Figure legends
Figure 1. Mean monthly temperature (°C) in ten polygons across the longitudinal axis in the south (a) and north (b) of the Palearctic biogeographic zone. Mean monthly temperature data (spatial resolution of 30 seconds) were based on WorldClim version 2.1 climate data for 1970-2000 (Fick & Hijmans 2017). Data processing and calculations were performed using the R package ’raster’ (Hijmans & van Etten 2016) and indicate a longer and colder winter in the Eastern Palearctic than in the Western Palearctic.
Figure 2. The phylogenetic tree of the 21 examined species included in the Phylogenetic Generalized Least Square (PGLS) analysis. The tree is based on an analysis of global bird diversity (Jetz et al. 2012) using 10,000 trees obtained from BirdTree.org (Ruboliniet al. 2015). The consensus tree was built using BEAST version 1.8.4. The tip colour represents the difference in (Δ) moult extent between the Eastern and the Western Palearctic samples (black = maximum Δ, white = minimum Δ). The scale (top left) represents 10 million years.
Figure 3. The difference in mean moult extent (number of moulted feathers) between the Eastern and Western Palearctic (n = 21 passerine species). The results indicate that moult extent among western populations is greater than among eastern populations of the same species. The boxplots display the minimum, 1st and 3rd quartiles, median and maximum of each tested region.
Figure 4. The effect of species-specific differences in (Δ) migration distance on the difference in (Δ) moult extent between Eastern and Western Palearctic passerines among the 21 examined species (± 95% confidence intervals). The results indicate a strong relationship between Δ migration distance and Δ moult extent: if an eastern population migrates a longer distance than the western population of the species, the extent of its moult decreases compared to the western population. The circled insets depict examples of Δ moult extent between Eastern and Western Palearctic regions in relation to the Δ migration distance for two species (right: Coal Tit Periparus ater , left: White Wagtail Motacilla alba ). The wing areas that are coloured depict the wing’s moulted feathers while non-coloured areas represent non-moulted feathers.
Figure 5. A schematic representation of major annual processes in Eastern (inner circle) and Western Palearctic (outer circle) passerines. The information is based on published data (Del Hoyoet al. 2019) and the authors’ field experience (Supplementary Note 1) and demonstrates a delay of about one month in spring and summer but not autumn events in Eastern Palearctic populations, causing a shorter post-juvenile moult period compared to the Western Palearctic. The overlap between migration, breeding and moult is represented by colour combinations.