Avoid, adapt or exploit: Re-visiting bird responses to urbanization using a novel landscape approach

Urbanization threatens global biodiversity, yet numerous species persist in cities, highlighting opportunities for habitat restoration and conservation. To facilitate conservation planning, species frequently are assigned to urban tolerance groups such as ‘avoiders’, ‘adapters’ and ‘exploiters’. To be useful, species in such groups must show consistent responses to major gradients of change in urban landscapes. We used a landscape-scale approach to investigate the relative influence of housing cover and canopy tree cover on individual bird species and compared the results for species independently classified as urban avoiders, adapters or exploiters. We surveyed birds in 30 landscapes, positioned along concurrent gradients of housing and canopy tree cover, in residential areas of Melbourne, Australia. We modelled relationships between reporting rate for 42 species and the landscape cover of housing and canopy trees, for each gradient independently and in combination. Responses typically were non-linear and varied between species but generally were consistent within groups. Many species (∼40%) responded to a combination of both housing and canopy tree cover. Responses to housing typically were negative, while responses to tree cover were positive for avoiders and adapters, but negative for exploiters. Relationships with tree cover were stronger in landscapes with higher housing density. Whilst species responses likely represent a continuum, the relative consistency within and differences between urban tolerance groups affirm such general classifications to be a useful framework for synthesizing responses of multiple species to urbanization. Importantly, however, the status of at least four species changed considerably over the last 20 years, highlighting that such classifications are dynamic and respond to both environmental change in urban environments and changing biotic relationships.