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Green areas in urban and peri-urban landscapes are increasingly recognized as important habitats for wild bees, but the extent to which species adjust their reproductive behavior to local conditions in these environments remains poorly studied. We studied the reproductive output, offspring’s body size, sex allocation, and larval diet (assessed using pollen morphology and DNA metabarcoding) of the solitary bee Osmia caerulescens (Linnaeus, 1758) in a Mediterranean peri-urban environment using trap nests and local-scale environmental descriptors. Based on a total of 215 nests and 648 brood cells, females increased their reproductive output with greater floral resource availability and increasing openness of the habitat, but this was accompanied by a reduction in offspring body size (ITD) and a sex allocation skewed toward males. These findings are consistent with a trade-off between offspring number and per-offspring investment and contrary to general expectations that higher resource availability should favor larger and more female-biased offspring. Together, these responses indicate a shift toward quantity-oriented reproductive strategies, in contrast to patterns reported for this species in agricultural systems, where higher resource availability is typically associated with greater per-offspring investment and more female-biased sex ratios. Pronounced among-site variation in pollen diet, reflecting differences in local floral resource composition, further highlights how use of floral resources varies at a fine spatial scale and may shape reproductive strategies and, ultimately, population persistence. Our results show that the ecological value of urban habitats for solitary bees depends not only on the amount of floral resources available, but also on how local conditions influence species-specific reproductive responses. These findings highlight the value of species-specific reproductive analyses in abundant, functionally important solitary bees as a complementary approach to evaluating habitat quality at local scales in urban and peri-urban environments. Understanding how solitary bees adjust reproductive strategies to urban conditions is essential for supporting their conservation and long-term persistence in urban green spaces.