Traits shaping urban tolerance in birds differ around the world

Area of Science:

• Ecology
• Urban Ecology
• Conservation Biology

Background:

• Biodiversity faces local extinction due to increasing human density and urbanization.
• Understanding species' tolerance to urban environments is crucial for conservation, but global patterns remain elusive.
• Existing research links urban tolerance to species traits, yet a generalizable predictive framework is lacking.

Purpose of the Study:

• To calculate an Urban Association Index (UAI) for bird species globally.
• To identify species-specific traits associated with urban tolerance.
• To investigate how city-specific variables influence trait-urban tolerance relationships.

Main Methods:

• Calculated the Urban Association Index (UAI) for 3,768 bird species across 137 cities worldwide.
• Assessed the relationship between UAI and ten species-specific traits (e.g., body mass, diet, dispersal ability).
• Tested how latitude and human population density modify the strength of these trait-urban tolerance relationships.

Main Results:

• Nine out of ten species traits were significantly associated with urban tolerance.
• Urban-associated species generally exhibit smaller body size, lower territoriality, higher dispersal ability, broader niches, larger clutch sizes, greater longevity, and lower elevational limits.
• The influence of traits like body mass and diet breadth on urban tolerance varied with latitude, while territoriality and longevity associations were affected by human population density.

Conclusions:

• Species traits predict urban tolerance, but the strength of these predictions varies geographically.
• Biogeographic variation in selection pressures explains challenges in finding global patterns of urban tolerance.
• A globally informed framework is needed to predict urban tolerance and guide conservation efforts in the face of widespread urbanization.