Climate change affects the spatial distribution and timing of seasonal events (i.e. phenology) of species. The phenology of ectotherms, such as sea turtles, is strongly influenced by rising temperatures associated with climate change, since most of their life history traits are dependent on temperature. Most studies on the phenology of sea turtles report changes in the duration and an earlier onset of the nesting season in response to warmer temperatures. However, inconsistencies remain among and within species in the direction and the strength of their responses. Some authors suggested that sea turtles' nesting phenology is triggered by a cue perceived before migration, whereas others suggested that environmental conditions at nesting sites are more important. Here, using nesting data collected over 26 nesting seasons, we tested whether the nesting phenology of loggerhead turtles Caretta caretta in Brazil is triggered by either temperature or demography, and if this occurs at nesting or foraging sites. We found that the onset of the nesting season was triggered by temperature at an important foraging site, while the duration of the nesting season was linked to the size of the nesting population. Our results suggest that loggerhead turtles respond to an environmental cue perceived at foraging sites before they start migrating. This finding will allow future research to assess the vulnerability of sea turtles in the context of climatic change by considering environmental conditions at foraging sites.