The Role of Predators in Shaping Urban Bird Populations: 1. Who Succeeds in Urban Landscapes?

Abstract

To date, urbanization has resulted in the establishment of peculiar, often extremely dense urban populations in some wild bird and mammal species. However, until the beginning of the 20th century, the number of such species remained small, only some full synanthropes and a few partial synanthropes reached an impressive abundance in cities. However, from the second third of the 20th century, urban populations began to appear in a steadily increasing number of species that previously either did not show a tendency to synanthropy at all, or inhabited the rural landscape. The importance of species interactions in this process later called synurbization are still rather poorly studied. There is the opinion that for birds one of the leading initial factors of ecological release in urban landscapes is the decreased predation pressure. However, this point of view is not shared by all experts. The purpose of this series of papers is to analyze ideas about the role of predators in the colonization of the urban environment by birds. This paper is devoted to a brief review of the categories and composition of urban species, the criteria for synurbization, and the proposed ecological mechanisms for the development of extremely high local densities of breeding birds in cities. Success in colonization of urban landscapes has been achieved by representatives of many orders and families, morphological types and ecological groups occupying different positions in food chains, species with different levels of plasticity of behavior, sociality, etc. There are many predators among synurbic species of the last 4–5 decades. Some of them, primarily bird-eating species, achieve not only high population density in cities, but also high rates of breeding success. High local population densities in synurbic species, in contrast to those in full synanthropes, can hardly be interpreted as a consequence of reduced species richness and filling of the “ecological vacuum” by individual urban species. For the most part, synurbic species are numerous not in continuous built-up areas, but in natural and green urban areas, which are characterized by an impressive range of species, so there is no reason to talk about competitive release. The hypothesis of an increase in density “on credit” also seems plausible mainly in relation to full synanthropes, however, not all of them. It remains unclear why the emergence and long-term (co)existence of highly dense local populations of synurbic species during the breeding season, including those of potential competitors, are not limited by those biotic factors that keep the population density of these species at lower levels in natural landscapes.