Urban heat islands elevate local temperatures by 2–5°C relative to surrounding rural areas, creating measurable selection differentials on breeding timing in resident passerine populations. In great tits (Parus major) and house sparrows (Passer domesticus), urban cohorts exhibit earlier clutch initiation by 8–14 days compared to rural counterparts — a gap too large to attribute solely to individual phenotypic plasticity within a single season. Common garden experiments with second-generation urban and rural chicks show heritable retention of advanced laying dates, indicating that UHI-driven selection is producing genuine microevolutionary divergence in breeding phenology within 3–5 generations (~15–25 years). If confirmed at scale, urban passerine populations are undergoing faster phenological evolution than climate-driven rural shifts. Does the heritability signal hold across species with longer generation times, or is this a passerine-specific artifact?