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Abstract Functional diversity is a multidimensional concept, with different dimensions capturing how functional traits are distributed within communities. Exploring how these dimensions scale with island area (functional diversity–area relationships [FDARs]) is key to understanding how habitat size shapes diversity and community assembly. Using avian abundance and richness data from three habitat‐island systems in southern Brazil, we analysed FDARs using three dimensions ‐ functional richness (FRic), divergence (FDiv) and regularity (FReg) ‐ to test how species abundance influences FDAR form, how FDARs differ among dimensions, and how standardized functional diversity values scale with island area. FDARs were modelled using a multimodel approach to assess their form, while null models were used to obtain standardised effect sizes (SES) of all indices. Abundance had little influence on FDAR form due to strong correlations with species richness. FRic increased more steeply with area than FDiv, while FReg declined, revealing distinct scaling patterns and showing that habitat loss affects FD dimensions differently. SES showed no relationship with area, underscoring that species richness is the main driver of FDARs in these habitat island systems. Community assembly seemed to be largely driven by neutral dynamics, with only a relatively small number of islands characterised by trait clustering and little evidence of overdispersion. However, the prevalence of non‐random assembly patterns varied with island size and across habitat island systems and functional diversity dimensions, the latter finding suggesting that different dimensions may capture distinct assembly processes. These findings underscore the importance of using multidimensional functional diversity approaches to disentangle complex assembly processes and guide conservation strategies in patchy ecosystems. Read the free Plain Language Summary for this article on the Journal blog.