Phenological data refers to the study and observation of natural events and processes in relation to the timing of seasonal changes. This includes the timing of plant flowering, leaf unfolding, bird migration, and insect emergence, among other biological phenomena. Phenological data is valuable for understanding how ecosystems and species are responding to environmental changes, such as climate change. Researchers collect phenological data through field observations, satellite imagery, and citizen science projects to track long-term trends and patterns in the timing of natural events.