Abstract
The impact of urbanization has been widely studied in the context of species diversity and life history evolution. Behavioural adaptation, by contrast, remains poorly understood because empirical studies rarely investigate the relative importance of two key mechanisms: plastic responses vs. non-random distributions of behavioural types. We propose here an approach that enables the simultaneous estimation of the respective roles of these distinct mechanisms. We investigated why risky behaviours are often associated with urbanisation, using an urban nest box population of great tits (Parus major) as a study system. We simultaneously and repeatedly quantified individual behaviour (aggression and flight initiation distance) as well as environmental factors characterizing level of urbanisation (numbers of pedestrians, cars and cyclists). This enabled us to statistically distinguish plastic responses from patterns of non-random distributions of behavioural types. Data analyses revealed that individuals did not plastically adjust their behaviour to the level of urbanization. Behavioural types were instead non-randomly distributed: bold birds occurred more frequently in areas with more cars and fewer pedestrians while shy individuals were predominantly found in areas with fewer cars and more pedestrians. These novel findings imply a major role for behavioural types in the evolutionary ecology of urban environments and call for the full integration of among- and within-individual variation in urban ecological studies.
Highlights
Urbanization represents one of the fastest environmental changes of our times (United Nations, Department of Economic and Social Affairs, and Population Division, 2017) and poses novel challenges to wildlife (Sih et al, 2011)
Our analyses of the sources of variation in behavior, which focussed on the simultaneous estimation of within-individual and among-individual effects of environmental variables related to urbanization (Figure 1), demonstrated that individuals did not plastically adjust their aggressiveness nor their flight initiation distance (FID) in response to withinindividual-among-day variation in PC1 or PC2 (Table 1)
We re-parameterised our models to directly estimate the difference ( ) between the among- and withinindividual effects of each focal gradient as a test for non-random distributions of behavioral types. This analysis produced strong evidence for non-random distributions of behavioral types with respect to FID because the difference ( ) in effect of PC2 among- vs. within-individuals was associated with 95% credible intervals (CIs) that did not overlap zero
Summary
Urbanization represents one of the fastest environmental changes of our times (United Nations, Department of Economic and Social Affairs, and Population Division, 2017) and poses novel challenges to wildlife (Sih et al, 2011). Urban habitats are characterized by a loss of natural resources (e.g., food) and increased anthropogenic levels of disturbance (e.g., pedestrian and vehicular traffic, and industrial noise). For instance, breeding densities are often higher in urban compared to natural habitats (Beissinger and Osborne, 1982). Behavioral Types Along Urban Gradients offspring production in urban compared to rural habitats (reviewed in Chamberlain et al, 2009). Research has focussed on behavioral modifications in urban wildlife (Lowry et al, 2013). The mechanisms responsible for associations between behavior and level of urbanization remain largely unknown
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