found that control of predators (including Carrion Crows) was ineffective and that population trends of waders at a managed site were no different to comparable trends elsewhere (Calladine et al. 2014a).