Urban landscapes are expanding at an ever-increasing pace, and this inevitably bears upon natural populations of plants and animals. One of the most common birds of urban environments, the House sparrow (Passer domesticus), recently showed a strong population decline in many western European cities. Such large-scale and highly-synchronized population decline is both intriguing and troublesome, as it may reflect a general deterioration of urban environmental quality. Understanding the underlying processes of this decline therefore is not only a prerequisite to safeguard the long-term viability of urban house sparrow populations, it may also reveal environmental issues of global interest to human inhabitants.

This study aims to explain the marked decline of urban house sparrow populations in the Belgian city of Ghent. As conservation theory predict stronger stochastic effects in smaller and more isolated groups of individuals, we use molecular techniques to estimate (sub)population sizes and dispersal rates both within the urban area and across urban-rural transitions. In addition, we use phenotypic markers such as fluctuating asymmetry and ptilochronology to assess presumed levels of environmental and genetic stress. To address these questions, we combine field observations (seasonal censuses, radio-telemetry), aviary experiments (to assess relationships between environmental stress and the expression of phenotypic markers), measurement of museum specimens (to compare pre- and post-decline morphometrics) and DNA sampling (to quantify genetic differentiation, inbreeding and gene flow).