Recent colonization and nest site selection of the Hooded Crow (Corvus corone cornix L.) in an urban environment

• We documented a 7-year population explosion of Hooded Crows in an urban environment.
• Crows preferred to nest high on pine, poplar and oak trees in open areas and parks.
• Population increase was linked with more flexible use of resources for nesting.
• Crows started to nest in tree rows and single trees of previously unused species.
• Crow population will likely grow further and represent challenges for urban planning.

The adjustment of Hooded Crows (Corvus corone cornix L.) to urban habitats has been ongoing in many European cities in the past decades, yet its causes and mechanisms remain largely unknown. The aims of this study were to study the colonization process and the nest site selection of this species in the city of Debrecen, Hungary. During seven years of our study (2006–2012), we localized 231 crow nests on 18 different tree species. The city area used for nesting and both the number and density of nests increased continuously. Crows avoided closed forests and built-up areas and had highest nesting densities in open forested areas, parks and tree rows. Hooded Crows preferred to nest high up on oaks (especially under mistletoes), pines and poplars. Crows built their nests higher in trees in the city than in rural areas and relatively higher in habitats with shorter trees and in conifers than in deciduous trees. Although the increasing use of less-preferred tree species and lower nesting heights indicated that pairs have recently started to use suboptimal nest sites, we detected no sign of saturation of the city nesting population. Our results identified preferences but also confirmed flexibility in nest site selection, which may explain why the Hooded Crow is a successful colonizer of urban habitats. We expect that the population will increase further, which may cause increased predation on songbirds and more complaints from people; thus, our study is important for urban planning, nature conservation and game/wildlife management.