Urban Bird Diversity Was More Strongly Linked to Forest Structure Than the Surrounding City
Bird diversity in Mediterranean urban forests was most strongly linked to the structure of the forest stand itself, while the total number of birds was more strongly linked to the surrounding landscape. A study of 12 urban and peri-urban forests in Florence, Rome and Campobasso recorded 2,035 birds from 52 species and compared species richness, abundance and functional diversity with vegetation and landscape characteristics.
The researchers combined bird surveys with detailed measurements of tree structure, dead wood, microhabitats and the composition and configuration of land cover around each forest. This allowed them to ask whether different dimensions of bird diversity respond primarily to conditions inside the woodland or to the larger urban landscape.
Local forest structure mattered most for diversity
Species richness and functional dispersion were associated mainly with the structure of the woodland itself. Functional dispersion measures how varied the birds’ ecological traits are, including where they forage and nest. A forest with both ground-feeding birds and species searching for insects on tree trunks or in the canopy can support a wider variety of ways of life than one dominated by birds using the same resources. Tree density was an influential predictor, but more trees did not consistently mean higher diversity.
Canopy development also mattered. Forests with a larger crown ratio, meaning a greater proportion of tree height occupied by the crown, generally had higher predicted species richness and functional diversity. But the relationship with variation in tree height was not simply positive: moderate height variation was more favourable for predicted richness than the highest variation measured. A varied woodland can provide feeding and nesting opportunities at several heights without maximum structural unevenness always being beneficial.
Abundance responded more to the surrounding landscape
The total abundance of birds showed a different pattern. Landscape-scale variables, including the aggregation of forest patches and diversity of surrounding land cover, contributed more strongly than local stand structure. This indicates that the broader urban matrix can influence movement, dispersal and access to resources even when local vegetation determines much of the diversity found within a forest.
The models predicted greater diversity at intermediate levels of variation in surrounding land cover and in how tightly forest patches were clustered. Neither highly uniform surroundings nor the most extreme patch arrangements consistently gave the best predictions within the studied range. This is not a universal prescription for an ideal urban forest: responses also differed among bird species and cities.
Familiar species formed part of a wider urban community
Several widespread species were recorded consistently across the study system. Eurasian Blackbird (Turdus merula), Eurasian Blackcap (Sylvia atricapilla), Eurasian Blue Tit (Cyanistes caeruleus) and Great Tit (Parus major) were among the recurring components of the urban-forest bird assemblages.
The study therefore shows why urban bird conservation cannot be reduced to the size of a green space alone. The internal structure of urban woodland was especially important for how many species and ecological strategies were supported, while the surrounding landscape helped determine how many birds used those forests in total.
Ecology
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