Despite the obvious shift in spring bird migration to earlier dates, the mechanism behind this process remains unclear, partly due to inappropriate climate predictors. Information on the influence of climate is not geographically representative, as it comes almost exclusively from the marginal parts of the continents.

The objective of this study is to investigate the mechanism for determining the timing of spring terrestrial bird migration based on a new concept of environmental phenology and phenological navigation, using data from Central Siberia as an example.

A literature review suggests that migrants follow the phenology of food resources, the dependence on which intensifies toward the end of migration with increasing seasonality of temperature conditions. We propose the concept of environmental phenology as a mediator between the influence of climate and the response of migrants. We defined the environmental phenology index as the average weighted temperature of the preceding days with weights decreasing exponentially in retrospect.

Using original data from the first arrival dates of 145 species in Central Siberia from 1976 to 2025, we demonstrated the effectiveness of the environmental phenology index in predicting arrival dates. This indicator reflects the accumulation of climate information by numerous ongoing phenological processes. Due to its high autocorrelation, it predicts future foraging conditions.

Despite significant warming at the study site (0.60 degrees per decade), bird arrival dates in the studied community responded to warming with a slight shift toward earlier dates: on average, by -0.91 days per degree annually and by -0. 86 days per decade. These estimates are lower than those known for other regions due to the pronounced continental climate and the steep rise in spring temperatures (1 degree in 3 days). We believe the compensated share of this impact (the response to impact ratio) to be a more adequate measure of the organism's response to the impact.

Based on the study results, we propose a mechanism of phenological navigation, in which a migrant at each stopover chooses whether to move on or wait for changes based on the relationship between environmental phenology and day length. This simple scheme opens new possibilities for comparing the phenological responses of different groups of organisms.

The proposed index of environmental phenology is theoretically and practically substantiated, expands the understanding of the mechanism of phenological navigation and can be used to solve similar phenological issues.