First arrival dates of spring migratory birds in Mirnoye, Central Siberia

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.