Deconstructing urban tolerance : a bird's-eye view : landscape-scale perspectives on the behavioural and community ecology of urban birds in Melbourne, Australia

Cities, while being the most anthropogenic of landscapes, often incorporate modified remnants of original habitats and also represent novel habitats for plants and animals. Urbanisation affects birds directly and indirectly, leading to changes in ecological processes, habitat, food supply, predator and competitor ecology, and disease epidemiology. The following questions were asked in relation to the birds of Melbourne, Victoria, Australia. Are there are distinct urban tolerator and urban avoider assemblages, and are these urban tolerant assemblages simplified when compared to the urban avoider assemblages? Does bird mass significantly differ between urban tolerant and avoider species? Do exotic and native urban tolerant species exhibit similar functional responses to urbanisation? Do the urban exploiter and suburban adapter assemblages within the broad urban tolerant grouping in Melbourne vary in their responses to predictor variables, and does habitat-of-origin have predictive utility in determining which urban tolerant birds become exploiters or adapters? Does the variability in birds' natural states of fearfulness of humans predict their likelihood of becoming successful urban species? For this study a sample of 141 species (including 13 non-indigenous species) and circa 220,000 individual data points within a 50km radius of the Melbourne General Post Office were drawn from BirdLife Australia's 'Atlas II' project. The data set was objectively classified into five assemblages. Two urban tolerant assemblages contain the more commonly encountered bird species of Melbourne; exploiters occurred widely across the whole city but adapters mostly in the eastern suburbs. Three avoider assemblages occurred within particular spatial and habitat nodes of the city; such as riparian and bush remnants of eastern Melbourne, wetland margins and in coastal vegetation, or on the margins of the urban matrix or in larger remnant native vegetation patches within it. Urban tolerant species were consistently larger in body size than most urban avoiders. Some combinations of foraging and nest substrate guild membership were exclusively urban avoiders. They tended to either nest on the ground, were gleaning species or frugivores, or were specialists such as brood-parasitic cuckoos. Urban tolerant species included many omnivorous or granivorous ground feeders that utilised cavities for nesting (including those in buildings). Nectarivores were often urban tolerant, whereas most raptors were not. All spring migrants were avoiders (except for the partial spring migrant Black-faced Cuckoo-shrike Coracina novaehollandiae, which was an adapter), as were most winter migrants, sedentary species, and nomadic species (more than expected by chance). The exotic species sort into three of Melbournes four urban bird assemblages, and exhibit similar functional and spatial responses to urbanisation as natives. The size differential between urban exploiter and urban avoider exotic birds also matches that in native urban birds, as do the nest substrate and foraging profiles. Two environmental or demographic factors that best explained the spatial and community structure of urban bird assemblages were Frequency Greenspace and IndexCombined. The former reflected structural habitat characteristics of the urban habitat matrix, while the latter represented its human demographic attributes. The spatial arrangement of most assemblages also showed a strong longitudinal gradient, with bird species and assemblage diversity increasing from west to east. The diversity of urban adapters along the gradient of urban intensity (measured by IndexCombined) follows a humped distribution, and the trend is more pronounced when viewed as landscape-scale preference for points on the gradient; resembling the trend seen for urban tolerant birds in other studies, and for bird species richness in response to environmental factors at a landscape scale. The inverted, humped curve for exploiters is atypical of urban tolerant bird species richness seen in other studies, and marks a strong divergence in response by exploiters and adapters to urbanisation intensity. The response of urban tolerant birds to increasing Frequency Greenspace resembles much more the broad trends observed in other cities, and closely mirrors the relationship observed between bird species richness and foliage height diversity observed in non-urban landscapes. The divergent responses of each group to urbanisation intensity are largely explained by their ecological histories. The clear partitioning of adapters and exploiters within the urban tolerant grouping in this study reveals the degree to which spatial and habitat origins of members of bird assemblages influence the degree to which they become urban tolerant. As in other world cities, bird species that showed greater variability in their fear of humans (cvFID) were more likely to be urban tolerant, though the best model had limited explanatory power. Urban bird assemblages of Melbourne are broadly analogous in their organisation to those in other world cities, but they differ in ways that caution against broad generalisations of (i) what constitutes an urban bird, or (ii) where and how abundantly they occur within cities. Whilst others have examined a panoply of physiological and behavioural traits that may predispose birds to urban adaptation, this study has examined the higher order habitat filtering mechanism that may be explanatory at a more fundamental mechanistic level, and point to some broadly generalisable concepts at the scale of the landscape and the assemblage. Species-poor subsets of urban tolerant bird assemblages prosper at sites that are at the extremes of urban habitat gradients. Such highly urbanised sites are species poor for both native and exotic bird species, and therefore exotic bird dominance may simply be a marker of particular urban habitat types where overall bird species richness and individual native species abundance is low, rather than being sites where exotics displace natives from their niches.