Relating phenotypic and genetic variation to urbanization in avian species: a case study on house sparrows (Passer domesticus)

Urbanization is expected to increase at staggering rates in the near future underpinning the importance of the urban habitat as a source of biodiversity. One of the urban species, the house sparrow, has always been living and thriving well in highly built-up areas. Yet, since a few decades this commensal of man has showed marked reductions in population size and numbers to such an extent that it has been vanished from sight in many large European cities. Home range size varied significantly along an urban-rural gradient where urban house sparrows were characterized by the smallest home ranges. Home range size in the urban habitat was linked to the spatial distribution of key vegetation (bushes, hedges). In contrast, patch connectivity did not affect home range size in neither suburban nor rural areas. Finally, urban sparrows were in poorest nutritional condition and these indices were positively associated with estimates of home range size. According to the heterzygosity hypothesis genetic diversity promotes homeostatic mechanisms to buffer developmental pathways against random perturbations. Our results corroborated this hypothesis as, in contrast to environmental stress, FA was inversely related to levels of heterozygosity. Heterozygosity at population-level was able to explain as much as 34% of the observed variation in FA but a single key locus was the main driver underlying this pattern, although we could not entirely rule out genome-wide effects. Population genetic analyses could not reveal strong urban bottleneck signatures although these could have been masked by ongoing dispersal events. Principal coordinate analyses showed a moderate distinction between either the urban/suburban populations and the rural ones. Hierarchical Dest values further suggested genetic drift as an important determinant of spatial genetic variation. Urban areas were characterized by a higher average relatedness and higher proportion of close kin. Spatial autocorrelograms further supported small-scale population structure but also reported a smaller extent of positive genetic structure in highly urbanized areas, suggesting genetic drift might be stronger here due to small population sizes. These results advocate to integrate both individual and population-based analyses when attempting to visualize small-scale population structures.