<p>Urbanisation is a major driver of global biodiversity loss, including functional biodiversity. This study characterises the functional space of urban bird and bat assemblages in the city of Poitiers, France. Specifically, our objectives were to quantify functional diversity across Urban Heat and Cool Islands (UHIs and UCIs), test the role of rare species, identify traits linked to urban tolerance or sensitivity, and examine whether birds and bats share ecological strategies in response to urban stressors. We sampled bird and bat assemblages across UHIs and UCIs within Poitiers. We compiled functional traits for each species, and we assessed relationships with fine-scale urban landscape variables using fourth-corner analysis. We quantified functional diversity metrics (alpha, rarity, beta) and trait distributions using Community Weighted Means. Birds and bats exhibited contrasting functional responses to urbanisation. In birds, functional alpha-diversity was higher in UHIs than in UCIs, driven mainly by functional rarity rather than local habitat variables. In contrast, bat functional diversity decreased in UHIs, with no detectable contribution of rare species but a strong influence of urban landscape structure. Urban tolerance was associated with high dispersal and longevity in birds, and with larger size and clutter-adapted echolocation in bats. Both groups shared key ecological adaptations to urbanisation, which likely enable persistence in densely urbanised environments. Overall, urbanisation acts as a strong ecological filter, but its influence differs across taxa in our urban system. This study confirms once again that functional approaches reveal hidden information in taxonomic-only approaches which reveals essential information for urban landscape management and conservation.</p>