Birds use local environmental cues to fine-tune the timing of egg laying such that the nestling period normally coincides with the local peak in food availability. Ambient temperature, vegetation phenology, and insect phenology are often considered the most likely cues, but no previous studies have explicitly compared and partitioned their relative effects. We used confirmatory path analyses and a long-term study of Blue Tits (Cyanistes caeruleus) to identify and measure the relative weighting of the causal paths that link laying date to spring phenology and temperature in deciduous and evergreen oak forests on Corsica. Path analysis showed that the effects of temperature and vegetation phenology vary between forest types and season. In deciduous oak forest, where females lay eggs early in spring, phenology of vegetation or insects sets the laying date. In evergreen oak forest, where breeding occurs later in the season, females shift from a predominantly phenology-based cue system to a predominantly temperature-based cue system. This plasticity in the decision process allows birds to minimize the risk of mismatching breeding date with the optimal time window and may be critical in allowing birds to track human-induced environmental change.