Editing Corvidae (section)

==Ecology==
Corvids occur in most climatic zones. Most are sedentary, and do not [[bird migration|migrate]] significantly. However, during a shortage of food, irruptive migration can occur.<ref name = robertson2000 /> When species are migratory, they will form large flocks in the fall (around August in the Northern Hemisphere) and travel south.<ref name = shadesfonight />

One reason for the success of crows, compared to ravens, is their ability to overlap breeding territory. During breeding season, crows were shown to overlap breeding territory six times as much as ravens. This invasion of breeding ranges allowed a related increase in local population density.<ref name = m&n2006 />

Since crows and magpies have benefited and even increased in numbers due to human development, it was suggested that this might cause increased rates of nest predation of smaller bird species, leading to declines. Several studies have shown this concern to be unfounded. One study examined [[American crow]]s, which had increased in numbers, were a suspect in nest predation of threatened [[marbled murrelet]]s. However, [[Steller's jay]]s, which are successful independently of human development, are more efficient in plundering small birds' nests than [[American crow]]s and [[common raven]]s. Therefore, the human relationship with crows and ravens did not significantly increase nest predation when compared to other factors, such as [[habitat destruction]].<ref name = m&n2006 /> Similarly, a study examining the decline of British songbirds found no link between [[Eurasian magpie]] numbers and population changes of 23 songbird species.<ref>{{cite journal|last1=Thompson|first1=D. L.|last2=Green|first2=R. E.|last3=Gregory|first3=R. D.|last4=Baillie|first4=S. R.|title=The widespread declines of songbirds in rural Britain do not correlate with the spread of their avian predators|journal=Proceedings of the Royal Society B|date=1998|volume=265|issue=1410|pages=2057–2062|doi=10.1098/rspb.1998.0540|pmc=1689492}}</ref>