Environmental phenology as a driver of spring migration timing

Despite the apparent shift of spring bird migration to earlier dates, the mechanism behind this process remains unclear partly because the climate predictors used are indirect or their causal relationships are not tested. We analysed first spring observations of 145 species during 1976–2022 in Central Siberia. Using this material, we showed that a combination of temperature accumulation and averaging methods represents an effective indicator of environmental phenology that may be a direct determinant of migration timing. This indicator reflected the accumulation of climatic information by multiple ongoing phenological processes responsible for future foraging conditions. In the studied bird community, arrival dates shifted annually by 1/4 of the deviation in environmental phenology and by 1/3 of the directed climate warming on a long-term scale. Several reasons for this low compensation have been considered, including the role of photoperiodic guidance, spatiotemporal autocorrelation of environmental phenology, and the continentality of regions. Some groups of species were found to be less successful in compensating for annual climate fluctuations: short-distance migrants, which are exposed to harsh conditions, and eastern migrants, which fly against the air mass movement and therefore lack climate information. The same groups lead the long-term shift, suggesting stronger annual selection pressures. Thus, the proposed index of environmental phenology is theoretically and practically justified and can change the understanding of migration mechanisms.