FIG 4. Blackbird is best considered as an ‘urban adopter’, a species that is opportunistic in the way that it uses gardens and the resources that they offer. (John Harding)
The species of birds associated with the urban environment – of which gardens are a key component – can be divided into three main types. These are the ‘urban exploiters’, the ‘urban adopters’ and the ‘urban relicts’. Exploiters are those species, like Feral Pigeon, that are typically abundant within urban areas and which depend upon the anthropogenic resources available within the built environment. Adopters also make use of the resources but are more opportunistic in how they do this, and many of our familiar garden birds can be regarded as being of this kind – think of the Blackbirds Turdus merula and Siskins Spinus spinus that move into gardens during the winter months. Urban relict species are those whose population has managed to hang on within a fragment of their former habitat that is now contained within a wider urbanised landscape; such species tend to be found outside of western Europe, where new cities have emerged quickly within formerly wildlife-rich habitats. Another term that may be encountered when discussing urban bird populations is ‘urban avoiders’, those species that are absent or very poorly represented within the urban bird community.
The Rock Dove/Feral Pigeon is one of the oldest and most cosmopolitan commensal species, whose huge global population reflects early domestication and the subsequent transportation and introduction to sites across the world. The very high Feral Pigeon densities encountered in many cities is, to a large degree, a consequence of supplementary feeding and discarded human food, and the presence of buildings and other structures with an abundance of suitable nest sites. In some cities, such as Singapore, these populations are derived from the rapid expansion of a genetically homogenous group of founder individuals (Tang et al., 2018), underlining how a species can very rapidly colonise an urban area given favourable conditions. Similar patterns may be seen in introduced populations of House Sparrow, Starling and Collared Dove.
The notion that species found in a high proportion of the world’s cities, such as Starling, are successful generalists and able to breed anywhere, does require some refinement. Work by Gwénaëlle Mennechez and Philippe Clergeau has, for example, revealed that while the abundance of breeding Starlings does appear to be similar throughout the urbanisation gradient, the degree of urbanisation still has a measurable negative impact on its breeding success. Working in western France, Mennechez and Clergeau (2006) found that the amount of food delivered to Starling nestlings in more urbanised areas was significantly lower than that delivered elsewhere along the urbanisation gradient, resulting in smaller nestling masses. These urban Starlings were found to produce fewer young, each of seemingly lower quality; while they were able to maintain breeding populations in these highly urban habitats, the species was not as successful as a simple measure of breeding abundance might suggest. This is something to which we will return in Chapter 3.
URBAN BIRD COMMUNITIES
Surprisingly perhaps, one in five of the world’s 10,000 or so bird species is found in the highly urbanised landscapes of major cities, including 36 species that have been identified by the IUCN global Red List as threatened with extinction. (Aronson et al., 2014). Globally, the species richness of urban bird communities ranges from 24 species to 368, with a global median of 112.5 species (Aronson et al., 2014). As just noted, a number of studies, typically involving work carried out within a single city, have suggested that the process of urbanisation results in urban bird communities that are becoming increasingly similar over time, and dominated by a few key species. This process is known as homogenisation and is thought to come about because urbanisation not only extirpates native species from an area but also promotes the establishment of non-native species capable of adapting to the new conditions (Luck & Smallbone, 2010). Although these new conditions tend to be similar in cities across the world, they do not in themselves promote the same species winners; instead, it is our preference for certain species (transporting them to new settlements) that has resulted in the homogenisation seen at a global scale (McKinney, 2006).
In their review of the bird communities of 54 cities, spread over 36 countries and 6 continents, Aronson et al. (2014) found that cities tended to retain similar compositional patterns within their distinct biogeographic regions. While certain non-native species were shared across many of the cities studied, the urban communities had not yet become taxonomically homogenised at a global scale – they retained the characteristics of their local region pool. Four species were found in more than 80 per cent of the cities examined; these being the familiar Feral Pigeon, House Sparrow, Starling and Barn Swallow Hirundo rustica. With the exception of Australasia, the proportion of non-native species found within each urban community was similar (at 3 per cent), Australasia being somewhat higher because of the large number of non-native species introduced into New Zealand by settlers. Work within Europe (Jokimäki & Kaisanlahti-Jokimäki, 2003; Clergeau et al., 2006) suggests that urbanisation here might bring about homogenisation by decreasing the abundance of ground-nesting species and those preferring bush-shrub habitats, but also noting that it is difficult to generalise because of the effects of latitude and diversity in the urban habitats studied.
What is interesting about species seemingly well adapted to the urban environment is that they tend to share a number of traits (Leveau, 2013). They tend to be omnivorous in their diet, largely sedentary in habits and able to utilise a range of artificial nesting sites (Máthé & Batáry, 2015). Table 1 highlights the broad ecological traits of the garden bird species considered in this book. As we have seen, omnivores and granivores (seed-eating species) tend to dominate the garden bird community, with insectivores often poorly represented. This suggests that invertebrate populations within gardens and the wider urban environment may not be sufficient to support populations of insect-eating birds. It also has implications for breeding success more widely, since most small birds feed their chicks on invertebrates. Work by Croci & Clergeau (2008) also suggests that ‘urban adapter’ species are those that are sedentary in habits and omnivorous, though with the additional traits of preferring forest environments, being widely distributed, and being high-nesters with large wingspans. Croci & Clergeau (2008) also note that ‘urban avoider’ species tend to be those that allocate more energy to their breeding attempts than ‘urban adaptor’ species, a trait that might make it difficult for them to adapt to new environments. Logically, given the absence of larger and older trees with natural cavities, you might predict cavity-nesting birds to be less common in urban environments. However, the presence of substantial numbers of nest boxes appears to help at least some of these species, most notably the tits, though those requiring large cavities often lose out.
TABLE 1. Common garden birds, their ecological traits and status. * Goldcrest and Long-tailed Tit are open-nesting species, whose nest is fully domed. Conservation status is derived from the Birds of Conservation Concern (BOCC) List, which classifies species as Red-, Amber- or Green-listed based on a number of criteria. Non-native species with feral populations are not assessed by BOCC.
That wild birds can take advantage of the opportunities afforded by human activities is evident from changes in urban-nesting gull populations within the UK, which have increased substantially over a short period of time; this increase has occurred despite the fact that coastal populations of the same species have been in decline. It is thought that much of this