Urbanization shapes bird communities and nest survival, but not their food quantity

Abstract

As urbanization accelerates, there is an increasing focus on understanding the ecological aspects of urbanization (both direct and indirect impacts on communities and biodiversity), monitoring qualitative and quantitative changes in the fauna of urbanized areas, and examining the underlying causes of these changes. To explore the effect of urban areas on avian communities, we examined differences in species richness and abundance of birds, and survival probabilities of artificial shrub nests along an urbanization gradient (rural – suburban - urban) in a medium-sized city in Hungary. Additionally, potential food sources (arthropods captured by sticky traps) and consumption rates by insectivorous birds (predation on dummy caterpillars) were compared along the gradient. Based on our observations, the diversity of species that form bird communities did not change significantly along the gradient. However, significantly more individuals were observed in urban areas than in suburban or rural ones, primarily due to the dominance of synanthropic species: the Domestic (Columba livia f. domestica) and Wood Pigeon (Columba palumbus), House Sparrow (Passer domesticus) and European Blackbird (Turdus merula). The daily survival rates of shrub nests were significantly higher in urban and suburban areas than in rural ones. Even so, we did not find any correlations between the species richness of shrub-nesters and urbanization, whilst, they were present with higher abundance in urban than rural habitats. Although the abundance of arthropods as potential food source was not related to the degree of urbanization, the caterpillar predation by birds was significantly higher in suburban than in urban areas. Nevertheless, there were no significant differences in richness of insectivores across urbanization gradients, but their abundance was significantly higher in urban and suburban habitats than in rural ones. In conclusion, no consistent negative or positive urbanization effect could be detected in the present study, but high-level urbanization seems to increase bird abundance. Our results also suggest that shrub-nesting in urban habitats may be safer, probably due to lower predation pressure and/or favourable nesting sites for shrub nesters in cities. Nevertheless, the food availability per se seemed to be not a key driver in habitat selection of birds along the studied urbanization gradient.