The establishment of an urban bird population

1. Despite the accelerating global spread of urbanized habitats and its associated implications for wildlife and humans, surprisingly little is known about the biology of urban ecosystems. 2. Using data from a 60-year study period, this paper provides a detailed description of how the northern goshawk Accipiter gentilis L. - generally considered a shy forest species - colonized the city of Hamburg, Germany. Six non-mutually exclusive hypotheses are investigated regarding the environmental factors that may have triggered this invasion. 3. The spatio-temporal analysis of 2556 goshawk chance observations (extracted from a total data set of 1 174 493 bird observations; 1946-2003) showed that hawks regularly visited the city centre decades before the first successful breeding attempts were recorded. Many observations were made in parts of the city where territories were established in later years, demonstrating that these early visitors had encountered, but not used, potential nest sites. 4. Pioneer settlement coincided with: (i) an increase in (legal) hunting pressure on goshawks in nearby rural areas; (ii) an increase in avian prey abundance in the city; and (iii) a succession of severe winters in the Greater Hamburg area. On the other hand, there was no evidence to suggest that the early stages of the invasion were due to: (i) decreasing food availability in rural areas; (ii) major habitat changes in the city; or (iii) rural intraguild dynamics forcing hawks into urban refugia. While breeding numbers of a potential rural source population were at a long-term low when the city was colonized, prior to first settlement there was a sharp increase of goshawk chance observations in the city and its rural periphery. 5. The urban population expanded rapidly, and pair numbers began to stabilize after about 10 years. Ringing data (219 ringed nestlings from 70 urban broods; 1996-2000) demonstrated that most urban recruits had fledged in the city, but also confirmed considerable gene flow between urban and rural habitats. Analysis of chance observations (as raw data or as detrended time series) suggested a tight coupling of population dynamics inside and outside the city. 6. City-colonizations such as the one described here provide a valuable opportunity to study some fundamental aspects of population ecology on a scale at which detailed monitoring is logistically feasible. Furthermore, a good understanding of urban ecology has become essential for efficient wildlife conservation in modern, human-altered environments.