This thesis examines whether urban residential areas, where small private gardens are the main wildlife resource, offer wild birds ecological opportunities as habitat or impose ecological constraints on them. Ecological opportunities should facilitate the establishment of an avifauna that does not depend on immigration, is similar in composition and structure to those in less-urbanised gardens and comprises individuals within normal ranges of biometrics, behaviour and condition. Ecological constraints may compromise the sustainability of this avifauna and restrict successful exploitation of the habitat to birds of particular characteristics. The analyses presented first investigate which bird species urbanise and why, and secondly which species use gardens and how. Patterns and trends in the use of gardens by British birds are examined, in general and in urban areas. A case study builds on these two themes, linking urban and garden ornithology by investigating an avifauna that uses gardens as its primary habitat but within a highly-urbanised context. Field studies of the breeding avifauna and some aspects of its ecology are presented, together with data from a ringing study on the use of feeding stations and the mobility and persistence of individual birds. More bird species urbanise than might be expected and urbanisation is not restricted to species with high behavioural flexibility. Gardens are an extension of natural habitat for many species, but in urban gardens utilisation rates are declining more than in gardens elsewhere, raising concerns for urban conservation policy. Small gardens in a highlyurbanised area support a breeding avifauna that is of low density but appears sustainable and of stable composition with individual birds maintaining normal territoriality and persisting over time. The spatial distribution of breeding territories is related to all-year artificial feeding but not to cat activity, and the mechanisms of birds' adaptation to urban life show interesting variation between species.