A review of the impacts of corvids on bird productivity and abundance

Abstract

Corvids are often viewed as efficient predators capable of limiting prey populations. Despite this widely held belief, a comprehensive review quantifying the effect of corvids is lacking. We examine the impacts of crows, ravens and magpies on other bird species. We summarise results from 42 studies (arising from a systematic search in Web of Science), which included 321 explicit evaluations. We examined “abundance” and “productivity” parameters from both experimental and correlative studies. Negative impacts were more likely for productivity than for abundance (49% vs 8%). Experimental studies removing only corvids were less likely to show a negative impact on productivity than those removing corvids alongside other predators (14% vs 63%). This suggests that the impact of corvids is smaller than that of other predators, or that compensatory predation occurs. The impact corvids was similar between different avian groups (such as waders, passerines and game birds; or ground-­nesting and non‐ground nesting species). Crows were more likely to have a negative impact on target species productivity than Magpies (58% vs 20%). We conclude that whilst corvids can have a negative impact on bird species, their impact was nearly 6 times more frequent for productivity than for abundance. These results suggest that in the vast majority of cases bird species are unlikely to be limited by corvid predation pressure and therefore that conservation measures may generally be better targeted at other limiting factors.