Abstract
Climate change is altering the thermal environment of nesting beaches worldwide, threatening species with temperature‐dependent sex determination (TSD) such as sea turtles. While models have predicted feminization of primary sex ratios—that is, a progressive increase in the proportion of females—empirical, population‐scale evidence across multiple species remains rare. Here we present the first broad‐scale, multi‐species evidence of long‐term changes in incubation duration (ID)—used as a proxy for temperature and primary sex ratio—across genetically distinct Management Units (MUs) of loggerhead ( Caretta caretta ), green ( Chelonia mydas ), and leatherback ( Dermochelys coriacea ) turtles nesting in Florida, USA. We introduce a simple, scalable method to assess population‐level feminization trends by identifying directional shifts in ID distributions over time, avoiding the uncertainty of model‐based primary sex ratio estimates. Using data from over 110,000 clutches laid between 2001 and 2022, we document significant declines in ID, spatial variation in embryo mortality across MUs, likely associated with greater exposure to lethal incubation temperatures, and the presence of seasonal and geographic male‐producing refugia. These findings provide robust empirical evidence of increasing feminization, early signs of temperature‐related lethal effects in at least one region, and highlight the importance of MU‐scale, species‐specific monitoring. This study underscores the need to protect male‐producing beaches and early‐ and late‐season clutches, which may be disproportionately vulnerable or overlooked. Given the simplicity and accessibility of ID data, we encourage its broader use in sea turtle conservation and recommend applying our approach to detect climate‐driven trends in incubation conditions and potential feminization across other rookeries.