Gardens occupy a significant component of the British land area (less so in Ireland) but are considered by many to be a minor and relatively unimportant habitat for wildlife. However, private gardens make a significant contribution to the amount of urban green space and are arguably the main contributors to urban biodiversity within developed countries (Cannon 1999).
Ongoing urbanisation underlines the need to find out more about the ecological communities that exist within urban landscapes and to understand the factors that determine their composition and conservation value.
Understanding avian communities in urban areas
There have been many studies on community composition in urbanised landscapes, often highlighting the dominance of medium-sized omnivores, granivores and sedentary or partially-migratory species in the more urbanised areas. The process of urbanisation may therefore alter the bird community at a given place, the resulting community determined by the proximity of other habitats, the structure of the urban environment itself and the distribution of resources like food and nesting opportunities. The systematic information collected through BTO Garden BirdWatch provides a mechanism for exploring some of these community level processes.
Determining the seasonal use of gardens
It is evident from BTO Garden BirdWatch that many bird species have a distinct seasonal pattern to their use of gardens and the resources they contain. The autumn trough in Blackbird use of gardens is one of the most pronounced patterns, often commented upon by garden birdwatchers, but many others are equally apparent. Examination of these patterns, sometimes alongside examination of patterns seen in other habitats, can improve our understanding of how and why birds use garden resources. We know, for example, that Coal Tits and Siskins increase their use of garden feeding stations in years when the Sitka Spruce seed crop is poor (McKenzie et al. 2007).
Supporting conservation assessments
Gardens may support populations of species considered to be of conservation importance; for example, Blackbird, Starling, House Sparrow and Song Thrush. Since the garden habitat may be difficult to monitor through other means, BTO Garden BirdWatch potentially provides a means of tracking the garden-based component of a wider breeding population, as highlighted by the BTO Garden Nesting Survey (Bland et al. 2004). Garden BirdWatch data have also been used in Government Indicators for measuring the health of urban bird communities.
Looking at breeding success in urban landscapes
Demographic measures (such as clutch size, timing of nesting, etc.) of birds nesting in urban areas may differ from those seen in other habitats. With increasing levels of urbanisation it is important to quantify and interpret any such differences in order to assess their implications for bird conservation. This is something that we have examined using BTO data and published as a review in the journal Ibis (Chamberlain et al. 2009).
Predation, mortality and disease
Processes like predation and disease will often vary in their causal agents between habitats. For example, while birds nesting in gardens may be more exposed to predation by domestic cats, those nesting in the wider countryside are less likely to face domestic cats but more likely to face corvids, Badgers and Weasels. Understanding the mortality agents operating within urbanised landscapes is another core component of our work. We have, for example, examined the impact of trichomonosis in finches (Robinson et al. 2010) and of Sparrowhawks on House Sparrows and other gardens birds (Chamberlain et al. 2009).