Climate-driven shifts in trans-Saharan bird migration illustrated by the spotted flycatcher (Muscicapa striata).

Records of European long-distance migrant birds wintering in the Western Palaearctic have increased recently, possibly in response to milder climatic conditions due to climate change. The spotted flycatcher (Muscicapa striata), traditionally a trans-Saharan migrant, has recently been observed wintering north of the Sahara Desert. This study examines the winter status of this species in the Western Palaearctic and identifies climatically favourable yet unoccupied wintering areas. We compiled wintering records from 2000 to 2024 using the citizen science platform eBird. We then modelled the distribution of these data across Europe and the Middle East using 19 climatic and two topographic variables. Our results provide the first evidence of a regular wintering pattern of the species in the Western Palaearctic over the last decade, with over 80% of the records located on the Iberian Peninsula and the Balearic Islands. This aligns with positive temperature anomalies in Europe between 2014 and 2023. We identified four climatic variables as significant drivers, with the diurnal temperature range and the temperature of the coldest quarter within the study area being the strongest predictors. Favourable wintering areas encompass much of the Mediterranean basin and large Mediterranean islands. Using fuzzy logic, we produced ensemble models of these variables under different hypotheses regarding their relative importance, tested against independent North African wintering records. The most supported hypothesis indicates that while favourable values in the four variables are important, they are not essential, meaning that favourable values in some variables can compensate for less favourable values in others. This species may serve as a model for other trans-Saharan migrants now wintering in the Western Palaearctic. If temperatures continue to increase, more individuals are expected to shift their wintering strategy from trans-Saharan to pre-Saharan migrants, leading to an increase in the populations regularly wintering along Mediterranean coasts and islands.