Theoretically, one might restrict studies of urbanization to only those sites which lack vegetation entirely in order to focus on species able to adapt to the most heavily modified areas.
However, in most cities such areas are used by only a few species, and it has been commented that only a small proportion of the bird species that are found in cities are well adapted to urban environments [ 18 ]. In Oslo, the most urbanized areas which lack trees and other vegetation are used by only a handful of land bird species mainly Columba livia , Apus apus , Corvus corone , Motacilla alba and Passer domesticus , S. Dale, personal observations and we do not think it is useful to restrict analyses aimed at understanding the process of urbanization to such a limited number of species.
Moreover, most previous studies have taken a similar approach as ours, focusing on the full range of species utilizing urban areas e. As expected, species nesting in safer sites or higher above ground were relatively more common in urban areas, in line with findings from other studies e. Furthermore, we found that breeding habitat selection influenced the likelihood of species occurring in urban sites. The most important finding was that farmland species were more often present in urban sites than species breeding in mixed or deciduous forest. Here it should be noted that several common species classified as belonging to farmland habitat use farm buildings and yards extensively such as Pica pica , Corvus corone , Sturnus vulgaris , Passer montanus , Motacilla alba , which possibly make them preadapted to exploiting urban areas.
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The finding that species from coniferous forest habitat were less likely to become urban was expected because the urban areas in Oslo generally lacked this habitat, but this result underlines that availability of suitable habitat is a prerequisite for urban invasion. Thus, the analyses simply showed that these species were more common in urban sites than their rural commonness would predict.
Methods and Additional file 1 : Table S2. Our analyses showed that body mass-corrected brain mass was not significantly associated with species occurrence in urban areas, even in an analysis in which frequency in surroundings was not included. We failed to find significant effects of relative brain mass in independent analyses from seven European cities.
In the comprehensive analysis of data from Oslo, the estimated slope of brain mass was negative but not significant , opposite to the prediction from the brain mass hypothesis. Even the upper confidence limits of the effect sizes of relative brain mass, although positive, were in general small, suggesting that increasing the power of the analyses would still lead to the conclusion that brain mass has a weak effect on urban commonness compared to rural commonness and ecological variables.
Our finding that relative brain mass was not significantly related to urban commonness is consistent with a number of recent studies [ 18 - 20 ]. However, it stands in contrast to other studies indicating that adaptation to urban life is related to brain size [ 13 , 14 ]. We suggest that if brain size is relatively larger among urban birds, this may be mediated through an effect of brain size on general commonness of species e.
Ducatez et al. On the other hand, to our knowledge there has been no specific analyses of the relationship between brain size and rural abundance, though at least one study has examined the relationship between population decline and relative brain size [ 69 ]. Interestingly, we also found that among lineages well known for their large brains e.
Corvidae and Paridae several species in our sample were actually rare or absent from the urban areas in Oslo despite being widespread in the surroundings Garrulus glandarius , Nucifraga caryocatactes , Corvus corax , Lophophanes cristatus , Poecile montanus.
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This contrasts with an earlier study which indicated that a larger proportion of species in large-brained families were recorded in cities [ 14 ]. More specifically, that study found that, among 82 passerine bird species occurring around 12 cities in France and Switzerland, species with larger brains were more likely to be classified as successful urban colonizers i. However, their analyses ignored ecological variables and the potential effect of rural abundance.
Thus, these data may also be compatible with the idea that rural abundance may affect urban occurrence.
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However, they also found that urban invasiveness was related to rural abundance, but they did not perform multivariate analyses to assess the relative effect of brain size versus rural abundance on urban abundance. Thus, pending more detailed analyses, these findings may also be compatible with the idea that relative brain size may affect rural abundance, and, subsequently, rural abundance determines urban abundance. In contrast, Kark and coworkers [ 18 ] did not find any significant changes in mean relative brain size of species along an urbanization gradient in Jerusalem, but suggested that ability to exploit urban areas depended on a combination of life history traits such as diet, sociality, migration and nest sites.
Similarly, Evans and coworkers [ 19 ] analysed densities of 55 species in about British urban and rural sites, but found no evidence that relative brain size influenced responses of species to urbanization. Instead, the authors concluded that generalist species were most favoured by urban development. Finally, Sol and coworkers found no effect of brain size when comparing species classified as urban exploiters or urban avoiders from 22 locations from five continents [ 20 ].
There is a vast literature suggesting that brain size may have profound ecological implications see e. From this, it is an obvious possibility that brain size may play a role in adaptation to urban life considering that this may require shifts in anti-predator behaviour [ 13 ], foraging behaviour [ 11 ], and nest site selection [ 17 ]. However, based on our review of previous studies on the relationship between brain size and urban life among birds, we find the evidence in favour of an effect of brain size to be equivocal, and our data clearly do not support such a hypothesis.
However, we acknowledge that discrepancies between studies regarding the effect of brain size on urban living could be due to geographical variation in the importance of this trait [ 20 ]. Finally, our data provided some insight into the importance of body mass for urban living. Large-bodied species might cope with urban environments better e.
Evans and coworkers [ 23 ] suggested that high population density in original habitat could promote invasion of urban areas. Our results provide strong support for this hypothesis. However, one important question is whether this means that urban areas are sinks relative to source populations outside cities. We think the answer may depend on the species in question. Urban establishment has been documented in detail in a few species; these studies show a pattern of multiple independent colonizations Columba palumbus [ 72 ]; Turdus merula [ 23 , 73 ] , suggesting that urban populations are no longer dependent on immigration from surroundings.
We believe this may be the case for several other widespread and common urban bird species. However, the urban bird community also contains a number of less common species, with much more variable occurrence. Unfortunately, there are no relevant studies providing information on whether these species have self-sustaining urban populations or depend on a continuous supply of immigrants from surrounding areas but see [ 74 ] for a review of differences in reproductive parameters of urban and rural populations.
We suggest that the latter may be the case, and that studies aimed at determining the origin of individuals of such species are needed. The strong quantitative relationship between commonness of rural and urban populations found in the present study suggests that regional spatial variation in source populations may be an important explanation for variation in bird communities between cities. We are not aware of any studies which have tested this idea. On the other hand, urbanization is a recent process, and there may also be time lags in adaptation to urban environments which may add further spatial variation in use of urban areas by species present in the regional species pool.
This may explain why some species did not occur within Oslo despite being common in the surroundings and common in urban areas of other parts of Europe e. Turdus philomelos , Garrulus glandarius. We predict that, over time, colonization pressure may cause such species to also become urban in Oslo.
Our results showed that urban commonness was primarily determined by how common species were in rural areas in combination with habitat preferences and nest site locations, and not by the relative brain mass or body mass of species. Thus, rural bird populations play an important role in shaping urban bird communities. The number of bird species which have become fully adapted to urban life is low [ 18 ], and this suggests that high urban bird diversity may depend on 1 source areas outside cities with high diversity, and 2 limitations on urban sprawl to take into account that immigration rates may be distance dependent [ 75 ].
Thus, green spaces in urbanized areas may be compared to islands, and basic biogeographical principles relating immigration rates from source mainland areas may determine species richness [ 24 , 25 ]. Together with well known effects of park size and vegetation structure [ 76 , 77 ], the present findings may help city planners and wildlife managers to better understand how urban bird diversity can be maintained or increased.
The data set supporting the results of this article is available in the Data Dryad repository, doi: Avian Ecology and Conservation in an Urbanizing World. Boston: Kluwer Academic Publishers; Urban effects on native avifauna: a review. Landscape Urban Plan. From patterns to emerging processes in mechanistic urban ecology. Trends Ecol Evol. Avifauna homogenisation by urbanisation: analysis at different European latitudes. Biol Conserv.
Urban birds have broader environmental tolerance. Biol Lett. Determinants of local extinction and turnover rates in urban bird communities.
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Ecol Appl. Does urbanization filter birds on the basis of their biological traits? Successful city dwellers: a comparative study of the ecological characteristics of urban birds in the Western Palearctic. Cassey P.