Birds in the City—Changes in Species Diversity along Urban Gradient and Time in Krakow, Poland ^†

Humans transform landscapes and replace natural habitats with anthropogenic ones. Some species can adapt to these new environments, increasing their abundance and range of occurrence. To conserve the biodiversity and manage populations of organisms inhabiting cities, monitoring is required. Birds are among animals used as bioindicators of landscape and environment quality. As research on urban avifauna has become popular in the 1960s, birds are one of the best-studied urban organisms. Their communities are very dynamic and vary among different cities. The aim of our study was to evaluate changes in bird species diversity along urban gradient and time. The study was conducted in Krakow city, southern Poland, characterized by various habitats useful for many different bird species. We reviewed over 140,000 scientific records, including data from academic theses (about 750 observations), publications (100), and an official online database (139,700) from the period of 1974-2020. The land use was divided into 3 zones i.e., urban, suburban, and exurban. Bird species were classified into exploiters, adapters, and avoiders, based on the adaptive behavioural traits. Also, we distinguished 3 seasons in the year i.e., winter, breeding, and migration. The total number of species observed in Krakow was 246, including 117 adapters, 103 avoiders, and 26 exploiters. There were 150 species in the breeding season, including 122 nesting ones, 199 in the migration season, and 136 in the winter. The number of species was positively correlated with the number of observations and time. Over decades, the highest increase in species abundance was observed in the exurban area. The diversity of exploiters was constant in time but an abundance of adapters and avoiders has risen. Avoiders preferred exurban areas, however, adapters and exploiters showed no differences in choice of habitat. In addition to the basic patterns of bird occurrence in Krakow along urban gradient and time, the study showed the importance of citizen science (online databases). The future data evaluation is planned.