Urban development from an avian perspective: causes of hooded crow (Corvus corone cornix) urbanisation in two Finnish cities

摘要

The hooded crow (Corvus corone cornix) colonised Turku and Helsinki, two cities in southern Finland, in the first half of the 20th century as a breeding species. Their urban population densities, however, remained low for decades, in spite of considerable changes in the urban environment. Since the 1960s, the breeding populations have increased very rapidly in both cities. The possible causes of the early colonisation of cities, the long period of low population density, and the recent rapid population increases are discussed based on environmental history data. The establishment of city parks in the 19th century was probably a prerequisite for initial colonisation, although it cannot alone explain the timing of colonisation. Hooded crow populations did not increase before the 1960s, although biological waste production increased considerably and large landfills were continuously available near both cities. In Turku, the rapid increase of the crow population took place after the availability of biological waste in the main landfill had decreased due to opening of a municipal waste incinerator. Suitable habitats created by urban expansion and a lack of predators in cities during the breeding season may have promoted urbanisation. The main factor promoting urban population growth has probably been decreased persecution in cities since the 1960s. Low levels of persecution in urban areas have facilitated the habituation of crows to humans and traffic, and probably explain the recent colonisation of city centres as breeding habitats.