FIG 8. The presence of green cover within urban areas is of particular importance to urban birds, shaping both their distribution and their ability to move between different parts of the built environment. (Mike Toms)
While there has been relatively little work examining how features within and around gardens influence the bird species present, rather more work has been done on other components of urban green space (Jokimäki, 1999). Some of the findings from these studies may provide an indication of how similar features may influence garden bird communities. Work on areas of public green space within London has, for example, underlined the importance of patch size, the presence of waterbodies and the provision of areas of scrubby cover and weedy patches (Chamberlain et al., 2004b). Importantly, some of this work has also underlined the importance of gardens in promoting the species richness of the bird communities using other types of urban green space, such as parks (Chamberlain et al., 2004b). Interactions between gardens, urban green space and the wider built environment may be complex, with the non-biotic components of a city (such as building height and architectural style) also a potential influence on the nature of the bird communities present. Work in Paris, for example, has demonstrated that building height can influence the abundance of particular avian guilds, such that the abundance of omnivorous species was found by Vincent Pellissier to be influenced by the interaction between building heterogeneity and the proportion of low and medium-height buildings present (Pellissier et al., 2012). The findings of this work have implications for urban planning.
Although our work examining the BTO Garden BirdWatch dataset (see Chapter 6) underlines the greater importance of features outside of a garden in determining its use, there do appear to be some garden characteristics that shape the extent to which a garden is visited by particular bird species. Several finch species appear to favour those gardens (and garden feeding stations) that are associated with the presence of tall and mature trees (either within the garden or nearby). Greenfinches, Chaffinches and Goldfinches appear to prefer to fly into tall trees before dropping down to garden feeding stations to feed. Similarly, the presence of thick shrubby cover near to feeding stations appears to increase their use by House Sparrows. The presence of berry-producing shrubs and trees will influence whether or not a garden is visited by wintering thrushes or Waxwings, while the presence of a pond appears to be a particularly attractive feature during the summer months for many different garden bird species.
HOW THE URBAN ENVIRONMENT IMPACTS ON BIRDS AND THEIR POPULATIONS
Over recent years there has been an increasing push to understand the finer-scale effects of the urban environment – of which gardens are a significant component – on the ecology, health, behaviour and physiology of birds. Such effects have been studied both at the level of the individual bird and at the level of the population or community. As we shall see in this section, some of these effects can have significant consequences, determining which species thrive in our town and city gardens and which struggle.
Three of the most striking ways in which urban habitats and gardens differ from more natural landscapes are in the availability, predictability and novelty of key resources, most notably food (provided at garden feeding stations) and nesting opportunities (provided through nest boxes). As we will discover in Chapter 2, the presence of food at garden feeding stations can lead to the creation of a more predictable environment (both through time, and spatially across the city or town). This can have profound consequences for urban birds, as work on Northern Cardinal Cardinalis cardinalis populations in Ohio has demonstrated. Rodewald & Arcese (2017) found that female cardinals, breeding in urban habitats within their study area, were more similar in their contribution to the next generation than was the case for those breeding in rural habitats, where a pattern of winners and losers was more evident. Importantly, this difference occurred despite comparable variation in body condition across the habitats studied. The urban cardinal population, then, is more homogenous in terms of its breeding performance than the rural population.
Living within an urban environment is thought to impact on the health of the organisms found there and we know, for example, that urban pollution elevates the levels of oxidative stress (see Chapter 4) seen in humans and birds (Isaksson, 2010). Researchers have measured the levels of oxidative stress in urban populations of study species and compared these with individuals living in other, more natural habitats. Other researchers have looked instead at the levels of particular hormones, again using these to tease out possible health effects. More recently, attention has turned to telomeres; these are the nucleoprotein structures found at the end of chromosomes. Telomeres are thought to promote genome stability and there is good evidence to associate the length of a telomere with lifespan, mortality rate and disease risk. This suggests that telomeres may prove a useful ‘biomarker’ for phenotypic quality and ageing. If the pressures of urban living modify an individual’s oxidative balance, then this may result in the shortening of telomere length and indicate a health effect.
FIG 9. Being raised in an urban environment significantly reduces telomere length in Great Tits. Telomeres are the nucleoprotein structures found at the end of chromosomes and thought to promote genome stability. They have been linked to health and longevity. (Jill Pakenham)
Through an experiment in which nestling Great Tits were cross-fostered, Salmón et al. (2016) found that being raised in an urban environment significantly reduced telomere length compared to that of nestlings raised in a rural environment. However, this finding could not be replicated by a similar study in France (Biard et al., 2017). A larger study, this time of Blackbirds, also found that urban-dwelling individuals had shorter telomeres than those living elsewhere, adding evidence to this growing area of research interest (Ibáñez-Álamo et al., 2018). While Clotilde Biard’s study failed to find differences in telomere length between urban and rural populations of Great Tits in France, the work did reveal differences in chick growth (rural chicks were larger and heavier) and in plumage colouration (the yellow carotenoid-based plumage was more colourful in rural chicks – see also Chapter 2), suggestive of developmental constraints within the urban populations. Measures of ‘damage’ in relation to the impacts of urbanisation have been shown to vary between species with, for example, Salmón et al. (2017) determining that sparrow species show greater overall damage than that seen in tits.
There is the suspicion then, that gardens and the wider urban habitat are sub-optimal for many bird species (though not all). If this is the case, then we might expect to see the presence of three features that have been found to be characteristic of sub-optimal habitats elsewhere. These are:
i) a greater proportion of young breeders and more floating individuals;
ii) less stable populations, whose numbers fluctuate from year to year; and
iii) low breeding density.
The idea that populations living within sub-optimal habitats are more likely to fluctuate from year to year is linked to the notion of ‘source-sink’ dynamics, where populations living within high-quality habitats produce a surplus of young each year, but competition for breeding territories in these high-quality habitats is high, resulting in many young and less dominant individuals moving away to occupy territories in less optimal habitats. This also explains the first characteristic that we mentioned – the greater proportion of young birds in sub-optimal habitats. We certainly see this in suburban tit populations (Cowie & Hinsley, 1987; Junker-Bornholdt & Schmidt, 1999) but it isn’t always the case. Work on farmland Blackbird populations has revealed the presence of greater numbers of young males in this habitat (Hatchwell et al., 1996a), and David Snow – in his classic treatise on urban Blackbirds – speculated that urban populations act as a source population for a rural sink (Snow, 1958). Snow had demonstrated that his urban Blackbirds were generating a significant surplus